Poles apart: why has Antarctic sea ice increased, and why can't coupled climate models reproduce observations?

Lead Research Organisation: National Oceanography Centre
Department Name: Science and Technology

Abstract

Due to its pale colour, sea ice reflects much of the incoming solar radiation back into space, keeping local temperatures relatively cold. However, if warming occurs and sea ice melts, it is replaced by darker ocean. This absorbs more solar energy, causing warming, and so the cycle, the so-called 'ice-albedo feedback' loop, continues. Sea ice also modifies the regional surface energy balance by capping the upper layer of the ocean, reducing its loss of heat to the atmosphere. In addition, sea ice is important because it plays a role in the exchange of carbon dioxide between the atmosphere and ocean, thereby affecting how much of this greenhouse gas is in the atmosphere and contributing to global warming. Moreover, sea ice formation is an important element in driving the global thermohaline circulation of heat and salt through the world's oceans. One component of this circulation is the North Atlantic Drift current that carries warm tropical water across the Atlantic and keeps the UK's winter temperatures much warmer than they would be otherwise.

The Intergovernmental Panel on Climate Change (IPCC) assessment reports are an important tool in drivng government policy around the world. However, the present generation of climate models, which are used to predict the future climate scenarios described in these reports, are unable to consistently reproduce the recent increase in Antarctic sea ice. As a result considerable uncertainty must be attached to their predictions of future climate.

This proposal aims to both advance our understanding of the Earth's climate and facilitate improved predictions of its future change to aid policy makers. This will be achieved through the following objectives:

1. To explain the key climate processes involved in the recent Antarctic sea ice increase. We know from observations that changes in the near-surface wind around Antarctica are predominantly responsible for the observed increase in sea ice but we don't know exactly how the wind and the ice interact. Using a state-of-the-art computer model of sea ice and the ocean forced by the latest atmospheric data we will establish the key processes through which changes in the wind are causing the ice to increase.

2. To establish the ultimate driver of the sea ice increase. Policymakers need to know whether we can attribute the observed changes in Antarctic sea ice to human activity. This might happen through changes in the near-surface winds around Antarctica caused by the 'ozone hole' or greenhouse gas increases for example. Alternatively, it may be simply due to natural variations in the Antarctic climate system. If the former is true, we must determine which human activities are responsible. If the latter is correct, we must try to understand connections between the key processes and wider aspects of the climate system.

3. To understand why current climate models fail to simulate the growth in Antarctic sea ice. We will examine the current UK climate model in detail to diagnose which components are to blame and, with our Met Office partner, we will design a development programme to ensure that our findings are transferred into future model improvements in time for the next IPCC report. To help other climate model developers around the world, we will also analyse whether the failings are common to the other models used in the IPCC reports.

Planned Impact

Due to the role of Antarctic sea ice in key components of the Earth's climate system, such as atmosphere-ocean CO2 exchange and the thermohaline circulation, at its highest level the science needs to be communicated to policymakers, the public and to anyone at risk from the effects of climate change. Clarity is required urgently on the issue of the Antarctic sea ice increase because failure to explain the opposing trends in the Arctic and Antarctic has the potential to increase public uncertainty about the validity of climate change: indeed, climate sceptics have already attempted to use the increase in Antarctic sea ice in their arguments. Furthermore, the latest climate models used as the basis for the IPCC reports fail to simulate the increase in Antarctic sea ice, bringing into question the value of their projections of future climate change in the polar regions.

As part of this proposal we will determine (i) whether we can attribute the recent observed increase in Antarctic sea ice to changes to human activity or natural climate variability, and (ii) why the current generation of coupled climate models are unable to reproduce the positive trend in sea ice. These outputs will lead to improved prediction of Antarctic sea ice over the 21st Century, help to inform both policymakers and public about future climate change, and enable the government to rebuff the claims of climate sceptics when being criticised for their current mitigation policies.

In the UK our findings will contribute directly to future climate model development to ensure that (i) the Met Office can continue in its remit to provide up-to-date, robust and traceable scientific evidence to government on climate variability and climate change, and (ii) UK climate modellers maintain their high-standing at the leading edge of climate research, thus allowing the UK to sustain its influential position in future climate negotiations. The two PDRAs will develop professional skills that have applicability to other employment sectors, such as adeptness in communication, gained from courses on communicating science to the public.

Publications

10 25 50
 
Description The climate mechanisms controlling the changes in the Antarctic sea ice. We discovered that melt water from the Antarctic, winds and mixing of heat from the warmer subsurface waters below sea ice are the principal mechanisms in controlling sea ice in the Southern Ocean.
Exploitation Route Understanding climate change in the Southern Ocean and globally.
Sectors Environment

URL http://meetingorganizer.copernicus.org/EGU2016/EGU2016-16982-6.pdf
 
Description The findings from the project inform the climate change policies, as they provided a better understanding of the climate change in the Antarctica, with consequences for the ocean and cryosphere and potential implications for the sea level rise.
First Year Of Impact 2017
Sector Environment
Impact Types Policy & public services

 
Description (IMMERSE) - Improving Models for Marine EnviRonment SErvices
Amount € 4,998,942 (EUR)
Funding ID 821926 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 12/2018 
End 11/2022
 
