ORCID Profile
0000-0002-5068-1591
Current Organisation
University of New South Wales - Randwick Campus
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Research Topics
In Research Link Australia (RLA), "Research Topics" refer to ANZSRC FOR and SEO codes. These topics are either sourced from ANZSRC FOR and SEO codes listed in researchers' related grants or generated by a large language model (LLM) based on their publications.
Top 5 Research Topics
ANZSRC Field of Research (FoR)
Palaeoclimatology | Physical Geography and Environmental Geoscience | Climate Change Processes | Chemical Oceanography | Quaternary Environments | Oceanography | Physical Oceanography | Glaciology
ANZSRC Socio-Economic Objective (SEO)
Climate Variability (excl. Social Impacts) | Expanding Knowledge in the Earth Sciences | Effects of Climate Change and Variability on Antarctic and Sub-Antarctic Environments (excl. Social Impacts) | Physical and Chemical Conditions of Water in Marine Environments | Climate and Climate Change not elsewhere classified | Atmospheric Composition (incl. Greenhouse Gas Inventory) | Global Effects of Climate Change and Variability (excl. Australia, New Zealand, Antarctica and the South Pacific) (excl. Social Impacts) | Expanding Knowledge in the Environmental Sciences |
Related Links
Publications
Supplementary material to Antarctic sea ice over the past 130,000 years, Part 1: A review of what proxy records tell us
Publisher: Copernicus GmbH
Date: 06-04-2022
Impact of oceanic circulation changes on atmospheric δ13CO2
Publisher: American Geophysical Union (AGU)
Date: 11-2015
DOI: 10.1002/2015GB005207
Abrupt changes in the southern extent of North Atlantic Deep Water during Dansgaard–Oeschger events
Publisher: Springer Science and Business Media LLC
Date: 12-10-2015
DOI: 10.1038/NGEO2558
The penultimate deglaciation: protocol for PMIP4 transient numerical simulations between 140 and 127 ka
Publisher: Copernicus GmbH
Date: 05-09-2018
DOI: 10.5194/CP-2018-106
Abstract: Abstract. The penultimate deglaciation (~ 138–128 thousand years before present, hereafter ka) is the transition from the penultimate glacial maximum to the Last Interglacial (LIG, ~ 129–116 ka). The LIG stands out as one of the warmest interglacials of the last 800 ka, with high-latitude temperature warmer than today and global sea level likely higher by at least 6 meters. The LIG therefore receives ever-growing attention, in particular to identify mechanisms and feedbacks responsible for such regional warmth that is comparable to that expected before 2100. Considering the transient nature of the Earth system, the LIG climate and ice-sheets evolution were certainly influenced by the changes occurring during the penultimate deglaciation. It is thus important to investigate the climate and environmental response to the large changes in boundary conditions (i.e. orbital configuration, atmospheric greenhouse gas concentrations, ice sheet geometry) occurring during this time interval. A deglaciation working group has recently been set up as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phase 4, with a protocol to perform transient simulations of the last deglaciation (19–11 ka). Similar to the last deglaciation, the disintegration of continental ice-sheets during the penultimate deglaciation led to significant changes in the oceanic circulation during Heinrich Stadial 11 (~ 136–129 ka). However, the two deglaciations bear significant differences in magnitude and temporal evolution of climate and environmental changes. Here, as part of the PAGES-PMIP working group on Quaternary Interglacials, we propose a protocol to perform transient simulations of the penultimate deglaciation to complement the PMIP4 effort. This design includes time-varying changes in orbital forcing, greenhouse gas concentrations, continental ice-sheets as well as freshwater input from the disintegration of continental ice-sheets. This experiment is designed to assess the coupled response of the climate system to all forcings. Additional sensitivity experiments are proposed to evaluate the response to each forcing. Finally, a selection of paleo records representing different parts of the climate system is presented, providing an appropriate benchmark for upcoming model-data comparisons across the penultimate deglaciation.
Exploring the oxygen isotope fingerprint of Dansgaard-Oeschger variability and Heinrich events
Publisher: Elsevier BV
Date: 03-2017
Tipping elements and amplified polar warming during the Last Interglacial
Publisher: Elsevier BV
Date: 04-2020
Natural carbon release over-rides anthropogenic carbon uptake when Southern Hemispheric westerlies strengthen
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-12282
Abstract: & & The Southern Ocean is one of today's largest sink of carbon, having absorbed about 10\\% of the anthropogenic carbon emissions. Southern Ocean's dynamics are principally modulated by the strength of the Southern Hemispheric westerlies, & which are projected to increase over the coming century. Here, using a high-resolution ocean-sea-ice-carbon cycle model, we explore the impact of idealized changes in Southern Hemispheric westerlies on the ocean carbon storage . We find that a 20\\% strengthening of the Southern Hemispheric westerlies leads to a $\\sim$25 Gt loss of natural carbon, while an additional 13 Gt of anthropogenic carbon is absorbed compared to the control run, thus resulting in a net loss of $\\sim$12 GtC from the ocean over a period of 42 years. This tendency is enhanced if the westerlies are also shifted polewards, with a total natural carbon loss of almost 37 GtC, and an additional anthropogenic carbon uptake of 18 GtC. While both experiments display a large natural carbon loss south of 10$^\\circ$S, the litude is three times greater in the poleward strengthening case, which is & not fully compensated by the increase in anthropogenic carbon content. However, the poleward wind shift leads to significant differences in the pattern of DIC change due to a weakening of the upper overturning cell, & which leads to an increase in natural and total carbon north of 35$^\\circ$S in the upper 2000 m.& &
A multi-model CMIP6-PMIP4 study of Arctic sea ice at 127 ka: sea ice data compilation and model differences
Publisher: Copernicus GmbH
Date: 11-01-2021
Abstract: Abstract. The Last Interglacial period (LIG) is a period with increased summer insolation at high northern latitudes, which results in strong changes in the terrestrial and marine cryosphere. Understanding the mechanisms for this response via climate modelling and comparing the models' representation of climate reconstructions is one of the objectives set up by the Paleoclimate Modelling Intercomparison Project for its contribution to the sixth phase of the Coupled Model Intercomparison Project. Here we analyse the results from 16 climate models in terms of Arctic sea ice. The multi-model mean reduction in minimum sea ice area from the pre industrial period (PI) to the LIG reaches 50 % (multi-model mean LIG area is 3.20×106 km2, compared to 6.46×106 km2 for the PI). On the other hand, there is little change for the maximum sea ice area (which is 15–16×106 km2 for both the PI and the LIG. To evaluate the model results we synthesise LIG sea ice data from marine cores collected in the Arctic Ocean, Nordic Seas and northern North Atlantic. The reconstructions for the northern North Atlantic show year-round ice-free conditions, and most models yield results in agreement with these reconstructions. Model–data disagreement appear for the sites in the Nordic Seas close to Greenland and at the edge of the Arctic Ocean. The northernmost site with good chronology, for which a sea ice concentration larger than 75 % is reconstructed even in summer, discriminates those models which simulate too little sea ice. However, the remaining models appear to simulate too much sea ice over the two sites south of the northernmost one, for which the reconstructed sea ice cover is seasonal. Hence models either underestimate or overestimate sea ice cover for the LIG, and their bias does not appear to be related to their bias for the pre-industrial period. Drivers for the inter-model differences are different phasing of the up and down short-wave anomalies over the Arctic Ocean, which are associated with differences in model albedo possible cloud property differences, in terms of optical depth and LIG ocean circulation changes which occur for some, but not all, LIG simulations. Finally, we note that inter-comparisons between the LIG simulations and simulations for future climate with moderate (1 % yr−1) CO2 increase show a relationship between LIG sea ice and sea ice simulated under CO2 increase around the years of doubling CO2. The LIG may therefore yield insight into likely 21st century Arctic sea ice changes using these LIG simulations.
Internally generated millennial-scale climate variability in an earth system model of intermediate complexity: sensitivity to ocean bathymetry and orbital forcing
Publisher: Copernicus GmbH
Date: 25-02-2010
Abstract: Abstract. The effect of orbital variations on simulated millennial-scale variability of the Atlantic Meridional Overturning Circulation (AMOC) is studied using the earth system model of intermediate complexity LOVECLIM. It is found that for present-day topographic boundary conditions low obliquity values (~22.1°) favor the triggering of internally generated millennial-scale variability in the North Atlantic region. Reducing the obliquity leads to changes of the pause-pulse ratio of the corresponding AMOC oscillations. Stochastic excitations of the density-driven overturning circulation in the Nordic Seas can create regional sea-ice anomalies and a subsequent reorganization of the atmospheric circulation. The resulting remote atmospheric anomalies over the Hudson Bay can release freshwater pulses into the Labrador Sea leading to a subsequent reduction of convective activity. The millennial-scale AMOC oscillations disappear if LGM bathymetry (with closed Hudson Bay) or Hudson Bay salinity is prescribed. Furthermore, our study documents the marine and terrestrial carbon cycle response to millennial-scale AMOC variability.
Quantification of factors impacting seawater and calcite δ18O during Heinrich Stadials 1 and 4
Publisher: American Geophysical Union (AGU)
Date: 07-2015
DOI: 10.1002/2014PA002751
Evaluating the Extent of North Atlantic Deep Water and the Mean Atlantic δ13C From Statistical Reconstructions
Publisher: American Geophysical Union (AGU)
Date: 06-2019
DOI: 10.1029/2019PA003589
Marine carbon cycle response to a warmer Southern Ocean: the case of the Last Interglacial
Publisher: Copernicus GmbH
Date: 17-08-2021
DOI: 10.5194/CP-2021-98
Abstract: Abstract. Recent studies investigating future warming scenarios have shown that the ocean carbon sink will weaken over the coming century due to ocean warming and changes in oceanic circulation. However, significant uncertainties remain regarding the magnitude of the oceanic carbon cycle response to warming. Here, we investigate the Southern Ocean’s (SO, south of 40° S) carbon cycle response to warmer conditions, as simulated under Last Interglacial boundary conditions (LIG, 129–115 thousand years ago, ka). We find a ∼150 % increase in carbon dioxide (CO2) outgassing over the SO at the LIG compared to pre-industrial (PI), due to a 0.5 °C increase in SO sea surface temperatures. This is partly compensated by an equatorward shift of the Southern Hemisphere westerlies and weaker Antarctic Bottom Water formation, which lead to an increase in dissolved inorganic carbon (DIC) in the deep ocean at the LIG compared to PI. These deep ocean DIC changes arise from increased deep and bottom water residence times, and higher remineralization rates due to higher temperatures. While our LIG simulation features a large reduction in SO sea-ice compared to PI, we find that changes in sea ice extent exert a minor control on the marine carbon cycle. Our results thus suggest that the projected poleward intensification of the SH westerlies, coupled with warmer conditions at the surface of the SO would weaken the SO carbon uptake over the coming century.
Past rapid warmings as a constraint on greenhouse-gas climate feedbacks
Publisher: Springer Science and Business Media LLC
Date: 30-08-2022
DOI: 10.1038/S43247-022-00536-0
Abstract: There are large uncertainties in the estimation of greenhouse-gas climate feedback. Recent observations do not provide strong constraints because they are short and complicated by human interventions, while model-based estimates differ considerably. Rapid climate changes during the last glacial period (Dansgaard-Oeschger events), observed near-globally, were comparable in both rate and magnitude to current and projected 21st century climate warming and therefore provide a relevant constraint on feedback strength. Here we use these events to quantify the centennial-scale feedback strength of CO 2 , CH 4 and N 2 O by relating global mean temperature changes, simulated by an appropriately forced low-resolution climate model, to the radiative forcing of these greenhouse gases derived from their concentration changes in ice-core records. We derive feedback estimates (expressed as dimensionless gain) of 0.14 ± 0.04 for CO 2, 0.10 ± 0.02 for CH 4 , and 0.09 ± 0.03 for N 2 O. This indicates that much lower or higher estimates of gains, particularly some previously published values for CO 2 , are unrealistic.
Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing
Publisher: Copernicus GmbH
Date: 14-01-2014
Abstract: Abstract. Millennial-scale variability associated with Dansgaard–Oeschger events is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-D intermediate complexity earth system model covering the period 50 to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice-sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing northern hemispheric ice volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model aleo data comparison. Our results are consistent with the idea that variations in ice-sheet calving and subsequent changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of glacial millennial-scale variability seen in paleorecords across the globe.
Poleward shift in the Southern Hemisphere westerly winds synchronous with the deglacial rise in CO2
Publisher: Research Square Platform LLC
Date: 16-04-2021
DOI: 10.21203/RS.3.RS-404786/V1
Abstract: The Southern Hemisphere westerly winds strongly influence deep ocean circulation and carbon storage 1 . While the westerlies are hypothesised to play a key role in regulating atmospheric CO 2 over glacial-interglacial cycles 2–4 , past changes in their position and strength remain poorly constrained 5–7 . Here, we use a compilation of planktic foraminiferal δ 18 O from across the Southern Ocean and constraints from an ensemble of climate models to reconstruct changes in the westerlies over the last deglaciation. We find a 4.7° (2.9-6.9°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (about 20,000 years ago) relative to the mid-Holocene (about 6,000 years ago). Our reconstruction shows that the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO 2 . Experiments with a 0.25° resolution ocean-sea-ice-carbon model demonstrate that shifting the westerlies equatorward substantially reduces the overturning rate of the abyssal ocean, leading to a suppression of CO 2 outgassing from the Southern Ocean. Our results establish a central role for the westerly winds in driving the deglacial CO 2 rise, and suggest natural CO 2 outgassing from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming 8–10 .
An Atlantic–Pacific ventilation seesaw across the last deglaciation
Publisher: Elsevier BV
Date: 08-2015
Southern Ocean link between changes in atmospheric CO2 levels and northern-hemisphere climate anomalies during the last two glacial periods
Publisher: Elsevier BV
Date: 02-2020
Supplementary material to "Evaluating seasonal sea-ice cover over the Southern Ocean from the Last Glacial Maximum"
Publisher: Copernicus GmbH
Date: 22-12-2020
Mechanisms of millennial-scale atmospheric CO2 change in numerical model simulations
Publisher: Elsevier BV
Date: 09-2019
Southern Ocean carbon sink enhanced by sea-ice feedbacks at the Antarctic Cold Reversal
Publisher: Springer Science and Business Media LLC
Date: 22-06-2020
Marine carbon cycle response to a warmer Southern Ocean: the case of the last interglacial
Publisher: Copernicus GmbH
Date: 16-03-2022
Abstract: Abstract. Recent studies investigating future warming scenarios have shown that the ocean carbon sink will weaken over the coming century due to ocean warming and changes in oceanic circulation. However, significant uncertainties remain regarding the magnitude of the oceanic carbon cycle response to warming. Here, we investigate the Southern Ocean's (SO, south of 40∘ S) carbon cycle response to warmer conditions, as simulated under last interglacial boundary conditions (LIG, 129–115 ka). We find a ∼150 % increase in carbon dioxide (CO2) outgassing over the SO at the LIG compared to pre-industrial conditions (PI), due to a 0.5 ∘C increase in SO sea surface temperatures. This is partly compensated for by an equatorward shift of the Southern Hemisphere (SH) westerlies and weaker Antarctic Bottom Water formation, which both lead to an increase in dissolved inorganic carbon (DIC) in the deep ocean at the LIG compared to PI. These deep-ocean DIC changes arise from increased deep- and bottom-water residence times and higher remineralization rates due to higher temperatures. While our LIG simulation features a large reduction in SO sea ice compared to the PI, we find that changes in sea ice extent exert a minor control on the marine carbon cycle. The projected future strengthening and poleward shift of the SH westerlies coupled to warmer conditions at the surface of the SO should thus weaken the capacity of the SO to absorb anthropogenic CO2 over the coming century.
