Retreat of Southern Hemisphere Sea Ice, 130 000 to 116 000 years BP

Lead Research Organisation: British Antarctic Survey
Department Name: Science Programmes


Sea ice is of huge importance to global climate, the Antarctic ice sheet, and to all life in the Southern Ocean. However the area covered by sea ice in the Southern Hemisphere is predicted to drastically decrease over the next century; the decrease by 2200 could be as much as 58%.

The models, on which predictions of sea ice loss are based, have never been tested against a documented retreat of Southern Hemisphere sea ice. A major retreat in Northern Hemisphere sea ice since 1978 has enabled climate and sea ice models to be evaluated for their ability to replicate retreat data. This has helped ensure that we have high confidence in Arctic sea ice predictions. In the Southern Hemisphere, there has been no similar sea retreat since 1978, which has meant that models cannot be tested against retreat data. This lack of sea ice retreat data has thus been a major contributor to the current lack of confidence in Southern Hemisphere sea ice forecasts.

Here we propose to address this lack of data which covers a retreat of the Southern Hemisphere sea ice. We have found a period during the Last Interglacial (LIG) between 130,000 and 116,000 years Before Present (BP), when there was a substantial retreat in Southern Hemisphere sea ice. By reconstructing and modelling a retreat during this LIG period, under a climate which was similar to that of today, this proposal aims to transform understanding of the structure and causes of a major Southern Hemisphere retreat event. The project will be the first ever quantitative investigation of a large Southern Hemisphere sea ice retreat, and will be be of considerable importance to both the sea ice and palaeoclimate science communities.

We will generate a new LIG sea ice dataset, using a robust new statistical approach. This will draw from, and build upon, the core expertise of our science team members. Our new Southern Hemisphere sea ice data data will then be used to help establish the cause of the LIG retreat and to test current climate and sea ice models. In addition to identifying the most likely cause of the decrease in sea ice during the LIG, this project will establish for the first time the degree of confidence that the sea ice community can have in our UK model-based predictions of a future retreat in Southern Hemisphere sea ice.

Planned Impact

Target audiences who will benefit from this research are: palaeoclimate and sea ice scientists (the main beneficiaries of the research as detailed in the above `Academic beneficiaries' section) as well as stakeholders (with a focus on policy-makers) and the general public (with a strong emphasis on schoolchildren at key stage 1 and 2).

The main users of the research outcomes who are outside of the immediate research community will be policy-makers and their science advisors. In particular, the Scientific Committee on Antarctic Research (SCAR) and the International Panel on Climate Change (IPCC) use evidence on polar and climate change to gauge the severity of anthropogenically enhanced global warming to help policy-makers develop an appropriate level of response.

POLICY-MAKERS. Submission of our previous publications to the appropriate IPCC AR5 Chapter editors ensured that our work forms part of past IPCC reports, thereby influencing UK and global policy. As an example of this influence, if this proposed research were to demonstrate that the UK Coupled Model Inter-comparison Project (CMIP6) Climate Model predictions of Southern Hemisphere sea ice changes are likely to be underestimated, it would increase the urgency for action proposed by SCAR and the IPCC. Similarly, if Southern Hemisphere sea ice changes are overestimated, this may allow attention to be focussed elsewhere.

CHILDREN AND TEACHERS. We hope to engage and inspire children with the spirit of Antarctic scientific endeavour - these young people will go on to become the future scientists and decision makers for NERC and for Antarctica. We propose to partner with the charity UK Antarctic Heritage Trust to produce inspirational resources for schools. The Antarctic Heritage Trust are dedicated to caring for, and promoting, Antarctic scientific heritage. Together, we will create a variety of learning resources based jointly on Trust archival material and our contemporary research outcomes. This approach will build on previous Trust successes in producing and distributing school resources. The learning resources will be aimed at teachers and pupils in schools to use in the classroom. A combination of digital and print resources will be written with the aid of a freelance educational specialist. The digital resources will be made freely available through existing child and teacher friendly sites, including and along with the tried and tested website. Printed resources will be particularly targeted at less privileged schools.

PRESSURE GROUPS / CLIMATE SCEPTICS. We will write at least one project RealClimate ( project blog post to reach the 'climate sceptic' pressure groups. The post will be co-written with the sea ice modellers Prof. D. Feltham and Dr. D. Schroeder. Post(s) will directly address a major line of attack on climate science: "Increases in Southern Hemisphere sea ice, particularly since 2012, undermine evidence for global warming". By talking with these users, using their own preferred means of communication, we will work to minimise the potential for any misunderstanding of Southern Hemisphere sea ice science.

LOCAL SCHOOLS, WI, AND U3A AUDIENCES. We will build on our record of public lectures by providing three public lectures per year of the project.

MEDIA AND THE GENERAL PUBLIC. Press releases, twitter, and a project web page will be used to both encourage and satisfy public interest in the work.