Description Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA)
Amount £7,094,230 (GBP)
Funding ID NE/N018095/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 03/2016 
End 03/2021
 
Title ORCA1-CICE with new mixing and sea ice melting schemes 
Description Results from the ORCA1-CICE model runs with different mixing schemes (TKE, GLS and modified GLS for wind and wave mixing) and different ice melting schemes using prognostic sea ice fragmentation, the list runs is below 1. Global NEMO1-control TKE, NEMO 3.6 stable + CICE 5.1, control 1with TKE vertical mixing lateral melting scheme with constant ice floes sizes, monthly output of U,V,T,S, W, vertical diffusivity and monthly output of sea ice (Hice Aice, Uice, Vice, Internal stresses, ice tendencies) and wave information (HS, Tp) 2. Global NEMO1-control GLS, NEMO 3.6 stable + CICE 5.1, control 2 with GLS vertical mixing, monthly output of U,V,T,S, W, vertical diffusivity and monthly output of sea ice (Hice Aice, Uice, Vice, Internal stresses, ice tendencies) and wave information (HS, Tp) 3. Global NEMO1-GLS, NEMO 3.6 stable + CICE 5.1, with modified GLS vertical mixing for wind effects, monthly output of U,V,T,S, W, vertical diffusivity and monthly output of sea ice (Hice Aice, Uice, Vice, Internal stresses, ice tendencies) and wave information (HS, Tp) 4. Global NEMO1-GLS, NEMO 3.6 stable + CICE 5.1, with modified GLS vertical mixing for wave effects, monthly output of U,V,T,S, W, vertical diffusivity and monthly output of sea ice (Hice Aice, Uice, Vice, Internal stresses, ice tendencies) and wave information (HS, Tp) 5. Global NEMO1-LM, NEMO 3.6 stable + CICE 5.1, with TKE vertical mixing and modified lateral melting scheme due to prognostics ice floes sizes, monthly output of U,V,T,S, W, vertical diffusivity and monthly output of sea ice (Hice Aice, Uice, Vice, Internal stresses, ice tendencies) and wave information (HS, Tp) 6. Global NEMO025-control, NEMO 3.6 stable + CICE 5.1, with TKE vertical mixing and modified lateral melting scheme with constant ice floes sizes, monthly output of U,V,T,S, W, vertical diffusivity and monthly output of sea ice (Hice Aice, Uice, Vice, Internal stresses, ice tendencies) and wave information (HS, Tp) 7. Global NEMO025-LM, NEMO 3.6 stable + CICE 5.1, with TKE vertical mixing and modified lateral melting scheme due to prognostics ice floes sizes, monthly output of U,V,T,S, W, vertical diffusivity and monthly output of sea ice (Hice Aice, Uice, Vice, Internal stresses, ice tendencies) and wave information (HS, Tp) 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Impact The results feeds in the ongoing NERC project ORCHESTRA. 
 
Title Mixing modules in NEMO 
Description Ocean mixing modules for the NEMO system 
Type Of Technology Software 
Year Produced 2017 
Open Source License? Yes  
Impact The modelling community is informed on the model development, which will be used as an open source for the scientific research. 
 
Description A Major International Congress, 13,650 participants 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk and discusssons at Session OS1.1 Open Session on Ocean Circulation and Air-Sea Interactionsthe In EGU General Assembly Conference 2016, 17-22 April 2016, Vienna, Austria. 13,650 participants in total.

Full title of the talk:
Aksenov, Y., Nurser, G., Bacon, S., Rye, C., Megann, A., Kjellsson, J., Holland, P., Ridley, J., Coward, A., Marshall, G. and Marsh, B., 2016, April. Response of the Southern Ocean dynamics to the changes in the Antarctic glacial runoff and icebergs discharge. In EGU General Assembly Conference Abstracts (Vol. 18, p. 16982).
Year(s) Of Engagement Activity 2016
URL http://meetingorganizer.copernicus.org/EGU2016/EGU2016-16982-6.pdf
 
Description European Geosciences Union General Assembly 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Results of the project were presented and discusssed atthe OS1.3 The Southern Ocean and its Role in the Global Climate System of the International Congress, ca. 11,837 people attended the meeting.

Full title of the talk:

Kjellsson, J., Holland, P., Marshall, G., Coward, A., Aksenov, Y., Bacon, S., ... & Ridley, J. (2015, April). Sensitivity of the recent increase in Antarctic sea ice in ocean models. In EGU General Assembly Conference Abstracts (Vol. 17, p. 5584).
Year(s) Of Engagement Activity 2015
URL http://meetingorganizer.copernicus.org/EGU2015/EGU2015-5584-2.pdf
 
Description Presentation at the EGU 2018: Aksenov, Y., Rynders, S., Hosekova, L., Feltham, D., Nurser, A. J., Madec, G., ... & Coward, A. (2018, April). Waves, Ice and Ocean in future projections of the Arctic and Southern Ocean. In EGU General Assembly Conference Abstracts (Vol. 20, p. 14180). 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Presentation delivered at the EGU General Assembly Conference Abstracts (Vol. 20, p. 14180). New data and results made available for the professionals and media. discussion followed the presentation helped to shape science directions.
Year(s) Of Engagement Activity 2018