Deglacial evolution of regional Antarctic climate and Southern Ocean conditions in transient climate simulations
Publisher: Copernicus GmbH
Date: 21-06-2018
DOI: 10.5194/CP-2018-69
Abstract: Abstract. Constraining Antarctica′s climate evolution since the end of the Last Glacial Maximum (∼18 kyr) remains a key challenge, but is important for accurately projecting future changes in Antarctic ice sheet mass balance. Here we perform spatial and temporal analysis of two transient deglacial climate simulations, one using a fully coupled GCM and one using an intermediate complexity model, to (1) better understand the mechanisms driving regional differences observed in paleoclimate records, and (2) identify the main strengths and limitations of the models in terms of parameters that impact ice sheet mass balance. The climate simulations show the greatest continental surface warming over the continental margins and regions with the greatest decrease in ice surface elevation, suggesting that sea ice-albedo feedbacks and ice sheet dynamics likely played strong roles in driving regional surface temperature differences during the deglaciation. The spatial distributions of simulated accumulation changes are quite distinct, with the intermediate complexity model experiencing resolution-related bias along the East Antarctic coast. Accumulation-temperature scaling relationships are fairly linear and constant further inland, but exhibit higher variability in the early to mid-Holocene over coastal regions. This climatic shift in the Holocene coincides with a weakening of the Amundsen Sea Low and a reduction in sea ice coverage. Circum-Antarctic coastal ocean temperatures at grounding line depths are highly sensitive to the meltwater forcings prescribed in each simulation, which are applied in different ways due to limited paleo-constraints. Although modelled centennial-scale rates of temperature and accumulation change are reasonable, clear model-proxy mismatches are observed with regard to the timing and duration of the Antarctic Cold Reversal (ACR) and Younger Dryas/early Holocene warming, suggesting that the Meltwater Pulse 1A and 1B events may be inadequately represented in these simulations. The incorporation of dynamic ice sheet models in future transient climate simulations could aid in improving meltwater forcing representation, and thus model-proxy agreement, through this time interval.
Enhanced climate instability in the North Atlantic and southern Europe during the Last Interglacial
Publisher: Springer Science and Business Media LLC
Date: 12-10-2018
DOI: 10.1038/S41467-018-06683-3
Abstract: Considerable ambiguity remains over the extent and nature of millennial/centennial-scale climate instability during the Last Interglacial (LIG). Here we analyse marine and terrestrial proxies from a deep-sea sediment sequence on the Portuguese Margin and combine results with an intensively dated Italian speleothem record and climate-model experiments. The strongest expression of climate variability occurred during the transitions into and out of the LIG. Our records also document a series of multi-centennial intra-interglacial arid events in southern Europe, coherent with cold water-mass expansions in the North Atlantic. The spatial and temporal fingerprints of these changes indicate a reorganization of ocean surface circulation, consistent with low-intensity disruptions of the Atlantic meridional overturning circulation (AMOC). The litude of this LIG variability is greater than that observed in Holocene records. Episodic Greenland ice melt and runoff as a result of excess warmth may have contributed to AMOC weakening and increased climate instability throughout the LIG.
Toward explaining the Holocene carbon dioxide and carbon isotope records: Results from transient ocean carbon cycle-climate simulations
Publisher: American Geophysical Union (AGU)
Date: 14-02-2012
DOI: 10.1029/2011PA002224
Impact of iron fertilisation on atmospheric CO2 during the last glaciation
Publisher: Copernicus GmbH
Date: 28-07-2023
Abstract: Abstract. While several processes have been identified to explain the decrease in atmospheric CO2 during glaciations, a better quantification of the contribution of each of these processes is needed. For ex le, enhanced aeolian iron input into the ocean during glacial times has been suggested to drive a 5 to 28 ppm atmospheric CO2 decrease. Here, we constrain this contribution by performing a set of sensitivity experiments with different aeolian iron input patterns and iron solubility factors under boundary conditions corresponding to 70 000 years before present (70 ka), a time period characterised by the first observed peak in glacial dust flux. We show that the decrease in CO2 as a function of Southern Ocean iron input follows an exponential decay relationship. This exponential decay response arises due to the saturation of the biological pump efficiency and levels out at ∼21 ppm in our simulations. We show that the changes in atmospheric CO2 are more sensitive to the solubility of iron in the ocean than the regional distribution of the iron fluxes. If surface water iron solubility is considered constant through time, we find a CO2 drawdown of ∼4 to ∼8 ppm. However, there is evidence that iron solubility was higher during glacial times. A best estimate of solubility changing from 1 % during interglacials to 3 % to 5 % under glacial conditions yields a ∼9 to 11 ppm CO2 decrease at 70 ka, while a plausible range of CO2 drawdown between 4 to 16 ppm is obtained using the wider but possible range of 1 % to 10 %. This would account for ∼12 %–50 % of the reconstructed decrease in atmospheric CO2 (∼32 ppm) between 71 and 64 ka. We further find that in our simulations the decrease in atmospheric CO2 concentration is solely driven by iron fluxes south of the Antarctic polar front, while iron fertilisation elsewhere plays a negligible role.
Removing the North Pacific halocline: Effects on global climate, ocean circulation and the carbon cycle
Publisher: Elsevier BV
Date: 02-2012
A multi-model assessment of the early last deglaciation (PMIP4 LDv1): The meltwater paradox reigns supreme
Publisher: Copernicus GmbH
Date: 10-08-2023
DOI: 10.5194/EGUSPHERE-2023-1802
Abstract: Abstract. Transient simulations of the last deglaciation have been increasingly performed to better understand the processes leading to both the overall deglacial climate trajectory as well as the centennial- to decadal- scale climate variations prevalent during deglaciations. The Paleoclimate Modelling Intercomparison Project (PMIP) has provided a framework for an internationally coordinated effort in simulating the last deglaciation (~20 – 11 ka BP) whilst encompassing a broad range of models. Here, we present a multi-model intercomparison of 17 simulations of the early part of the last deglaciation (~20 – 15 ka BP) from nine different climate models spanning a range of model complexities and uncertain boundary conditions/forcings. A main contrasting element between the simulations is the method by which groups implement freshwater fluxes from the melting ice sheets and how this forcing then impacts ocean circulation and surface climate. We find that the choice of meltwater scenario heavily impacts the deglacial climate evolution, but the response of each model depends largely on the sensitivity of the model to the freshwater forcing as well as to other aspects of the experimental design (e.g., CO2 forcing or ice sheet reconstruction). There is agreement throughout the ensemble that warming begins in the high latitudes associated with increasing insolation and delayed warming in the tropics aligned with the later increases in atmospheric CO2 concentration. The delay in this warming in the tropics is dependent on the timescale of the CO2 reconstruction used by the modelling group. Simulations with freshwater forcings greater than 0.1 Sverdrup (Sv) after 18 ka BP experience delayed warming in the North Atlantic, whereas simulations with smaller freshwater forcings begin deglaciating sooner. All simulations show a strong correlation between North Atlantic temperatures, atmospheric CO2 concentrations, and the AMOC. In simulations with a freshwater forcing greater than 0.1 Sv, North Atlantic temperatures correlate strongly with changes in the AMOC. Simulations with a smaller freshwater forcing show stronger correlations with atmospheric CO2. This indicates that the amount of meltwater strongly controls the climate trajectory of the deglaciation. Comparing multiple simulations run by the same model demonstrate model biases by showing similar surface climate spatial patterns despite the use of different ice sheet reconstructions and/or meltwater flux scenarios. Simulations run with different models, but similar boundary conditions, have provided insight into the sensitivity of in idual models to particular forcings, such as the amount freshwater forcing, which has been highly debated in previous studies. This debate has stemmed from the so-called ‘meltwater paradox’ that exists in choosing how much meltwater to input into simulations of the last deglaciation (i.e., large and geologically inconsistent meltwater forcings that successfully produce abrupt climate events versus glaciologically realistic meltwater fluxes that do not). The results of this research highlight how important this decision is.
Supplementary material to A multi-model assessment of the early last deglaciation (PMIP4 LDv1): The meltwater paradox reigns supreme
Publisher: Copernicus GmbH
Date: 10-08-2023
Land-sea temperature contrasts at the Last Interglacial and their impact on the hydrological cycle
Publisher: Copernicus GmbH
Date: 21-04-2021
Abstract: Abstract. Due to different orbital configurations, high northern latitude summer insolation was higher during the Last Interglacial period (LIG 129–116 thousand years before present, ka) than during the pre-industrial period (PI), while high southern latitude summer insolation was lower. The climatic response to these changes is studied here with focus on the Southern Hemisphere monsoons, by performing an equilibrium experiment of the LIG at 127 ka with the Australian Earth System Model, ACCESS-ESM1.5, as part of the Paleoclimate Model Intercomparison Project 4 (PMIP4). Simulated mean surface air temperature between 40 and 60∘ N over land during boreal summer is 6.5 ∘C higher at the LIG compared to PI, which leads to a northward shift of the Intertropical Convergence Zone (ITCZ) and a strengthening of the North African and Indian monsoons. Despite 0.4 ∘C cooler conditions in austral summer in the Southern Hemisphere (0–90∘ S), annual mean air temperatures are 1.2 ∘C higher at southern mid-latitudes to high latitudes (40–80∘ S). These differences in temperature are coincident with a large-scale reorganisation of the atmospheric circulation. The ITCZ shifts southward in the Atlantic and Indian sectors during the LIG austral summer compared to PI, leading to increased precipitation over the southern tropical oceans. However, weaker Southern Hemisphere insolation during LIG austral summer induces a significant cooling over land, which in turn weakens the land–sea temperature contrast, leading to an overall reduction (−20 %) in monsoonal precipitation over the Southern Hemisphere's continental regions compared to PI. The intensity and areal extent of the Australian, South American and South African monsoons are consistently reduced in LIG. This is associated with greater pressure and subsidence over land due to a strengthening of the Southern Hemisphere Hadley cell during austral summer.
2and Orbital Forcing in Driving Southern Hemispheric Temperature Variations during the Last 21 000 Yr*
Publisher: American Meteorological Society
Date: 04-2009
Abstract: Transient climate model simulations covering the last 21 000 yr reveal that orbitally driven insolation changes in the Southern Hemisphere, combined with a rise in atmospheric pCO2, were sufficient to jump-start the deglacial warming around Antarctica without direct Northern Hemispheric triggers. Analyses of sensitivity experiments forced with only one external forcing component (greenhouse gases, ice-sheet forcing, or orbital forcing) demonstrate that austral spring insolation changes triggered an early retreat of Southern Ocean sea ice starting around 19–18 ka BP. The associated sea ice–albedo feedback and the subsequent increase of atmospheric CO2 concentrations helped to further accelerate the deglacial warming in the Southern Hemisphere. Implications for the interpretation of Southern Hemispheric paleoproxy records are discussed.
PMIP-carbon: towards a multi-models comparison of climate-carbon interactions at the Last Glacial Maximum
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-13279
Abstract: & & More and more climate models now include the carbon cycle, but multi-models studies of climate-carbon simulations within the Climate Model Intercomparison Project (CMIP) are limited to present and future time periods. In addition, the carbon cycle is not considered in the simulations of past periods analysed within the Paleoclimate Modelling Intercomparison Project (PMIP). Yet, climate-carbon interactions are crucial to anticipate future atmospheric CO& sub& & /sub& concentrations and their impact on climate. Such interactions can change depending on the background climate, it is thus necessary to compare model results among themselves and to data for past periods with different climates such as the Last Glacial Maximum (LGM).& & & & The Last Glacial Maximum, around 21,000 years ago, was about 4& #176 C colder than the pre-industrial, and associated with large ice sheets on the American and Eurasian continents. It is one of the best documented periods thanks to numerous paleoclimate archives such as marine sediment cores and ice cores. Despite this period having been studied for years, no consensus on the causes of the lower atmospheric CO& sub& & /sub& concentration at the time (around 180 ppm) has been reached and models still struggle to simulate these low CO& sub& & /sub& values. The ocean, which contains around 40 times more carbon than the atmosphere, likely plays a key role, but models tend to simulate ocean circulation changes in disagreement with proxy data, such as carbon isotopes.& & & & This new project aims at comparing, for the first time, the carbon cycle representation at the Last Glacial Maximum from general circulation models and intermediate complexity models. We will explain the protocol and present first results in terms of carbon storage in the main reservoirs (atmosphere, land and ocean) and their link to key climate variables such as temperature, sea ice and ocean circulation. The use of coupled climate-carbon models will not only allow to compare changes in the carbon cycle in models and analyse their causes, but it will also enable us to better compare to indirect data related to the carbon cycle such as carbon isotopes.& &
Influence of Southern Ocean dynamics on Antarctic temperatures and on the global carbon cycle over the past two millennia.
Publisher: Copernicus GmbH
Date: 03-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-1180
Abstract: & & Reconstructions of Antarctic surface temperature covering the past millennia display a large centennial variability that is not synchronous with fluctuations recorded on other continents and which is generally not well simulated by models. Many processes can be at the origin of these temperature variations such as teleconnections with tropical oceans and changes in the Southern Ocean. The focus here will be on the latter, in particular on the influence of westerly winds that have a large impact on the exchange of heat and carbon between the ocean and atmosphere. Changes in the Southern Ocean circulation and stratification also influence the carbon cycle at global scale. It is generally suggested that atmospheric CO& sub& & /sub& variations over the past two millennia were mainly controlled by land processes but the Southern Ocean might have also played a role. We will thus test whether the joint analysis of Antarctic temperature and atmospheric CO& sub& & /sub& concentration fluctuations can inform us on the origin of the observed changes over this period. In this purpose, we use the climate model LOVECLIM which includes a representation of the global carbon cycle. Experiments over the last two millennia will address the sensitivity to realistic perturbations of the wind stress. Finally, experiments with data assimilation will allow assessing what constraints are needed for model results to better reproduce the atmospheric CO& sub& & /sub& concentration and reconstructed temperature history.& &
Transient response of Southern Ocean ecosystems during Heinrich stadials
Publisher: Authorea, Inc.