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Description Our 2018 ice core-model comparison indicates a last interglacial maximum winter sea ice retreat of 67, 59, and 43% relative to preindustrial in the Atlantic, Indian, and Pacific sectors of the Southern Ocean. Our compilation of Southern Ocean sea ice proxy data provides support for this ice core based reconstruction. We find that published marine core sites are located too far north of the 128,000 years B.P. sea ice edge, preventing independent corroboration for this sea ice reconstruction. These results may have had serious implications for the stability of marine terminating glaciers around the Antarctic Ice Sheet and their contribution to the last interglacial sea level rise. Further we demonstrate that climate models are capable of simulating this sort of sea ice retreat when run for 2-3000 years under norther hemisphere meltwater forcing.

In 2020/2021 we demonstrated that the UK model HadGEM3 is the first coupled climate model to simulate an ice-free Arctic during the Last Interglacial (LIG), 127 000 years ago. This simulation appears to yield accurate Arctic surface temperatures during the summer season. We find that the summer ice melt is predominantly driven by thermodynamic processes: atmospheric and ocean circulation changes do not significantly contribute to the ice loss. We demonstrate these thermodynamic processes are significantly impacted by melt ponds, which form on average 8 days earlier during the LIG than during the pre-industrial control (PI) simulation. This relatively small difference significantly changes the LIG surface energy balance, and strengthens the albedo feedback. Compared to the PI simulation: in mid-June, of the absorbed flux at the surface over ice-covered cells (ice concentration > 0.15), ponds account for 45-50 %, open water 45 %, and bare ice and snow 5-10 %. We show that the simulated ice loss leads to large Arctic sea surface salinity and temperature changes. The sea surface temperature and salinity signals we identify here provide a means to verify, in marine observations, if and when an ice-free Arctic occurred during the LIG. Strong LIG correlations between spring melt pond and summer ice area indicate that, as Arctic ice continues to thin in future, the spring melt pond area will likely become an increasingly reliable predictor of the September sea-ice area. Finally, we note that our multi-model work shows that climate models with explicitly modelled melt ponds seem to simulate particularly low LIG sea ice extent. These results show that models with explicit (as opposed to parameterised) melt ponds can simulate very different sea-ice behaviour under forcings other than the present-day. This is of concern for future projections of sea-ice loss.
Exploitation Route These finding are primarily of use to the sea ice modelling community. They are being further explored under the EU-TiPES program.
Sectors Energy,Environment,Government, Democracy and Justice,Security and Diplomacy

Description Outreach through Maker Workshops.
First Year Of Impact 2019
Sector Culture, Heritage, Museums and Collections
Impact Types Cultural,Societal