Date: 18-09-2023
Large-scale features of Last Interglacial climate: Results from evaluating the <i>lig127k</i> simulations for CMIP6-PMIP4
Publisher: Copernicus GmbH
Date: 21-01-2020
DOI: 10.5194/CP-2019-174
Abstract: Abstract. The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-s le test of the models that are used for the projection of future climate changes. New to CMIP6 is the Tier 1 lig127k experiment, designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models and following a common experimental protocol. We present a multi-model ensemble of 17 climate models, all of which (except for two) have also completed the CMIP6 DECK experiments. The Equilibrium Climate Sensitivity (ECS) of these models varies from 2.1 to 5.6 °C. The seasonal character of the insolation anomalies results in strong warming over the Northern Hemisphere (NH) continents in the lig127k ensemble as compared to the piControl in June–July–August and a much-reduced minimum (August–September) summer sea ice extent in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation and areal extent in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than midHolocene simulations as expected from the larger insolation anomalies at 127 ka than 6 ka. New syntheses for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and precipitation over the Northern Hemisphere continents. The model-data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the specified boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions. The CMIP6-PMIP4 lig127k simulations, in combination with the proxy record, have potential implications for confidence in future projections of monsoons, surface temperature, Arctic sea ice, and the stability of the Greenland ice sheet.
Deepwater circulation variation in the South China Sea since the Last Glacial Maximum
Publisher: American Geophysical Union (AGU)
Date: 19-08-2016
DOI: 10.1002/2016GL070342
Millennial and centennial CO2 release from the Southern Ocean during the last deglaciation
Publisher: Springer Science and Business Media LLC
Date: 17-03-2022
Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing
Publisher: Copernicus GmbH
Date: 16-08-2013
Abstract: Abstract. Millennial-scale variability associated with Dansgaard–Oeschger (DO) and Heinrich events (HE) is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-dimensional intermediate complexity Earth system model covering the period 50 ka BP to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing Northern Hemisphere ice-volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model aleo data comparison. Our analysis is consistent with the idea that variations in ice sheet calving and related changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of DO and HE variability seen in paleorecords across the globe.
Evaluating seasonal sea-ice cover over the Southern Ocean at the Last Glacial Maximum
Publisher: Copernicus GmbH
Date: 20-04-2022
Abstract: Abstract. Southern hemispheric sea-ice impacts ocean circulation and the carbon exchange between the atmosphere and the ocean. Sea-ice is therefore one of the key processes in past and future climate change and variability. As climate models are the only tool available to project future climate change, it is important to assess their performance against observations for a range of different climate states. The Last Glacial Maximum (LGM, ∼21 000 years ago) represents an interesting target as it is a relatively well-documented period with climatic conditions very different from preindustrial conditions. Here, we analyze the LGM seasonal Southern Ocean sea-ice cover as simulated in numerical simulations as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phases 3 and 4. We compare the model outputs to a recently updated compilation of LGM seasonal Southern Ocean sea-ice cover and summer sea surface temperature (SST) to assess the most likely LGM Southern Ocean state. Simulations and paleo-proxy records suggest a fairly well-constrained glacial winter sea-ice edge between 50.5 and 51∘ S. However, the spread in simulated glacial summer sea-ice is wide, ranging from almost ice-free conditions to a sea-ice edge reaching 53∘ S. Combining model outputs and proxy data, we estimate a likely LGM summer sea-ice edge between 61 and 62∘ S and a mean summer sea-ice extent of 14–15×106 km2, which is ∼20 %–30 % larger than previous estimates. These estimates point to a higher seasonality of southern hemispheric sea-ice during the LGM than today. We also analyze the main processes defining the summer sea-ice edge within each of the models. We find that summer sea-ice cover is mainly defined by thermodynamic effects in some models, while the sea-ice edge is defined by the position of Southern Ocean upwelling in others. For models included in both PMIP3 and PMIP4, this thermodynamic or dynamic control on sea-ice is consistent across both experiments. Finally, we find that the impact of changes in large-scale ocean circulation on summer sea-ice within a single model is smaller than the natural range of summer sea-ice cover across the models considered here. This indicates that care must be taken when using a single model to reconstruct past climate regimes.
An ice–climate oscillatory framework for Dansgaard–Oeschger cycles
Publisher: Springer Science and Business Media LLC
Date: 03-11-2020
Natural carbon release compensates for anthropogenic carbon uptake when Southern Hemispheric westerlies strengthen
Publisher: Wiley
Date: 31-08-2020
Towards a quantitative understanding of millennial-scale Antarctic warming events
Publisher: Elsevier BV
Date: 2010
Weak Southern Hemispheric monsoons during the Last Interglacial period
Publisher: Copernicus GmbH
Date: 25-11-2020
DOI: 10.5194/CP-2020-149
Abstract: Abstract. Due to different orbital configurations, high northern latitude boreal summer insolation was higher during the Last Interglacial period (LIG 129–116 thousand years before present, ka) than during the preindustrial period (PI), while high southern latitude austral summer insolation was lower. The climatic response to these changes is studied here with focus on the southern hemispheric monsoons, by performing an equilibrium experiment of the LIG at 127 ka with the Australian Earth System Model, ACCESS-ESM1.5, as part of the Paleoclimate Model Intercomparison Project 4 (PMIP4). In our simulation, mean surface air temperature increases by 6.5 °C over land during boreal summer between 40° N and 60° N in the LIG compared to PI, leading to a northward shift of the Inter-Tropical Convergence Zone (ITCZ) and a strengthening of the North African and Indian monsoons. Despite 0.4 °C cooler conditions in austral summer in the Southern Hemisphere (0–90° S), annual mean air temperatures are 1.2 °C higher at southern mid-to-high latitudes (40° S–80° S). These differences in temperature are coincident with a large-scale reorganisation of the atmospheric circulation. The ITCZ shifts southward in the Atlantic and Indian sectors during the LIG austral summer compared to PI, leading to increased precipitation over the southern tropical oceans. However, the decline in Southern Hemisphere insolation during austral summer induces a significant cooling over land, which in turn weakens the land-sea temperature contrast, leading to an overall reduction (−20 %) in monsoonal precipitation over the Southern Hemisphere's continental regions. The intensity and areal extent of the Australian, South American and South African monsoons are consistently reduced. This is associated with greater pressure and subsidence over land due to a strengthening of the southern hemispheric Hadley cell during austral summer.
Stalagmite-inferred European westerly drift in the early Weichselian with centennial-scale variability in marine isotope stage 5a
Publisher: Elsevier BV
Date: 07-2022
Holocene centennial to millennial shifts in North-Atlantic storminess and ocean dynamics
Publisher: Springer Science and Business Media LLC
Date: 24-08-2018
DOI: 10.1038/S41598-018-29949-8
Abstract: The forcing mechanisms responsible for centennial to millennial variability of mid-latitude storminess are still poorly understood. On decadal scales, the present-day geographic variability of North-Atlantic storminess responds to latitudinal shifts of the North-Atlantic westerly wind-belt under the prime control of the North-Atlantic Oscillation (NAO). An equivalent mechanism operating at centennial to millennial time scales during the Holocene is still to be ascertained, especially owing to the lack of high-resolution and continuous records of past-storminess extending far enough in time. Here we present a reconstruction of past storminess activity based on a high-resolution record of wind-blown sand retrieved from a near-coastal wetland. Our record extends back to ca. 10,000 B.P. and allows to continuously document fluctuations in the frequency of Holocene storm-force winds at our study-site at a mean high temporal resolution of 40 years. Large similarities between our record and palaeo-oceanographic records of Holocene climate changes in the North-Atlantic suggest that our past-storminess record reproduces a signal of significance for the North-Eastern Atlantic realm. We find that Holocene North-Atlantic storminess is dominated by robust millennial (≈2,200-year) to centennial (≈450, 300 and 200-year) periodicities. These changes in storminess were accompanied by changes in the precipitation regimes over northern Europe, evidencing large-scale shifts in the latitudinal positions of the Atlantic westerlies akin to present-day NAO patterns. We propose that these shifts originate from changes in the position and extent of the Azores high-pressure system and Polar vortex, as supported by climate model simulations. Finally, we demonstrate that enhanced zonal storminess activity over the North-Atlantic was the driver of millennial and centennial-scale changes in North-Atlantic oceanic circulation, while ocean dynamics most likely influenced back the atmospheric circulation at millennial time-scales. This may vouch for the instrumental role played by North-Atlantic storminess in triggering abrupt climate change at centennial scales during the Holocene.
Supplementary material to "The Atmospheric Bridge Communicated the δ<sup>13</sup>C Decline during the Last Deglaciation to the Global Upper Ocean"
Publisher: Copernicus GmbH
Date: 28-07-2020
ACCESS Datasets for CMIP6: Methodology & Idealised Experiments
Publisher: CSIRO Publishing
Date: 14-07-2022
DOI: 10.1071/ES21031
Abstract: The Australian Community Climate and Earth System Simulator (ACCESS) has contributed to the World Climate Research Programme’s Coupled Model Intercomparison Project Phase 6 (CMIP6) using two fully coupled model versions (ACCESS-CM2 and ACCESS-ESM1.5) and two ocean–sea-ice model versions (1° and 0.25° resolution versions of ACCESS-OM2). The fully coupled models differ primarily in the configuration and version of their atmosphere components (including the aerosol scheme), with smaller differences in their sea-ice and land model versions. Additionally, ACCESS-ESM1.5 includes biogeochemistry in the land and ocean components and can be run with an interactive carbon cycle. CMIP6 comprises core experiments and associated thematic Model Intercomparison Projects (MIPs). This paper provides an overview of the CMIP6 submission, including the methods used for the preparation of input forcing datasets and the post-processing of model output, along with a comprehensive list of experiments performed, detailing their initialisation, duration, ensemble number and computational cost. A small selection of model output is presented, focusing on idealised experiments and their variants at global scale. Differences in the climate simulation of the two coupled models are highlighted. ACCESS-CM2 produces a larger equilibrium climate sensitivity (4.7°C) than ACCESS-ESM1.5 (3.9°C), likely a result of updated atmospheric parameterisation in recent versions of the atmospheric component of ACCESS-CM2. The idealised experiments run with ACCESS-ESM1.5 show that land and ocean carbon fluxes respond to both changing atmospheric CO2 and to changing temperature. ACCESS data submitted to CMIP6 are available from the Earth System Grid Federation (0.22033/ESGF/CMIP6.2281 and 0.22033/ESGF/CMIP6.2288). The information provided in this paper should facilitate easier use of these significant datasets by the broader climate community.
Meridional reorganizations of marine and terrestrial productivity during Heinrich events
Publisher: American Geophysical Union (AGU)
Date: 18-01-2008
DOI: 10.1029/2007PA001445
Modelled response of marine ecosystems to Last Glacial Maximum forcing
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-1370
Abstract: & & Marine plankton play a key role in climatic transitions through their ability to transfer atmospheric carbon dioxide (CO& sub& & /sub& ) to the deep ocean via the biological pump. It has been suggested that the lower atmospheric CO& sub& & /sub& concentrations during the Last Glacial Maximum (LGM) might have resulted from enhanced export production triggered by higher micronutrient (Fe, Si) availability from continental dust, particularly in the Southern Ocean. Such a scenario is consistent with higher sediment accumulation rates observed during the LGM.& & & & In this study we use a new competition-driven ecosystem model that includes four major plankton types (diazotrophs, coccolithophores, diatoms and other general phytoplankton) to investigate their response to LGM climatic boundary conditions and to reconstructed micronutrient (Fe, Si) availability. We apply different dust fluxes, based on two plausible reconstructions (Mahowald et al., 2006 and Ohgaito et al., 2018). We compare LGM simulations with preindustrial simulations and disentangle the simulated ecosystem response due to climate forcing from the response due to micronutrient availability. We find that the ecosystem responses are complex and spatially heterogenic.& &
Iron sources and pathways into the Pacific Equatorial Undercurrent
Publisher: American Geophysical Union (AGU)
Date: 28-09-2016
DOI: 10.1002/2016GL070501
A multi-model CMIP6 study of Arctic sea ice at 127 ka: Sea ice data compilation and model differences
Publisher: Copernicus GmbH
Date: 23-01-2020
DOI: 10.5194/CP-2019-165
Abstract: Abstract. The Last interglacial (LIG) is a period with increased summer insolation at high northern latitudes, which results in strong changes in the terrestrial and marine cryosphere. Understanding the mechanisms for this response via climate modelling and comparing the models’ representation of climate reconstructions is one of the objectives set up by the Paleoclimate Mod-elling Intercomparison Project for its contribution to the sixth phase of the Coupled Model Intercomparison Project. Here we analyse the results from 12 climate models in terms of Arctic sea ice. The mean pre-industrial to LIG reduction in minimum sea ice area (SIA) reaches 59 % (multi-model mean LIG area is 2.21 mill. km2, compared to 5.85 mill. km2 for the PI), and the range of model results for LIG minimum sea ice area (from 0.02 to 5.65 mill. km2) is larger than for PI (from 4.10 to 8.30 mill. km2). On the other hand there is little change for the maximum sea ice area (which is 12 mill. km2 for both the PI and the LIG, with a standard deviation of 1.04 mill. km2 for PI and 1.21 mill. km2 for LIG). To evaluate the model results we synthesize LIG sea ice data from marine cores collected in the Arctic Ocean, Nordic Seas and northern North Atlantic. South of 78° N in the Atlantic and Nordic seas the LIG was seasonally ice-free. North of 78° N there are some discrepancies between sea-ice re-constructions based on dinocysts/foraminifers/ostracods and IP25: some sites have both seasonal and perennial interpretations based on the same core, but different indicators. Because of the conflicting interpretations it is not possible for any one model to match every data point in our data synthesis, or say whether the Arctic was seasonally ice-free. Drivers for the inter-model differences are: different phasing of the up and down short-wave anomalies over the Arctic ocean, associated with differences in model albedo possible cloud property differences, in terms of optical depth LIG ocean circulation changes which occur for some, but not all, LIG simulations. Finally we note that inter-comparisons between the LIG simulations, and simulations with moderate CO2 increase (during the transition to high CO2 levels), may yield insight into likely 21C Arctic sea ice changes using these LIG simulations.
Poorly ventilated deep ocean at the Last Glacial Maximum inferred from carbon isotopes: A data‐model comparison study
Publisher: American Geophysical Union (AGU)
Date: 2017
DOI: 10.1002/2016PA003024
Supplementary material to "Weak Southern Hemispheric monsoons during the Last Interglacial period"
Publisher: Copernicus GmbH
Date: 25-11-2020
A First Intercomparison of the Simulated LGM Carbon Results Within PMIP‐Carbon: Role of the Ocean Boundary Conditions
Publisher: American Geophysical Union (AGU)
Date: 29-09-2021
DOI: 10.1029/2021PA004302
Abstract: Model intercomparison studies of coupled carbon‐climate simulations have the potential to improve our understanding of the processes explaining the drawdown at the Last Glacial Maximum (LGM) and to identify related model biases. Models participating in the Paleoclimate Modeling Intercomparison Project (PMIP) now frequently include the carbon cycle. The ongoing PMIP‐carbon project provides the first opportunity to conduct multimodel comparisons of simulated carbon content for the LGM time window. However, such a study remains challenging due to differing implementation of ocean boundary conditions (e.g., bathymetry and coastlines reflecting the low sea level) and to various associated adjustments of biogeochemical variables (i.e., alkalinity, nutrients, dissolved inorganic carbon). After assessing the ocean volume of PMIP models at the pre‐industrial and LGM, we investigate the impact of these modeling choices on the simulated carbon at the global scale, using both PMIP‐carbon model outputs and sensitivity tests with the iLOVECLIM model. We show that the carbon distribution in reservoirs is significantly affected by the choice of ocean boundary conditions in iLOVECLIM. In particular, our simulations demonstrate a GtC effect of an alkalinity adjustment on carbon sequestration in the ocean. Finally, we observe that PMIP‐carbon models with a freely evolving and no additional glacial mechanisms do not simulate the drawdown at the LGM (with concentrations as high as 313, 331, and 315 ppm), especially if they use a low ocean volume. Our findings suggest that great care should be taken on accounting for large bathymetry changes in models including the carbon cycle.
What Drives Climate Flip-Flops?
Publisher: American Association for the Advancement of Science (AAAS)
Date: 17-07-2009
Abstract: A numerical modeling study questions the validity of a key paradigm in rapid climate change studies.
Evaluating seasonal sea-ice cover over the Southern Ocean from the Last Glacial Maximum
Publisher: Copernicus GmbH
Date: 22-12-2020
DOI: 10.5194/CP-2020-155
Abstract: Abstract. Sea-ice cover over the Southern Ocean responds to and impacts Southern Ocean dynamics and, thus, mid to high latitude climate in the Southern Hemisphere. In addition, sea-ice cover can significantly modulate the carbon exchange between the atmosphere and the ocean. As climate models are the only tool available to project future climate changes, it is important to assess their performance in simulating past changes. The Last Glacial Maximum (LGM, ∼21,000 years ago) represents an interesting target as it is a relatively well documented period with climatic conditions and a carbon cycle very different from pre-industrial conditions. Here, we study the changes in seasonal Antarctic sea-ice cover as simulated in numerical PMIP3 and LOVECLIM simulations of the LGM, and their relationship with windstress and ocean temperature. Simulations and paleo-proxy records suggest a fairly well constrained glacial winter sea-ice edge at 51.5° S (1 sigma range: 50°–55.5° S). Simulated glacial summer sea-ice cover however differs widely between models, ranging from almost no sea ice to a sea-ice edge reaching 55.5° S. The austral summer multi-model mean sea-ice edge lies at ∼60.5° S (1 sigma range: 57.5°–70.5° S). Given the lack of strong constraints on the summer sea-ice edge based on sea-ice proxy records, we extend our model-data comparison to summer sea-surface temperature. Our analysis suggests that the multi-model mean summer sea ice provides a reasonable, albeit upper end, estimate of the austral summer sea-ice edge allowing us to conclude that the multi-model mean of austral summer and winter sea-ice cover seem to provide good estimates of LGM conditions. Using these best estimates, we find that there was a larger sea-ice seasonality during the LGM compared to the present day.
The impact of bathymetry on the simulated carbon at the Last Glacial Maximum
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-7297
Abstract: & & Understanding the processes causing variations in the carbon cycle is critical to accurately simulate the future carbon cycle and climate. Paleoclimate models can provide insights about these processes since they are used under different conditions than present-day& #8217 s and evaluated against paleoproxy data. In particular, the Last Glacial Maximum (LGM) has been a focus of the Paleoclimate Modelling Intercomparison Project (PMIP) as it is well-documented thanks to numerous paleoclimate archives. Around 21,000 years ago, the LGM was a colder period with extensive ice sheets in the Northern Hemisphere and a resulting lower sea-level. Although this period has been studied for years, the causes of the lower atmospheric CO& sub& & /sub& concentration at the time (around 186 ppm, against 280 ppm at the pre-industrial) remain unclear, and models struggle to simulate this low CO& sub& & /sub& value. The ocean is thought to have played a significant role due to different processes (through changes of the biological pump efficiency, ocean circulation, sea-ice, and CO& sub& & /sub& solubility due to colder temperatures), but no consensus has been reached yet as to their contribution (Khatiwala et al. [2019], Yu et al. [2016], Marzocchi and Jansen [2019]).& & & & Despite the carbon cycle being simulated by more and more climate models, it has not been systematically analysed within the framework of PMIP multimodel comparisons. In this context, the ongoing PMIP-carbon project aims at comparing climate-carbon interactions in LGM simulations, and includes results from both intermediate complexity models and general circulation models. The PMIP protocol proposes standardized forcing parameters and boundary conditions (Kageyama et al. [2017]) and specifies a few recommendations for ocean biogeochemistry models (adjustment of salinity, dissolved inorganic carbon, alkalinity, and nutrients to account for the change in ocean volume). Indeed, the bathymetric changes associated with a sea-level drop of 133 m entail a change of the reservoir size and potential technical issues concerning the conservation of carbon.& & & & In this study, we use outputs from PMIP-carbon models and other models available on the ESGF (MIROC4m-COCO, MIROC-ES2L, CESM, IPSL-CM5A2, UVic, LOVECLIM, iLOVECLIM, CLIMBER_2P GISS-E2-R, MRI-CGCM3, MPI-ESM-P, CNRM-CM5, MIROC-ESM) to compute total ocean volumes and compare them to high resolution topographic data (etopo1 for the PI, GLAC-1D and ICE-6G-C for the LGM). We show that the deglacial volume change is rarely accurate. We then use the iLOVECLIM model with a new bathymetry implementation method (Lhardy et al. [in review, 2020]) to demonstrate the effect of an improved ocean volume on the simulated oceanic carbon content, resulting in an increase of the already overestimated atmospheric CO& sub& & /sub& concentration. We also quantify the effect of the mentioned adjustments of salinity, alkalinity, and carbon. The results reinforce the idea that a realistic ocean volume is needed, as well as consistency between models in dealing with large changes in bathymetry.& &
Climate and biogeochemical response to a rapid melting of the West Antarctic Ice Sheet during interglacials and implications for future climate
Publisher: American Geophysical Union (AGU)
Date: 12-2010
DOI: 10.1029/2009PA001892
The penultimate deglaciation: protocol for Paleoclimate Modelling Intercomparison Project (PMIP) phase 4 transient numerical simulations between 140 and 127 ka, version 1.0
Publisher: Copernicus GmbH
Date: 22-08-2019
Abstract: Abstract. The penultimate deglaciation (PDG, ∼138–128 thousand years before present, hereafter ka) is the transition from the penultimate glacial maximum (PGM) to the Last Interglacial (LIG, ∼129–116 ka). The LIG stands out as one of the warmest interglacials of the last 800 000 years (hereafter kyr), with high-latitude temperature warmer than today and global sea level likely higher by at least 6 m. Considering the transient nature of the Earth system, the LIG climate and ice-sheet evolution were certainly influenced by the changes occurring during the penultimate deglaciation. It is thus important to investigate, with coupled atmosphere–ocean general circulation models (AOGCMs), the climate and environmental response to the large changes in boundary conditions (i.e. orbital configuration, atmospheric greenhouse gas concentrations, ice-sheet geometry and associated meltwater fluxes) occurring during the penultimate deglaciation. A deglaciation working group has recently been set up as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phase 4, with a protocol to perform transient simulations of the last deglaciation (19–11 ka although the protocol covers 26–0 ka). Similar to the last deglaciation, the disintegration of continental ice sheets during the penultimate deglaciation led to significant changes in the oceanic circulation during Heinrich Stadial 11 (∼136–129 ka). However, the two deglaciations bear significant differences in magnitude and temporal evolution of climate and environmental changes. Here, as part of the Past Global Changes (PAGES)-PMIP working group on Quaternary interglacials (QUIGS), we propose a protocol to perform transient simulations of the penultimate deglaciation under the auspices of PMIP4. This design includes time-varying changes in orbital forcing, greenhouse gas concentrations, continental ice sheets as well as freshwater input from the disintegration of continental ice sheets. This experiment is designed for AOGCMs to assess the coupled response of the climate system to all forcings. Additional sensitivity experiments are proposed to evaluate the response to each forcing. Finally, a selection of paleo-records representing different parts of the climate system is presented, providing an appropriate benchmark for upcoming model–data comparisons across the penultimate deglaciation.
Modelling the impact of biogenic particle flux intensity and composition on sedimentary Pa/Th
Publisher: Elsevier BV
Date: 07-2020
Assessing the Spatial Origin of Meltwater Pulse 1A Using Oxygen‐Isotope Fingerprinting
Publisher: American Geophysical Union (AGU)
Date: 12-2019
DOI: 10.1029/2019PA003599
Greenland ice mass loss during the Younger Dryas driven by Atlantic Meridional Overturning Circulation feedbacks
Publisher: Springer Science and Business Media LLC
Date: 09-08-2018
DOI: 10.1038/S41598-018-29226-8
Abstract: Understanding feedbacks between the Greenland Ice Sheet (GrIS) and the Atlantic Meridional Overturning Circulation (AMOC) is crucial for reducing uncertainties over future sea level and ocean circulation change. Reconstructing past GrIS dynamics can extend the observational record and elucidate mechanisms that operate on multi-decadal timescales. We report a highly-constrained last glacial vertical profile of cosmogenic isotope exposure ages from Sermilik Fjord, a marine-terminating ice stream in the southeast sector of the GrIS. Our reconstruction reveals substantial ice-mass loss throughout the Younger Dryas (12.9-11.7 ka), a period of marked atmospheric and sea-surface cooling. Earth-system modelling reveals that southern GrIS marginal melt was likely driven by strengthening of the Irminger Current at depth due to a weakening of the AMOC during the Younger Dryas. This change in North Atlantic circulation appears to have drawn warm subsurface waters to southeast Greenland despite markedly cooler sea surface temperatures, enhancing thermal erosion at the grounding lines of palaeo ice-streams, supporting interpretation of regional marine-sediment cores. Given current rates of GrIS meltwater input into the North Atlantic and the vulnerability of major ice streams to water temperature changes at the grounding line, this mechanism has important implications for future AMOC changes and northern hemisphere heat transport.
Millennial atmospheric CO2 changes linked to ocean ventilation modes over past 150,000 years
Publisher: Springer Science and Business Media LLC
Date: 26-10-2023
Lower oceanic 𝛿<sup>13</sup>C during the Last Interglacial compared to the Holocene
Publisher: Copernicus GmbH
Date: 11-06-2020
DOI: 10.5194/CP-2020-73
Abstract: Abstract. The last time in Earth’s history when the high latitudes were warmer than during pre-industrial times was the last interglacial (LIG, 129–116 ka BP). Since the LIG is the most recent and best documented warm time period, it can provide insights into climate processes in a warmer world. However, some key features of the LIG are not well constrained, notably the oceanic circulation and the global carbon cycle. Here, we use a new database of LIG benthic 𝛿13C to investigate these two aspects. We find that the oceanic mean 𝛿13C was ~ 0.2 ‰ lower during the LIG (here defined as 125–120 ka BP) when compared to the mid-Holocene (7–4 ka BP). As the LIG was slightly warmer than the Holocene, it is possible that terrestrial carbon was lower, which would have led to both a lower oceanic 𝛿13C and atmospheric 𝛿13CO2 as observed in paleo-records. However, given the multi-millennial timescale, the lower oceanic 𝛿13C most likely reflects a long-term imbalance between weathering and burial of carbon. The 𝛿13C distribution in the Atlantic Ocean suggests no significant difference in the latitudinal and depth extent of North Atlantic Deep Water (NADW) between the LIG and the mid-Holocene. Furthermore, the data suggests that the multi-millennial mean NADW transport was similar between these two time periods.
Modelling the impact of biogenic particle flux intensity and composition on sedimentary Pa/Th
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-290
Abstract: & & There is compelling evidence of a strong relation between the Atlantic Meridional Overturning Circulation (AMOC) and millennial scale climate variability during the last glacial period. Part of the advances in understanding the underlying mechanisms rely on the analysis of the sedimentary Pa/Th ratio, which can be used to qualitatively infer past flow rates in the Atlantic. The compilation of existing North Atlantic records indicates repeated, consistent and significant Pa/Th increases across millennial-scale events, indicating significant reductions of deep-water formation in the Northwest Atlantic. However, the use of sedimentary Pa/Th as a pure kinematic circulation proxy is challenging because Pa and Th are also highly sensitive to changes in particulate flux intensity and composition that have probably occurred across these millennial scale events. A primary control of particles on the available Pa/Th records has been ruled out ensuring the absence of correlation between the reconstructed particle fluxes (e.g. Th-normalized opal fluxes) and the sedimentary Pa/Th. However, quantitative estimates of the impact of particles on the available paleo Pa/Th are still missing.& & & & In this study, we use the Pa/Th enabled iLOVECLIM Earth System Model of Intermediate Complexity to decipher the impact of particles on the sedimentary Pa/Th. We evaluate the impact of imposed changes in biogenic particle flux intensity and composition on the Atlantic Pa/Th in a 3-D geographical perspective. We find that up to 30% of the observed Pa/Th increase across Heinrich Stadial 1 could be explained by changes in particle fluxes and composition. Besides, changes in the Particulate Organic Carbon (POC) most efficiently affects the sedimentary Pa/Th, followed by biogenic opal. Last but not least, the global Atlantic sedimentary Pa/Th response is very sensitive to shifts in the geographical distribution of particles and high scavenging areas. In our simulations, a decrease of the opal production in the Northwest Atlantic can induce a far field Pa/Th increase in a large part of the North Atlantic basin, suggesting that a local monitoring of the particle fluxes might not be enough to rule out any influence of the particles on paleo sedimentary Pa/Th records.& &
The southern amplifier
Publisher: American Association for the Advancement of Science (AAAS)
Date: 08-03-2019
Abstract: Greater Southern Ocean stratification may have lowered the atmospheric CO 2 concentration and prolonged ice ages
Warming Seas in the Coral Triangle: Coral Reef Vulnerability and Management Implications
Publisher: Informa UK Limited
Date: 15-09-2010
Supplementary material to "Large-scale features of Last Interglacial climate: Results from evaluating the <i>lig127k</i> simulations for CMIP6-PMIP4"
Publisher: Copernicus GmbH
Date: 21-01-2020
Sea ice variability in the southern Norwegian Sea during glacial Dansgaard-Oeschger climate cycles
Publisher: American Association for the Advancement of Science (AAAS)
Date: 03-2019
Abstract: New biomarker sea ice reconstructions document that sea ice changes shaped past abrupt climate change.