Description EPSRC-PEN feasibility study
Amount £8,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2017 
End 12/2017
Title Global monthly outputs of orography, surface air temperature and water stable isotopes for the last interglacial for idealised Antarctic Ice Sheet simulations run by the isotope-enabled HadCM3 
Description Global monthly outputs of orography, surface air temperature and water stable isotopes (d18O) were run by the isotope-enabled atmosphere/ocean coupled model HadCM3 for the last interglacial (128 ka). An ensemble of ten idealised Antarctic Ice Sheet (AIS) simulations were processed, included a pre-industrial and a last interglacial control simulations. The eight other simulations used changed topography of the AIS relative to Dome C to ensure the preservation of the atmospheric pathways. The simulations were run 100 years and the last 50 years were used for the analyses. This work was funding through the European Research Council under the Horizon 2020 research and innovation programme (grant agreement No 742224, WACSWAIN). 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact Unknown at this time. 
Title HadCM3 and HadGEM3 LIG model outputs: A sea ice-free Arctic 
Description The HadGEM3 (HadGEM3-GC3.1 or HadGEM3-GC3.1-N96ORCA1) PI simulation was initialized using the standard CMIP6 protocol using constant 1850 GHGs, ozone, solar, tropospheric aerosol, stratospheric volcanic aerosol and land-use forcing. The PI spin-up was 700 model-years, which allowed the land and oceanic masses to attain approximate steady state. The HadGEM3 LIG (Last Interglacial) simulation was initialized from the end of the spin-up phase of the equivalent pre-industrial (PI) simulation. After initialization, the LIG was run for 350 model-years. This 350 LIG spin-up permits the model to reach atmospheric equilibrium and to achieve an upper-ocean equilibrium. The model was then run for a further 200 model-years of LIG production run. This has been demonstrated to be an adequate run length to appropriately capture the model internal variability. This dataset contains outputs from the 200 years of production run of the period. The HadCM3 PI simulation was run for a period of over 600 years. The HadCM3 LIG simulation was initialized from the end of a previous CMIP5 LIG simulation, which was of length 400 years and initiated from the end of the corresponding PI, and run for further 250 years. The total spin-up phase for the HadCM3 LIG simulation used in this study was thus 600 model-years, and the length of the production (at atmospheric and upper-oceanic equilibrium) LIG HadCM3 simulation is 50 model-years. This work was funded by NERC standard research grant no. NE/P013279/1. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
Impact The simulations were carried out using the HadGEM3 and HadCM3 climate models. These model simulations were run on the ARCHER UK National Supercomputing Service ( and processed on the JASMIN data analysis platform ( They are Tier 1 LIG simulations, on the basis of the standard CMIP6-PMIP4 LIG experimental protocol. The prescribed LIG (127,000 yr) protocol differs from the CMIP6 PI simulation protocol in astronomical parameters and the atmospheric trace GHG concentrations. LIG astronomical parameters are prescribed according to orbital constants43, and atmospheric trace GHG concentrations are based on ice-core measurements. other boundary conditions, including solar activity, ice sheets and aerosol emissions, are identical to the PI simulation. We run two LIG simulations, one using the UK CMIP6 HadGEM3 model and the other using the CMIP3 HadCM3 model. HadGEM3 is the lowest-resolution version of the UK CMIP6 physical climate model. It is a global coupled atmosphere-land-ocean-ice model that comprises the Unified Model atmosphere model, the JULES land surface model, the NEMO ocean model and the CICE sea-ice model. Impacts arise from future useage of this data. 
Title Last Interglacial summer air temperature observations for the Arctic 
Description These 21 Last Interglacial (LIG) summer surface air temperature (SSAT) observations were compiled to assess LIG Arctic sea ice (Guarino et al 2020). Twenty of the observations were also previously used in the IPCC-AR5 report. Each observation is thought to be of summer LIG air temperature anomaly relative to present day and is located in the circum-Arctic region. All sites are from north of 51N. There are 7 terrestrial based temperature records; 8 lacustrine records; 2 marine pollen-based records; and 3 ice core records included in the original compilation. This compilation includes 1 additional ice core record. This work was funded by NERC standard research grant no. NE/P013279/1. 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
Title Southern Hemisphere winter sea ice concentration simulated by HadCM3 to best explain the early last interglacial Antarctic isotope peak 
Description The file contains Southern Hemisphere winter (September) sea ice concentration (sic) from a simulation performed using the isotope-enabled HadCM3 climate model forced with early last interglacial boundary conditions, centred approximately 128,000 years ago. The resulting sic represents a reduction in winter sea ice area of approximately 54% relative to pre-industrial and is proposed as the best explanation for the Antarctic ice core data from 128,000 years ago. The spatial pattern of sea ice retreat was determined using a large ensemble of model experiments and a pattern search optimization approach to match the last interglacial ice core isotope peak. Further details can be found in the published manuscript ( 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Title Surface elevation of 69 Greenland Ice Sheet morphologies and associated d18O anomalies (with respect to Pre-industrial) simulated by HadCM3 
Description The text file (.csv) contains d18O changes simulated at six Greenland deep ice cores (NEEM, NGRIP, GRIP, GISP2, Camp Century and DYE3) from 69 simulations performed using the isotope-enabled HadCM3 climate model forced with mid last interglacial boundary conditions, centred at 125,000 years ago. HadCM3 is used to reproduce the d18O response to 69 modified Last Interglacial (LIG) Greenland Ice Sheet (GIS) morphologies at the ice-core sites. To parameterise the set of 69 GIS morphologies, we undertake a Principal Component Analysis (PCA) approach. The text file also contains the 8PC coefficients for each of the 69 morphologies. The netcdf file (.nc) contains the 8PC shapes and the average shape. To obtain any of the 69 GIS morphologies: (1) store the 8 PC coefficients of a specific GIS morphology and, (2) take a linear combination of the PC shapes (according to those coefficients) and add the average shape. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact Recovery of the past shape/volume of the Greenland ice sheet. 
Description Eric J. Steig Professor, Earth & Space Sci. And IsbLab 
Organisation University of Washington
Department Department of Biostatistics
Country United States 
Sector Academic/University 
PI Contribution Provision of Climate Model isotopic output for location of possible US led Hercules Dome ice coring site.
Collaborator Contribution Provision of Climate Model isotopic output for location of possible US led Hercules Dome ice coring site.
Impact Papers in preparation.
Start Year 2016
Description The Past Earth Network (PEN) 
Organisation University of Leeds
Country United Kingdom 
Sector Academic/University 
PI Contribution Co-leader of the PEN Model-Data comparison group. Myself and my research group will be helping to organise PEN workshops in the coming years.
Collaborator Contribution Organising conference sessions and PhD studentships: earth sciences and statisticians
Impact Conference sessions and PhD studentships: earth sciences and statisticians
Start Year 2015
Description Maker workshops in London 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Free creative workshops using art, crafts and model making to share ideas about responding to the impacts of climate change.

2:30pm 24 January, 21 February, 20 March

The news is full of fires and floods and it is difficult to know how to help. These workshops will imagine and invent ways we can respond to the experience of
climate change in different ways around the world.
Year(s) Of Engagement Activity 2019,2020