Natural carbon release compensates for anthropogenic carbon uptake when Southern Hemispheric westerlies strengthen
Publisher: Wiley
Date: 02-10-2021
Greenhouse-gas feedbacks estimated from Dansgaard-Oeschger events
Publisher: Copernicus GmbH
Date: 03-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-3660
Abstract: & & There are large uncertainties in the estimation of greenhouse-gas feedbacks: model-based estimates vary considerably recent observations are too short provide strong constraints. Rapid climate changes during the last glacial period (Dansgaard-Oeschger, D-O, events) are potentially valuable because they are comparable in rate and magnitude to projected future climate warming, and are registered near-globally. Here we use D-O events to quantify the centennial-scale feedback strength of feedbacks involving CO& sub& & /sub& , CH& sub& & /sub& and N& sub& & /sub& O. We use climate model simulations of the D-O events to estimate the relationship between global mean and Greenland temperature. We then relate global mean temperature changes to changes in greenhouse-gas concentrations derived from ice-core records, and then estimate the associated radiative forcing. We found the magnitude of the feedbacks (expressed in gain, with 95 % confidence interval) to be 0.07 & #177 0.02 for CO& sub& ,& /sub& 0.04 & #177 0.01 for CH& sub& & /sub& , 0.04 & #177 0.01 for N& sub& & /sub& O. These estimates are more constrained than previous model-based estimates but comparable to estimates based on recent observations.& &
Drivers of the evolution and amplitude of African Humid Periods
Publisher: Research Square Platform LLC
Date: 19-07-2021
DOI: 10.21203/RS.3.RS-665330/V1
Abstract: During orbital precession minima, the Sahara was humid and hosted tropical plant species thus providing a corridor for Hominins migration. Uncertainties remain over the climatic processes controlling the initiation, demise and litude of these African Humid Periods (AHPs). Here we present transient simulations of the penultimate deglaciation and Last Interglacial period (LIG), and compare them to transient simulations of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) at the end of the deglacial millennial-scale events exerts a dominant control on the abrupt initiation of AHPs, as the AMOC modulates the position of the Intertropical Convergence Zone (ITCZ). In addition, residual Northern Hemispheric (NH) ice-sheets can delay the AHP peak. Through its impact on NH ice-sheets disintegration and thus AMOC variations, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt LIG AHP onset. Finally, we show that the background climate state modulates precipitation variability with higher variability under wetter background conditions.
Climate and marine carbon cycle response to changes in the strength of the Southern Hemispheric westerlies
Publisher: American Geophysical Union (AGU)
Date: 04-10-2008
DOI: 10.1029/2008PA001604
The mechanism behind internally generated centennial-to-millennial scale climate variability in an earth system model of intermediate complexity
Publisher: Copernicus GmbH
Date: 25-08-2010
Abstract: Abstract. The mechanism triggering centennial-to-millennial-scale variability of the Atlantic Meridional Overturning Circulation (AMOC) in the earth system model of intermediate complexity LOVECLIM is investigated. It is found that for several climate boundary conditions such as low obliquity values (~22.1°) or LGM-albedo, internally generated centennial-to-millennial-scale variability occurs in the North Atlantic region. Stochastic excitations of the density-driven overturning circulation in the Nordic Seas can create regional sea-ice anomalies and a subsequent reorganization of the atmospheric circulation. The resulting remote atmospheric anomalies over the Hudson Bay can release freshwater pulses into the Labrador Sea and significantly increase snow fall in this region leading to a subsequent reduction of convective activity. The millennial-scale AMOC oscillations disappear if LGM bathymetry (with closed Hudson Bay) is prescribed or if freshwater pulses are suppressed artificially. Furthermore, our study documents the process of the AMOC recovery as well as the global marine and terrestrial carbon cycle response to centennial-to-millennial-scale AMOC variability.
Millennial to orbital-scale variations of drought intensity in the Eastern Mediterranean
Publisher: Elsevier BV
Date: 02-2016
Enhanced Southern Ocean CO2 outgassing as a result of stronger and poleward shifted southern hemispheric westerlies
Publisher: Copernicus GmbH
Date: 07-03-2023
DOI: 10.5194/EGUSPHERE-2023-390
Abstract: Abstract. While the Southern Ocean (SO) provides the largest oceanic sink of carbon, some observational studies have suggested that the total SO CO2 uptake exhibited large (~0.3 GtC/yr) decadal-scale variability over the last 30 years, with a similar SO CO2 uptake in 2016 than in the early 1990s. Here, using an eddy-rich ocean, sea-ice, carbon cycle model, with a nominal resolution of 1/10th degree, we explore the changes in total, natural and anthropogenic CO2 fluxes in the Southern Ocean over the period 1970–2021 and the processes leading to the CO2 flux variability. Over that period, the simulated total CO2 uptake increases by 0.5 GtC/yr, half of which occurs between 1970 and 1982. The simulated total CO2 flux exhibits decadal-scale variability with an litude of ~0.1 GtC/yr in phase with observations and with variability in the Southern Annular Mode (SAM). Notably, a stagnation of the total CO2 uptake is simulated between 1982 and 2000, while a re-invigoration is simulated between 2000 and 2012. This decadal-scale variability results from enhanced outgassing of natural CO2 south of the sub-Antarctic front due to the strengthening and poleward shift of the southern hemispheric (SH) westerlies. These wind changes also lead to enhanced anthropogenic CO2 uptake south of the polar front, even though the correlation is low and the litude 75 % smaller than for natural CO2 changes. The total SO CO2 uptake capability thus reduced since 1970 in response to a shift towards positive phases of the SAM. Both the multi-decadal and annual changes in SO fluxes can be mostly explained by variations in surface dissolved inorganic carbon (DIC) brought about by a combination of Ekman-driven vertical advection and DIC diffusion at the base of the mixed layer, thus indicating that even in an eddy-rich ocean model a strengthening and/or poleward shift of the southern hemispheric westerlies enhance CO2 outgassing. The projected poleward strengthening of the SH westerlies over the coming century will thus reduce the capability of the SO to mitigate the increase in atmospheric CO2.
Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning
Publisher: Springer Science and Business Media LLC
Date: 29-09-2014
DOI: 10.1038/NCOMMS6107
Abstract: During the last glacial termination, the upwelling strength of the southern polar limb of the Atlantic Meridional Overturning Circulation varied, changing the ventilation and stratification of the high-latitude Southern Ocean. During the same period, at least two phases of abrupt global sea-level rise--meltwater pulses--took place. Although the timing and magnitude of these events have become better constrained, a causal link between ocean stratification, the meltwater pulses and accelerated ice loss from Antarctica has not been proven. Here we simulate Antarctic ice sheet evolution over the last 25 kyr using a data-constrained ice-sheet model forced by changes in Southern Ocean temperature from an Earth system model. Results reveal several episodes of accelerated ice-sheet recession, the largest being coincident with meltwater pulse 1A. This resulted from reduced Southern Ocean overturning following Heinrich Event 1, when warmer subsurface water thermally eroded grounded marine-based ice and instigated a positive feedback that further accelerated ice-sheet retreat.
Supplementary material to "Lower oceanic 𝛿<sup>13</sup>C during the Last Interglacial compared to the Holocene"
Publisher: Copernicus GmbH
Date: 11-06-2020
Author Correction: Volcanic influence on centennial to millennial Holocene Greenland temperature change
Publisher: Springer Science and Business Media LLC
Date: 06-03-2018
DOI: 10.1038/S41598-018-21307-Y
Abstract: A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.
A weaker Atlantic Meridional Overturning Circulation at the Last Glacial Maximum led to a greater deep ocean carbon content
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-2920
Abstract: & & While paleoproxy records and modelling studies consistently suggest that North Atlantic& Deep Water (NADW) was shallower at the Last Glacial Maximum (LGM) than during pre-industrial times, its strength is still subject to debate partly due to different signals across the North Atlantic. Here, using a series of LGM experiments performed with a carbon isotopes enabled Earth system model, we show that proxy records are consistent with a shallower and weaker NADW. A significant equatorward advance of sea-ice over the Labrador Sea and the Nordic Seas shifts the NADW convection sites to the south of the Norwegian Sea. While the deep western boundary current in the Northwest Atlantic weakens with NADW, a change in density gradients strengthens the deep southward flow in the Northeast Atlantic. A shoaling and weakening of NADW further allow penetration of Antarctic Bottom Water in the North Atlantic despite its transport being reduced. This resultant globally weaker oceanic circulation leads to an increase in deep ocean carbon of ~500 GtC, thus significantly contributing to the lower LGM atmospheric CO& sub& & /sub& concentration.& & & & & & &
More efficient North Atlantic carbon pump during the Last Glacial Maximum
Publisher: Springer Science and Business Media LLC
Date: 15-05-2019
DOI: 10.1038/S41467-019-10028-Z
Abstract: During the Last Glacial Maximum (LGM ~20,000 years ago), the global ocean sequestered a large amount of carbon lost from the atmosphere and terrestrial biosphere. Suppressed CO 2 outgassing from the Southern Ocean is the prevailing explanation for this carbon sequestration. By contrast, the North Atlantic Ocean—a major conduit for atmospheric CO 2 transport to the ocean interior via the overturning circulation—has received much less attention. Here we demonstrate that North Atlantic carbon pump efficiency during the LGM was almost doubled relative to the Holocene. This is based on a novel proxy approach to estimate air–sea CO 2 exchange signals using combined carbonate ion and nutrient reconstructions for multiple sediment cores from the North Atlantic. Our data indicate that in tandem with Southern Ocean processes, enhanced North Atlantic CO 2 absorption contributed to lowering ice-age atmospheric CO 2 .
The Sensitivity of the Antarctic Ice Sheet to a Changing Climate: Past, Present, and Future
Publisher: American Geophysical Union (AGU)
Date: 11-11-2020
DOI: 10.1029/2019RG000663
Abstract: The Antarctic Ice Sheet (AIS) is out of equilibrium with the current anthropogenic‐enhanced climate forcing. Paleoenvironmental records and ice sheet models reveal that the AIS has been tightly coupled to the climate system during the past and indicate the potential for accelerated and sustained Antarctic ice mass loss into the future. Modern observations by contrast suggest that the AIS has only just started to respond to climate change in recent decades. The maximum projected sea level contribution from Antarctica to 2100 has increased significantly since the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report, although estimates continue to evolve with new observational and theoretical advances. This review brings together recent literature highlighting the progress made on the known processes and feedbacks that influence the stability of the AIS. Reducing the uncertainty in the magnitude and timing of the future sea level response to AIS change requires a multidisciplinary approach that integrates knowledge of the interactions between the ice sheet, solid Earth, atmosphere, and ocean systems and across time scales of days to millennia. We start by reviewing the processes affecting AIS mass change, from atmospheric and oceanic processes acting on short time scales (days to decades), through to ice processes acting on intermediate time scales (decades to centuries) and the response to solid Earth interactions over longer time scales (decades to millennia). We then review the evidence of AIS changes from the Pliocene to the present and consider the projections of global sea level rise and their consequences. We highlight priority research areas required to improve our understanding of the processes and feedbacks governing AIS change.
Southern Ocean convection amplified past Antarctic warming and atmospheric CO2 rise during Heinrich Stadial 4
Publisher: Springer Science and Business Media LLC
Date: 22-09-2020
DOI: 10.1038/S43247-020-00024-3
Abstract: The record of past climate highlights recurrent and intense millennial anomalies, characterised by a distinct pattern of inter-polar temperature change, termed the ‘thermal bipolar seesaw’, which is widely believed to arise from rapid changes in the Atlantic overturning circulation. By forcing a suppression of North Atlantic convection, models have been able to reproduce many of the general features of the thermal bipolar seesaw however, they typically fail to capture the full magnitude of temperature change reconstructed using polar ice cores from both hemispheres. Here we use deep-water temperature reconstructions, combined with parallel oxygenation and radiocarbon ventilation records, to demonstrate the occurrence of enhanced deep convection in the Southern Ocean across the particularly intense millennial climate anomaly, Heinrich Stadial 4. Our results underline the important role of Southern Ocean convection as a potential lifier of Antarctic warming, and atmospheric CO 2 rise, that is responsive to triggers originating in the North Atlantic.
Fingerprints of changes in the terrestrial carbon cycle in response to large reorganizations in ocean circulation
Publisher: Copernicus GmbH
Date: 28-03-2011
Abstract: Abstract. CO2 and carbon cycle changes in the land, ocean and atmosphere are investigated using the comprehensive carbon cycle-climate model NCAR CSM1.4-carbon. Ensemble simulations are forced with freshwater perturbations applied at the North Atlantic and Southern Ocean deep water formation sites under pre-industrial climate conditions. As a result, the Atlantic Meridional Overturning Circulation reduces in each experiment to varying degrees. The physical climate fields show changes qualitatively in agreement with results documented in the literature, but there is a clear distinction between northern and southern perturbations. Changes in the physical variables, in turn, affect the land and ocean biogeochemical cycles and cause a reduction, or an increase, in the atmospheric CO2 concentration by up to 20 ppmv, depending on the location of the perturbation. In the case of a North Atlantic perturbation, the land biosphere reacts with a strong reduction in carbon stocks in some tropical locations and in high northern latitudes. In contrast, land carbon stocks tend to increase in response to a southern perturbation. The ocean is generally a sink of carbon although large reorganizations occur throughout various basins. The response of the land biosphere is strongest in the tropical regions due to a shift of the Intertropical Convergence Zone. The carbon fingerprints of this shift, either to the south or to the north depending on where the freshwater is applied, can be found most clearly in South America. For this reason, a compilation of various paleoclimate proxy records of Younger Dryas precipitation changes are compared with our model results. The proxy records, in general, show good agreement with the model's response to a North Atlantic freshwater perturbation.
Mechanisms for the Onset of the African Humid Period and Sahara Greening 14.5–11 ka BP*
Publisher: American Meteorological Society
Date: 15-05-2010
Abstract: The mechanisms leading to the onset of the African Humid Period (AHP) 14 500–11 000 yr ago are elucidated using two different climate–vegetation models in a suite of transient glacial–interglacial simulations covering the last 21 000 yr. A series of sensitivity experiments investigated three key mechanisms (local summer insolation and ice sheet evolution, vegetation–albedo–precipitation feedback, and CO2 increase via radiative forcing and fertilization) that control the climate–vegetation history over North Africa during the last glacial termination. The simulations showed that neither orbital forcing nor the remote forcing from the retreating ice sheets alone was able to trigger the rapid formation of the AHP. Only both forcing factors together can effectively lead to the formation of the AHP. The vegetation–albedo–precipitation feedback enhances the intensity of the monsoon and further accelerates the onset of the AHP. The experiments indicate that orbital forcing and vegetation–albedo–precipitation feedback alone are insufficient to trigger the rapid onset of the AHP. The sensitivity experiments further show that the increasing radiative forcing from rising CO2 concentrations had no significant impact on the temporal evolution of the African monsoon during the last deglaciation. However, the fertilization effect of CO2 is important for the terrestrial carbon storage. The modeling results are discussed and compared with paleoproxy records of the African monsoon system. It is concluded that the model results presented here do not lend support to the notion that simple insolation thresholds govern the abrupt transitions of North African vegetation during the early to middle Holocene.
Drivers of the evolution and amplitude of African Humid Periods
Publisher: Springer Science and Business Media LLC
Date: 18-11-2021
DOI: 10.1038/S43247-021-00309-1
Abstract: During orbital precession minima, the Sahara was humid and more vegetated, providing potential corridors for Hominins migration. Uncertainties remain over the climatic processes controlling the initiation, demise and litude of these African Humid Periods. Here we study these processes using a series of transient simulations of the penultimate deglaciation and Last Interglacial period, and compare the results with a transient simulation of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation at the end of deglacial millennial-scale events exerts a dominant control on the abrupt initiation of African Humid Periods as the Atlantic Meridional Overturning Circulation modulates the position of the Intertropical Convergence Zone. In addition, residual Northern Hemispheric ice-sheets can delay the peak of the African Humid Period. Through its impact on Northern Hemispheric ice-sheets disintegration and thus Atlantic Meridional Overturning Circulation, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt onset of the African Humid Period during the Last Interglacial period. Finally, we show that the mean climate state modulates precipitation variability, with higher variability under wetter background conditions.
Changes in atmospheric CO2 concentration over the past two millennia: contribution of climate variability, land-use and Southern Ocean dynamics
Publisher: Springer Science and Business Media LLC
Date: 07-01-2022
Response to Reviewer
Publisher: Copernicus GmbH
Date: 12-06-2019
Deconstructing the Last Glacial termination: the role of millennial and orbital-scale forcings
Publisher: Elsevier BV
Date: 05-2011
Response to Editor
Publisher: Copernicus GmbH
Date: 12-06-2019
Deep South Atlantic carbonate chemistry and increased interocean deep water exchange during last deglaciation
Publisher: Elsevier BV
Date: 04-2014
A first intercomparison of the simulated LGM carbon results within PMIP-carbon: role of the ocean boundary conditions
Publisher: Wiley
Date: 11-05-2021
Is there warming in the pipeline? A multi-model analysis of the zero emission commitment from CO<sub>2</sub>
Publisher: Copernicus GmbH
Date: 20-01-2020
DOI: 10.5194/BG-2019-492
Abstract: Abstract. The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 emissions, and as such is a critical parameter for calculating the remaining carbon budget. The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) was established to gain a better understanding of the potential magnitude and sign of ZEC, in addition to the processes that underlie this metric. Eighteen Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO2 concentration increases exponentially until 1000 PgC has been emitted. Thereafter emissions are set to zero and models are configured to allow free evolution of atmospheric CO2 concentration. Many models conducted additional second priority simulations with different cumulative emissions totals and an alternative idealized emissions pathway with a gradual transition to zero emissions. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is − 0.36 to 0.29 ºC with a model ensemble mean of −0.06 ºC, median of −0.05 ºC and standard deviation of 0.19 ºC. Models exhibit a wide variety of behaviours after emissions cease, with some models continuing to warm for decades to millennia and others cooling substantially. Analysis shows that both ocean carbon uptake and carbon uptake by the terrestrial biosphere are important for counteracting the warming effect from reduction in ocean heat uptake in the decades after emissions cease. Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments.
Large-scale features of Last Interglacial climate: results from evaluating the <i>lig127k</i> simulations for the Coupled Model Intercomparison Project (CMIP6)–Paleoclimate
Publisher: Copernicus GmbH
Date: 11-01-2021
Abstract: Abstract. The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-s le test of the models that are used for the projection of future climate changes. New to the Coupled Model Intercomparison Project (CMIP6) is the Tier 1 Last Interglacial experiment for 127 000 years ago (lig127k), designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models as for the future and following a common experimental protocol. Here we present a first analysis of a multi-model ensemble of 17 climate models, all of which have completed the CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) experiments. The equilibrium climate sensitivity (ECS) of these models varies from 1.8 to 5.6 ∘C. The seasonal character of the insolation anomalies results in strong summer warming over the Northern Hemisphere continents in the lig127k ensemble as compared to the CMIP6 piControl and much-reduced minimum sea ice in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than the CMIP6 midHolocene simulations as expected from the larger insolation anomalies at 127 than 6 ka. New synthesis for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and for precipitation over the Northern Hemisphere continents. The model–data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions. The CMIP6–Paleoclimate Modeling Intercomparison Project (PMIP4) lig127k simulations, in combination with the proxy record, improve our confidence in future projections of monsoons, surface temperature, and Arctic sea ice, thus providing a key target for model evaluation and optimization.
Representative Transmission Coefficient for Evaluating the Wave Attenuation Performance of 3D Floating Breakwaters in Regular and Irregular Waves
Publisher: MDPI AG
Date: 06-04-2021
DOI: 10.3390/JMSE9040388
Abstract: Wave attenuation performance is the prime consideration when designing any floating breakwater. For a 2D hydrodynamic analysis of a floating breakwater, the wave attenuation performance is evaluated by the transmission coefficient, which is defined as the ratio between the transmitted wave height and the incident wave height. For a 3D breakwater, some researchers still adopted this evaluation approach with the transmitted wave height taken at a surface point, while others used the mean transmission coefficient within a surface area. This paper aims to first examine the rationality of these two evaluation approaches via verified numerical simulations of 3D heave-only floating breakwaters in regular and irregular waves. A new index—a representative transmission coefficient—is then presented for one to easily compare the wave attenuation performances of different 3D floating breakwater designs.
The Atmospheric Bridge Communicated the δ<sup>13</sup>C Decline during the Last Deglaciation to the Global Upper Ocean
Publisher: Copernicus GmbH
Date: 28-07-2020
DOI: 10.5194/CP-2020-95
Abstract: Abstract. During the early last glacial termination (17.2–15 ka) atmospheric δ13C declined sharply by 0.3–0.4 ‰ as atmospheric pCO2 rose. This was the initial part of the atmospheric δ13C excursion that lasted for multiple thousand years. A similar δ13C decline has been documented in marine proxy records from both surface and thermocline-dwelling planktic foraminifera. The foraminiferal δ13C decline has previously been attributed to a flux of respired carbon from the deep ocean that was subsequently transported within the upper ocean (i.e. bottom up transport) to sites where the signal is recorded. Here, we provide modeling evidence that when respired carbon upwells in the Southern Ocean, negative δ13C anomalies in the global upper ocean were instead transferred from the atmosphere (i.e. top down transport). Due to this efficient atmospheric bridge, the pathway of δ13C transport was likely to be different from nutrient transport during the early deglaciation. This implies that the usage of planktic δ13C records for identifying the carbon source(s) responsible for the atmospheric pCO2 rise during the early deglaciation is limited. The model results also suggest that thermocline waters in upwelling systems like the eastern equatorial Pacific, and even upper deep waters above 2000 m, can be affected by this atmospheric bridge during the early deglaciation. Our results imply that caution must be applied when interpreting early deglacial marine δ13C records from depths that are potentially affected by the atmosphere.
Author Correction: Millennial and centennial CO2 release from the Southern Ocean during the last deglaciation
Publisher: Springer Science and Business Media LLC
Date: 19-04-2022
Supplementary material to "Deglacial evolution of regional Antarctic climate and Southern Ocean conditions in transient climate simulations"
Publisher: Copernicus GmbH
Date: 21-06-2018
Deepwater Formation in the North Pacific During the Last Glacial Termination
Publisher: American Association for the Advancement of Science (AAAS)
Date: 09-07-2010
Abstract: Most of the densest, deepest water at the bottom of the oceans comes from two regions, the North Atlantic and the circum-Antarctic. Have other regions been able to produce significant quantities of deep water in the past? For decades, researchers have looked, with limited success, for evidence of deepwater formation in the North Pacific since the time of the Last Glacial Maximum, about 23,000 years ago. Okazaki et al. (p. 200 ) combine published observational evidence from the North Pacific with model simulations to suggest that deep water did form in the North Pacific during the early part of the Last Glacial Termination, between about 17,500 and 15,000 years ago. The switch between deep-water formation in the North Atlantic and the North Pacific is likely to have had an important effect on heat transport and climate.
Volcanic influence on centennial to millennial Holocene Greenland temperature change
Publisher: Springer Science and Business Media LLC
Date: 03-05-2017
DOI: 10.1038/S41598-017-01451-7
Abstract: Solar variability has been hypothesized to be a major driver of North Atlantic millennial-scale climate variations through the Holocene along with orbitally induced insolation change. However, another important climate driver, volcanic forcing has generally been underestimated prior to the past 2,500 years partly owing to the lack of proper proxy temperature records. Here, we reconstruct seasonally unbiased and physically constrained Greenland Summit temperatures over the Holocene using argon and nitrogen isotopes within trapped air in a Greenland ice core (GISP2). We show that a series of volcanic eruptions through the Holocene played an important role in driving centennial to millennial-scale temperature changes in Greenland. The reconstructed Greenland temperature exhibits significant millennial correlations with K + and Na + ions in the GISP2 ice core (proxies for atmospheric circulation patterns), and δ 18 O of Oman and Chinese Dongge cave stalagmites (proxies for monsoon activity), indicating that the reconstructed temperature contains hemispheric signals. Climate model simulations forced with the volcanic forcing further suggest that a series of large volcanic eruptions induced hemispheric-wide centennial to millennial-scale variability through ocean/sea-ice feedbacks. Therefore, we conclude that volcanic activity played a critical role in driving centennial to millennial-scale Holocene temperature variability in Greenland and likely beyond.
Deglacial evolution of regional Antarctic climate and Southern Ocean conditions in transient climate simulations
Publisher: Copernicus GmbH
Date: 30-01-2019
Abstract: Abstract. Constraining Antarctica's climate evolution since the end of the Last Glacial Maximum (∼18 ka) remains a key challenge, but is important for accurately projecting future changes in Antarctic ice sheet mass balance. Here we perform a spatial and temporal analysis of two transient deglacial climate simulations, one using a fully coupled GCM (TraCE-21ka) and one using an intermediate complexity model (LOVECLIM DGns), to determine regional differences in deglacial climate evolution and identify the main strengths and limitations of the models in terms of climate variables that impact ice sheet mass balance. The greatest continental surface warming is observed over the continental margins in both models, with strong correlations between surface albedo, sea ice coverage, and surface air temperature along the coasts, as well as regions with the greatest decrease in ice surface elevation in TraCE-21ka. Accumulation–temperature scaling relationships are fairly linear and constant in the continental interior, but exhibit higher variability in the early to mid-Holocene over coastal regions. Circum-Antarctic coastal ocean temperatures at grounding line depths are highly sensitive to the meltwater forcings prescribed in each simulation, which are applied in different ways due to limited paleo-constraints. Meltwater forcing associated with the Meltwater Pulse 1A (MWP1A) event results in subsurface warming that is most pronounced in the Amundsen and Bellingshausen Sea sector in both models. Although modelled centennial-scale rates of temperature and accumulation change are reasonable, clear model–proxy mismatches are observed with regard to the timing and duration of the Antarctic Cold Reversal (ACR) and Younger Dryas–early Holocene warming, which may suggest model bias in large-scale ocean circulation, biases in temperature reconstructions from proxy records, or that the MWP1A and 1B events are inadequately represented in these simulations. The incorporation of dynamic ice sheet models in future transient climate simulations could aid in improving meltwater forcing representation, and thus model–proxy agreement, through this time interval.
Antarctic sea ice over the past 130 000 years – Part 1: a review of what proxy records tell us
Publisher: Copernicus GmbH
Date: 02-08-2022
Abstract: Abstract. Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat and freshwater fluxes, air–sea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging and, therefore, projecting future sea-ice changes and their influence on the global climate system is uncertain. Reconstructing past changes in sea-ice cover can provide additional insights into climate feedbacks within the Earth system at different timescales. This paper is the first of two review papers from the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) working group. In this first paper, we review marine- and ice core-based sea-ice proxies and reconstructions of sea-ice changes throughout the last glacial–interglacial cycle. Antarctic sea-ice reconstructions rely mainly on diatom fossil assemblages and highly branched isoprenoid (HBI) alkenes in marine sediments, supported by chemical proxies in Antarctic ice cores. Most reconstructions for the Last Glacial Maximum (LGM) suggest that winter sea ice expanded all around Antarctica and covered almost twice its modern surface extent. In contrast, LGM summer sea ice expanded mainly in the regions off the Weddell and Ross seas. The difference between winter and summer sea ice during the LGM led to a larger seasonal cycle than today. More recent efforts have focused on reconstructing Antarctic sea ice during warm periods, such as the Holocene and the Last Interglacial (LIG), which may serve as an analogue for the future. Notwithstanding regional heterogeneities, existing reconstructions suggest that sea-ice cover increased from the warm mid-Holocene to the colder Late Holocene with pervasive decadal- to millennial-scale variability throughout the Holocene. Studies, supported by proxy modelling experiments, suggest that sea-ice cover was halved during the warmer LIG when global average temperatures were ∼2 ∘C above the pre-industrial (PI). There are limited marine (14) and ice core (4) sea-ice proxy records covering the complete 130 000 year (130 ka) last glacial cycle. The glacial–interglacial pattern of sea-ice advance and retreat appears relatively similar in each basin of the Southern Ocean. Rapid retreat of sea ice occurred during Terminations II and I while the expansion of sea ice during the last glaciation appears more gradual especially in ice core data sets. Marine records suggest that the first prominent expansion occurred during Marine Isotope Stage (MIS) 4 and that sea ice reached maximum extent during MIS 2. We, however, note that additional sea-ice records and transient model simulations are required to better identify the underlying drivers and feedbacks of Antarctic sea-ice changes over the last 130 ka. This understanding is critical to improve future predictions.
Estimated strength of the Atlantic overturning circulation during the last deglaciation
Publisher: Springer Science and Business Media LLC
Date: 10-02-2013
DOI: 10.1038/NGEO1723
Fast and slow components of millennial-scale climate changes
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-20608
Abstract: & & Despite a substantial body of evidence on millennial-scale climate variability during Marine Isotope Stage 3, uncertainty remains over the precise sequence of changes in different parts of the climate system, and ultimately their causes.& Here, we present results of joint marine and terrestrial proxy analyses from the Portuguese Margin, showing the typical succession of cold stadials and warm interstadials over the interval 35-57 ka, with most extreme changes occurring during Heinrich Stadials (HS). & The planktonic and benthic foraminiferal isotope records map onto Greenland and Antarctic temperature variations, respectively, while the pollen record bears a close similarity to changes in the Asian summer monsoon, atmospheric methane and dust concentrations, indicating coupled changes in hydroclimate in middle-to-low latitudes. & Closer inspection of HS4 and HS5 reveals considerable structure, with a relatively fast transition to maximum cooling and aridity associated with a peak in ice-rafted detritus, containing detrital carbonate grains originating from the Hudson Strait.& This was followed by an interval of slowly increasing sea-surface temperatures (SST) and moisture availability, in line with evidence indicating a gradual evolution in low-latitude hydroclimate. & A climate model experiment closely reproduces the gradual increase in SST and precipitation in W. Iberia during the final part of HS4 as a result of the recovery of the Atlantic overturning circulation, but does not capturethe abrupt warming in Greenland.& What emerges is a ersity of response timescales, from centuries in low-to-mid latitude SST and precipitation to decades in Greenland temperatures.& &
Simulating atmospheric CO2, 13C and the marine carbon cycle during the Last Glacial–Interglacial cycle: possible role for a deepening of the mean remineralization depth and an increase in the oceanic nutrient inventory
Publisher: Elsevier BV
Date: 11-2012
Southern Ocean Ecosystem Response to Last Glacial Maximum Boundary Conditions
Publisher: American Geophysical Union (AGU)
Date: 07-2021
DOI: 10.1029/2020PA004075
Abstract: Phytoplankton exert a significant control on the marine carbon cycle and can thus impact atmospheric CO 2 concentration. Here we use a new ecosystem model to analyze the response of diatoms and coccolithophores in the Southern Ocean to Last Glacial Maximum (LGM) climate conditions, and changes in aeolian iron (Fe) input in the Southern Ocean. We find that LGM climate conditions without changes in Fe input lead to a large increase in diatoms north of the winter sea ice edge in the South Atlantic (19%) and the South Pacific (26%), and a 31% and 9% increase within the seasonal sea‐ice zone in the South Atlantic and Indian oceans, respectively, while diatoms decrease in the Ross and Weddell Seas, and in the South Pacific (62%) south of the winter sea ice edge. Coccolithophores increase by 11% in the South West Atlantic near 45°S but are outcompeted by diatoms within the seasonal sea‐ice zone, where they decrease by 21%. Overall, this results in a 11% decrease in Southern Ocean net primary productivity (NPP) and a 2.4% decrease in export production (EP). A series of sensitivity experiments with different aeolian Fe input are compared to available paleo‐proxy records. The best fit is obtained for a simulation forced with dust fluxes from Lambert et al. (2015), 0.1002/2015gl064250 and reduced Antarctic Bottom Water formation in the Weddell Sea. The 78% increase in aeolian Fe input in the Southern Ocean in this simulation increases the Southern Ocean EP by 4.4%, while NPP remains 8.7% weaker compared to preindustrial.
Sequestration of carbon in the deep Atlantic during the last glaciation
Publisher: Springer Science and Business Media LLC
Date: 08-02-2016
DOI: 10.1038/NGEO2657
Antarctic ice sheet discharge driven by atmosphere-ocean feedbacks at the Last Glacial Termination
Publisher: Springer Science and Business Media LLC
Date: 05-01-2017
DOI: 10.1038/SREP39979
Abstract: Reconstructing the dynamic response of the Antarctic ice sheets to warming during the Last Glacial Termination (LGT 18,000–11,650 yrs ago) allows us to disentangle ice-climate feedbacks that are key to improving future projections. Whilst the sequence of events during this period is reasonably well-known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records, making it difficult to assess relationships between Antarctic ice-sheet (AIS) dynamics, climate change and sea level. Here we present results from a highly-resolved ‘horizontal ice core’ from the Weddell Sea Embayment, which records millennial-scale AIS dynamics across this extensive region. Counterintuitively, we find AIS mass-loss across the full duration of the Antarctic Cold Reversal (ACR 14,600–12,700 yrs ago), with stabilisation during the subsequent millennia of atmospheric warming. Earth-system and ice-sheet modelling suggests these contrasting trends were likely Antarctic-wide, sustained by feedbacks lified by the delivery of Circumpolar Deep Water onto the continental shelf. Given the anti-phase relationship between inter-hemispheric climate trends across the LGT our findings demonstrate that Southern Ocean-AIS feedbacks were controlled by global atmospheric teleconnections. With increasing stratification of the Southern Ocean and intensification of mid-latitude westerly winds today, such teleconnections could lify AIS mass loss and accelerate global sea-level rise.
Atlantic-Pacific seesaw and its role in outgassing CO2 during Heinrich events
Publisher: American Geophysical Union (AGU)
Date: 2014
DOI: 10.1002/2013PA002542
Supplementary material to "The penultimate deglaciation: protocol for PMIP4 transient numerical simulations between 140 and 127 ka"
Publisher: Copernicus GmbH
Date: 05-09-2018
Lower oceanic <i>δ</i><sup>13</sup>C during the last interglacial period compared to the Holocene
Publisher: Copernicus GmbH
Date: 25-02-2021
Abstract: Abstract. The last time in Earth's history when high latitudes were warmer than during pre-industrial times was the last interglacial period (LIG, 129–116 ka BP). Since the LIG is the most recent and best documented interglacial, it can provide insights into climate processes in a warmer world. However, some key features of the LIG are not well constrained, notably the oceanic circulation and the global carbon cycle. Here, we use a new database of LIG benthic δ13C to investigate these two aspects. We find that the oceanic mean δ13C was ∼ 0.2 ‰ lower during the LIG (here defined as 125–120 ka BP) when compared to the Holocene (7–2 ka BP). A lower terrestrial carbon content at the LIG than during the Holocene could have led to both lower oceanic δ13C and atmospheric δ13CO2 as observed in paleo-records. However, given the multi-millennial timescale, the lower oceanic δ13C most likely reflects a long-term imbalance between weathering and burial of carbon. The δ13C distribution in the Atlantic Ocean suggests no significant difference in the latitudinal and depth extent of North Atlantic Deep Water (NADW) between the LIG and the Holocene. Furthermore, the data suggest that the multi-millennial mean NADW transport was similar between these two time periods.
In-phase millennial-scale glacier changes in the tropics and North Atlantic regions during the Holocene
Publisher: Springer Science and Business Media LLC
Date: 17-03-2022
DOI: 10.1038/S41467-022-28939-9
Abstract: Based on new and published cosmic-ray exposure chronologies, we show that glacier extent in the tropical Andes and the north Atlantic regions (TANAR) varied in-phase on millennial timescales during the Holocene, distinct from other regions. Glaciers experienced an early Holocene maximum extent, followed by a strong mid-Holocene retreat and a re-advance in the late Holocene. We further explore the potential forcing of TANAR glacier variations using transient climate simulations. Since the Atlantic Meridional Overturning Circulation (AMOC) evolution is poorly represented in these transient simulations, we develop a semi-empirical model to estimate the “AMOC-corrected” temperature and precipitation footprint at regional scales. We show that variations in the AMOC strength during the Holocene are consistent with the observed glacier changes. Our findings highlight the need to better constrain past AMOC behavior, as it may be an important driver of TANAR glacier variations during the Holocene, superimposed on other forcing mechanisms.
Global temperature and hydroclimate in warmer climates of the past and future: the Last Interglacial versus greenhouse scenarios
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15417
Abstract: & & Past climates contain precious information about the workings of the climate system, and about what can be expected in a changed climate. The Last Interglacial (LIG ca. 125,000 years ago) is the most recent period of climate warmer than modern, at least in the Northern Hemisphere. Because of this, it has been often proposed that the LIG holds a partial analogy with a future warmer climate forced by enhanced greenhouse effect. Still, such analogy has never been examined in a quantitative manner. Here we address the question: for which scenario, time horizon, regions and season is the climate of the LIG a useful analogue of the future? We use the results of 13 climate models that performed the standard experiments of PMIP4 and CMIP6, and present a comparison of hemispheric temperature and precipitation between the LIG and SSP scenarios of the future. We also two independent assessments of models performance, by comparing their temperature and precipitation to climate reanalysis of the last decades and to proxies of the LIG. Insights gained from this comparison can inform studies in disciplines beyond climate studies, such as hydrology and ecology.& &
Southern Hemisphere westerlies as a driver of the early deglacial atmospheric CO2 rise
Publisher: Springer Science and Business Media LLC
Date: 27-06-2018
DOI: 10.1038/S41467-018-04876-4
Abstract: The early part of the last deglaciation is characterised by a ~40 ppm atmospheric CO 2 rise occurring in two abrupt phases. The underlying mechanisms driving these increases remain a subject of intense debate. Here, we successfully reproduce changes in CO 2 , δ 13 C and Δ 14 C as recorded by paleo-records during Heinrich stadial 1 (HS1). We show that HS1 CO 2 increase can be explained by enhanced Southern Ocean upwelling of carbon-rich Pacific deep and intermediate waters, resulting from intensified Southern Ocean convection and Southern Hemisphere (SH) westerlies. While enhanced Antarctic Bottom Water formation leads to a millennial CO 2 outgassing, intensified SH westerlies induce a multi-decadal atmospheric CO 2 rise. A strengthening of SH westerlies in a global eddy-permitting ocean model further supports a multi-decadal CO 2 outgassing from the Southern Ocean. Our results highlight the crucial role of SH westerlies in the global climate and carbon cycle system with important implications for future climate projections.
Contribution of enhanced Antarctic Bottom Water formation to Antarctic warm events and millennial-scale atmospheric CO2 increase
Publisher: Elsevier BV
Date: 03-2015
Impact of iron fertilisation on atmospheric CO2 during the last glaciation
Publisher: Copernicus GmbH
Date: 28-06-2022
DOI: 10.5194/CP-2022-46
Abstract: Abstract. While several processes have been identified to explain the decrease in atmospheric CO2 during glaciations, a better quantification of the contribution of each of these processes is needed. For ex le, enhanced aeolian iron input into the ocean during glacial times has been suggested to drive a 5 to 28 ppm atmospheric CO2 decrease. Here, we constrain this contribution by performing a set of sensitivity experiments with different aeolian iron input patterns and iron solubility factors under boundary conditions corresponding to 70 thousand years before present (70 ka BP), a time period characterised by the first observed peak in glacial dust flux. We show that the decrease in CO2 as a function of the Southern Ocean iron input follows an exponential decay relationship. This exponential decay response arises due to the saturation of the biological pump efficiency and levels out at ∼21 ppm in our simulations. Using a best estimate of surface water iron solubility between 3 and 5 %, a ∼9 to 11 ppm CO2 decrease is simulated at 70 ka BP, while a plausible range of CO2 draw-down between 4 to 16 ppm is obtained using the wider but possible range of 1 to 10 %. This would account for ∼12–50 % of the reconstructed decrease in atmospheric CO2 (∼32 ppm) between 71 and 64 ka BP. We further find that in our simulations the decrease in atmospheric CO2 concentrations is solely driven by iron fluxes south of the Antarctic polar front, while iron fertilization elsewhere plays a negligible role.
Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation
Publisher: Springer Science and Business Media LLC
Date: 28-05-2014
DOI: 10.1038/NATURE13397
Abstract: Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum (26,000-19,000 years ago) is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise.
Nitrate deficits by nitrification and denitrification processes in the Indian Ocean
Publisher: Elsevier BV
Date: 2006
Response to Reviewer 2
Publisher: Copernicus GmbH
Date: 08-02-2019
Response to K. Grant comments
Publisher: Copernicus GmbH
Date: 08-02-2019
The atmospheric bridge communicated the <i>δ</i><sup>13</sup>C decline during the last deglaciation to the global upper ocean
Publisher: Copernicus GmbH
Date: 19-07-2021
Abstract: Abstract. During the early part of the last glacial termination (17.2–15 ka) and coincident with a ∼35 ppm rise in atmospheric CO2, a sharp 0.3‰–0.4‰ decline in atmospheric δ13CO2 occurred, potentially constraining the key processes that account for the early deglacial CO2 rise. A comparable δ13C decline has also been documented in numerous marine proxy records from surface and thermocline-dwelling planktic foraminifera. The δ13C decline recorded in planktic foraminifera has previously been attributed to the release of respired carbon from the deep ocean that was subsequently transported within the upper ocean to sites where the signal was recorded (and then ultimately transferred to the atmosphere). Benthic δ13C records from the global upper ocean, including a new record presented here from the tropical Pacific, also document this distinct early deglacial δ13C decline. Here we present modeling evidence to show that rather than respired carbon from the deep ocean propagating directly to the upper ocean prior to reaching the atmosphere, the carbon would have first upwelled to the surface in the Southern Ocean where it would have entered the atmosphere. In this way the transmission of isotopically light carbon to the global upper ocean was analogous to the ongoing ocean invasion of fossil fuel CO2. The model results suggest that thermocline waters throughout the ocean and 500–2000 m water depths were affected by this atmospheric bridge during the early deglaciation.
Response to Reviewer 1
Publisher: Copernicus GmbH
Date: 08-02-2019
Beyond the Principle of Plentitude: A Review of Terrestrial Planet Habitability
Publisher: Mary Ann Liebert Inc
Date: 04-2005
Abstract: We review recent work that directly or indirectly addresses the habitability of terrestrial (rocky) planets like the Earth. Habitability has been traditionally defined in terms of an orbital semimajor axis within a range known as the habitable zone, but it is also well known that the habitability of Earth is due to many other astrophysical, geological, and geochemical factors. We focus this review on (1) recent refinements to habitable zone calculations (2) the formation and orbital stability of terrestrial planets (3) the tempo and mode of geologic activity (e.g., plate tectonics) on terrestrial planets (4) the delivery of water to terrestrial planets in the habitable zone and (5) the acquisition and loss of terrestrial planet carbon and nitrogen, elements that constitute important atmospheric gases responsible for habitable conditions on Earth's surface as well as being the building blocks of the biosphere itself. Finally, we consider recent work on evidence for the earliest habitable environments and the appearance of life itself on our planet. Such evidence provides us with an important, if nominal, calibration point for our search for other habitable worlds.
Confirmation of PMIP4 endorsed protocol
Publisher: Copernicus GmbH
Date: 10-12-2018
Last Interglacial subsurface warming on the Antarctic shelf triggered by reduced deep-ocean convection
Publisher: Research Square Platform LLC
Date: 21-02-2022
DOI: 10.21203/RS.3.RS-1274081/V1
Abstract: The global mean sea-level was likely ∼3-6 m higher at the Last Interglacial (LIG) compared to pre-industrial (PI), with an Antarctic contribution estimated at 3 to 5 m sea-level equivalent. Antarctic ice-sheet modelling studies suggest that such an ice-mass loss from Antarctica requires a subsurface warming on the Antarctic shelf of ∼3◦C compared to PI. Here we show that such a subsurface warming is simulated in an equilibrium experiment of the LIG performed with a comprehensive Earth System Model. Reduced deep-ocean convection in the Weddell and Ross Seas, arising from reduced sea-ice cover, are the primary drivers of this subsurface warming, reaching +2.4◦C at 430 m depth. The associated changes in meridional density gradients and surface winds lead to a weakened Antarctic Circumpolar Current but strengthened Antarctic Slope Current, which further impact subsurface temperatures around both East and West Antarctica, with a maximum warming of +3.1◦C at 125 m depth on the East Antarctic shelf. Higher SST and reduced sea-ice formation in the Southern Ocean thus increase ocean stratification and lead to a subsurface warming on the Antarctic shelf, with the potential to trigger ice-mass loss from the Antarctic ice-sheet.
Response to the Editor
Publisher: Copernicus GmbH
Date: 08-02-2019
Challenges and research priorities to understand interactions between climate, ice sheets and global mean sea level during past interglacials
Publisher: Elsevier BV
Date: 09-2019
Supplementary material to "The penultimate deglaciation: protocol for PMIP4 transient numerical simulations between 140 and 127 ka, version 1.0"
Publisher: Copernicus GmbH
Date: 06-03-2019
Supplementary material to Enhanced Southern Ocean CO2 outgassing as a result of stronger and poleward shifted southern hemispheric westerlies
Publisher: Copernicus GmbH
Date: 07-03-2023
Corrigendum to “Millennial to orbital-scale variations of drought intensity in the Eastern Mediterranean” [Quat. Sci. Rev. 133 (2016) 77–95]
Publisher: Elsevier BV
Date: 10-2016
The penultimate deglaciation: protocol for PMIP4 transient numerical simulations between 140 and 127 ka, version 1.0
Publisher: Copernicus GmbH
Date: 06-03-2019
DOI: 10.5194/GMD-2019-41
Abstract: Abstract. The penultimate deglaciation (PDG, ~ 138–128 thousand years before present, hereafter ka) is the transition from the penultimate glacial maximum to the Last Interglacial (LIG, ~ 129–116 ka). The LIG stands out as one of the warmest interglacials of the last 800 ka, with high-latitude temperature warmer than today and global sea level likely higher by at least 6 meters. Considering the transient nature of the Earth system, the LIG climate and ice-sheets evolution were certainly influenced by the changes occurring during the penultimate deglaciation. It is thus important to investigate, with coupled Atmosphere-Ocean General Circulation Models (AOGCMs), the climate and environmental response to the large changes in boundary conditions (i.e. orbital configuration, atmospheric greenhouse gas concentrations, ice-sheet geometry, and associated meltwater fluxes) occurring during the penultimate deglaciation. A deglaciation working group has recently been set up as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phase 4, with a protocol to perform transient simulations of the last deglaciation (19–11 ka although the protocol covers 26–0 ka). Similar to the last deglaciation, the disintegration of continental ice-sheets during the penultimate deglaciation led to significant changes in the oceanic circulation during Heinrich Stadial 11 (~ 136–129 ka). However, the two deglaciations bear significant differences in magnitude and temporal evolution of climate and environmental changes. Here, as part of the PAGES-PMIP working group on Quaternary Interglacials, we propose a protocol to perform transient simulations of the penultimate deglaciation under the auspices of PMIP4. This design includes time-varying changes in orbital forcing, greenhouse gas concentrations, continental ice-sheets as well as freshwater input from the disintegration of continental ice-sheets. This experiment is designed for AOGCMs to assess the coupled response of the climate system to all forcings. Additional sensitivity experiments are proposed to evaluate the response to each forcing. Finally, a selection of paleo records representing different parts of the climate system is presented, providing an appropriate benchmark for upcoming model-data comparisons across the penultimate deglaciation.
Fast and slow components of interstadial warming in the North Atlantic during the last glacial
Publisher: Springer Science and Business Media LLC
Date: 13-08-2020
DOI: 10.1038/S43247-020-0006-X
Abstract: The abrupt nature of warming events recorded in Greenland ice-cores during the last glacial has generated much debate over their underlying mechanisms. Here, we present joint marine and terrestrial analyses from the Portuguese Margin, showing a succession of cold stadials and warm interstadials over the interval 35–57 ka. Heinrich stadials 4 and 5 contain considerable structure, with a short transitional phase leading to an interval of maximum cooling and aridity, followed by slowly increasing sea-surface temperatures and moisture availability. A climate model experiment reproduces the changes in western Iberia during the final part of Heinrich stadial 4 as a result of the gradual recovery of the Atlantic meridional overturning circulation. What emerges is that Greenland ice-core records do not provide a unique template for warming events, which involved the operation of both fast and slow components of the coupled atmosphere–ocean–sea-ice system, producing adjustments over a range of timescales.
Enhanced Mid‐depth Southward Transport in the Northeast Atlantic at the Last Glacial Maximum Despite a Weaker AMOC
Publisher: American Geophysical Union (AGU)
Date: 02-2020
DOI: 10.1029/2019PA003793
Paleoclimate constrains future El Niño/Southern Oscillation increase
Publisher: Research Square Platform LLC
Date: 20-09-2022
DOI: 10.21203/RS.3.RS-2062789/V1
Abstract: IPCC projections indicate that El Niño/Southern Oscillation (ENSO) variability will increase throughout the 21st century under global warming. However, the magnitude of the increase greatly varies among models. Here we use paleoclimate model simulations of the 4th phase of the Paleoclimate Model Intercomparison Project (PMIP4) to constrain the projected ENSO increase and propose a common mechanism to explain ENSO changes throughout climate states. We show that the projected ENSO increase is unprecedented in warm climates of the past ~3.3 million years and that ENSO operates in a bi-stable regime dictated by the position of the tropical Pacific Ocean convection centers, namely the Intertropical and South Pacific Convergence Zones. Constraining ENSO variability with the positions of the tropical convection centers indicates a 12% (3 to 19%) ENSO increase in the 21st century. Our results elucidate how tropical Pacific mean state has modulated ENSO activity and alerts for the unprecedented nature of the 21st century global warming.
Drivers of the evolution and amplitude of African Humid Periods
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-6415
Abstract: & & During orbital precession minima, the Sahara was humid and more vegetated. Uncertainties remain over the climatic processes controlling the initiation, demise and litude of these African Humid Periods (AHPs). Here we study these processes using a series of transient simulations of the penultimate deglaciation and Last Interglacial period performed with an Earth system model of intermediate complexity (LOVECLIM). These results are compared to a transient simulation of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) at the end of deglacial millennial-scale events exerts a dominant control on the abrupt initiation of AHPs, as the AMOC modulates the position of the Intertropical Convergence Zone. In addition, residual Northern Hemispheric ice-sheets can delay the peak of the AHPs. Through its impact on Northern Hemispheric ice-sheets disintegration and thus AMOC, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt onset of the AHP during the Last Interglacial period. Finally, we show that the mean climate state modulates precipitation variability, with higher variability under wetter background conditions.& &
Antarctic sea ice over the past 130,000 years, Part 1: A review of what proxy records tell us
Publisher: Copernicus GmbH
Date: 06-04-2022
DOI: 10.5194/EGUSPHERE-2022-99
Abstract: Abstract. Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat, and freshwater fluxes, air-sea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity, and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging, and therefore projecting future sea-ice changes and their influence on the global climate system is uncertain. Reconstructing past changes in sea-ice cover can provide additional insights into climate feedbacks within the Earth system at different timescales. This paper is the first of two review papers from the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) Working Group. In this first paper, we review marine- and ice core-based sea-ice proxies and reconstructions of sea-ice changes throughout the last glacial-interglacial cycle. Antarctic sea-ice reconstructions rely mainly on diatom fossil assemblages and highly branched isoprenoid (HBI) alkenes in marine sediments, supported by chemical proxies in Antarctic ice cores. Most reconstructions for the Last Glacial Maximum (LGM) suggest winter sea-ice expanded all around Antarctica and covered almost twice its modern surface extent. In contrast, LGM summer sea-ice expanded mainly in the regions off the Weddell and Ross seas. The difference between winter and summer sea ice during the LGM led to a larger seasonal cycle than today. More recent efforts have focused on reconstructing Antarctic sea-ice during warm periods, such as the Holocene and the Last Interglacial (LIG), which may serve as an analogue the future. Notwithstanding regional heterogeneities, existing reconstructions suggest sea-ice cover increased from the warm mid-Holocene to the colder Late Holocene, with pervasive decadal-to-millennial scale variability throughout the Holocene. Sparse marine and ice core data, supported by proxy modelling experiments, suggest that sea-ice cover was halved during the warmer LIG, when global average temperatures were ~2 °C above the pre-industrial (PI). There are limited marine (14) and ice core (4) sea-ice proxy records covering the complete 130,000 year (130 ka) last glacial cycle. The glacial-interglacial pattern of sea-ice advance and retreat appears relatively similar in each basin of the Southern Ocean. Rapid retreat of sea ice occurred during Terminations II and I, while the expansion of sea ice during the last glaciation appears more gradual, especially in cores data sets. Marine records suggest that the first prominent expansion occurred during Marine Isotope Stage (MIS) 4 and that sea ice reached maximum extent during MIS 2. We however note that additional sea-ice records and transient model simulations are required to better identify the underlying drivers and feedbacks of Antarctic sea-ice changes over the last 130 ka. This understanding is critical to improve future predictions.
Atlantic inflow and low sea-ice cover in the Nordic Seas promoted Fennoscandian Ice Sheet growth during the Last Glacial Maximum
Publisher: Springer Science and Business Media LLC
Date: 21-10-2023
Author Correction: Holocene centennial to millennial shifts in North-Atlantic storminess and ocean dynamics
Publisher: Springer Science and Business Media LLC
Date: 06-08-2020
DOI: 10.1038/S41598-020-69870-7
Abstract: An amendment to this paper has been published and can be accessed via a link at the top of the paper.
Contribution of climate variability, land-use and Southern Ocean dynamics to changes in atmospheric CO2 concentration over the past two millennia
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-1525
Abstract: & & By performing an ensemble of sensitivity experiments with the LOVECLIM model, we confirm the earlier results showing that temperature changes had a large influence on the land carbon cycle over the preindustrial Common Era. However, this process alone cannot explain the magnitude of the reconstructed atmospheric CO& sub& & /sub& and & #948 & sup& & /sup& CO& sub& & /sub& variations. In particular, even when the model is constrained to follow reconstructed temperature changes by data assimilation, and when applying relatively large values of the climate-carbon feedback parameter, it can only explain about 50% of the atmospheric CO& sub& & /sub& decrease between the 12& sup& th& /sup& and the 17& sup& th& /sup& century. We find that land use changes are likely responsible for most of the observed long term atmospheric CO& sub& & /sub& trend over the first millennium of the Common Era, and for up to 30 % of the decrease observed after 1600 CE. In addition, in our experiments, changes in southern hemisphere westerly winds induce slightly smaller changes in atmospheric CO& sub& & /sub& concentrations than those associated with land use change, and variations in & #948 & sup& & /sup& CO& sub& & /sub& of the same order of magnitude as the observed ones. Those wind changes also have a strong impact on the difference in & sup& & /sup& C between the northern and southern hemisphere, presenting strong similarities with observed changes. Combining the effects of changes in temperature, land use and winds over the Southern Ocean provides a reasonable agreement with reconstructions for atmospheric CO& sub& & /sub& concentrations and & #948 & sup& & /sup& CO& sub& & /sub& , especially for the low CO& sub& & /sub& values observed during the 17& sup& th& /sup& century. This underlines the important contribution of both land and ocean carbon processes. Nevertheless, some uncertainties remain on the origin of the relatively high CO& sub& & /sub& concentrations reconstructed during the 11& sup& th& /sup& and 16& sup& th& /sup& centuries.& & & & & / &
Last glacial atmospheric CO2 decline due to widespread Pacific deep-water expansion
Publisher: Springer Science and Business Media LLC
Date: 20-07-2020
Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO<sub>2</sub>
Publisher: Copernicus GmbH
Date: 15-06-2020
Abstract: Abstract. The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 emissions and as such is a critical parameter for calculating the remaining carbon budget. The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) was established to gain a better understanding of the potential magnitude and sign of ZEC, in addition to the processes that underlie this metric. A total of 18 Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO2 concentration increases exponentially until 1000 PgC has been emitted. Thereafter emissions are set to zero and models are configured to allow free evolution of atmospheric CO2 concentration. Many models conducted additional second-priority simulations with different cumulative emission totals and an alternative idealized emissions pathway with a gradual transition to zero emissions. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is −0.36 to 0.29 ∘C, with a model ensemble mean of −0.07 ∘C, median of −0.05 ∘C, and standard deviation of 0.19 ∘C. Models exhibit a wide variety of behaviours after emissions cease, with some models continuing to warm for decades to millennia and others cooling substantially. Analysis shows that both the carbon uptake by the ocean and the terrestrial biosphere are important for counteracting the warming effect from the reduction in ocean heat uptake in the decades after emissions cease. This warming effect is difficult to constrain due to high uncertainty in the efficacy of ocean heat uptake. Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments and simple theory.
Poleward Shift in the Southern Hemisphere Westerly Winds Synchronous With the Deglacial Rise in CO2
Publisher: American Geophysical Union (AGU)
Date: 28-06-2023
DOI: 10.1029/2023PA004666
Abstract: The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesized to play a key role in regulating atmospheric CO 2 over glacial‐interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ 18 O from across the Southern Ocean and emergent relationships within an ensemble of climate models to reconstruct changes in the Southern Hemisphere surface westerlies over the last deglaciation. We infer a 4.8° (2.9–7.1°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (20 ka) relative to the mid‐Holocene (6.5 ka). Climate models from the Palaeoclimate Modeling Intercomparison Project substantially underestimate this inferred equatorward wind shift. According to our reconstruction, the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO 2 ( R 2 = 0.98). Experiments with a 0.25° resolution ocean‐sea‐ice‐carbon model suggest that shifting the westerlies equatorward reduces the overturning rate of the ocean below 2 km depth, leading to a suppression of CO 2 outgassing from the polar Southern Ocean. Our results support a role for the westerly winds in driving the deglacial CO 2 rise, and suggest outgassing of natural CO 2 from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming.
Variability in North Pacific intermediate and deep water ventilation during Heinrich events in two coupled climate models
Publisher: Elsevier BV
Date: 02-2012
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View Funded ActivitySpecial Research Initiatives - Grant ID: SR200100008
Start Date: 08-2021
End Date: 12-2027
Amount: $20,000,000.00
Funder: Australian Research Council
View Funded ActivityDiscovery Early Career Researcher Award - Grant ID: DE150100107
Start Date: 04-2015
End Date: 12-2018
Amount: $369,536.00
Funder: Australian Research Council
View Funded Activity