Ocean, sea ice, life and atmosphere
Sea ice is an integral part of the high latitude oceans and a key Earth system indicator for climate change. Sea ice is frozen seawater, it expands during each hemisphere’s winter; and melts in the summer. The white surface of sea ice reflects solar energy (albedo), cooling the planet. Sea-ice strongly influences ocean-air momentum, heat and gas exchange. When it melts, the darker ocean absorbs more heat, amplifying the cycle of melting sea ice. Sea ice is also a habitat to huge diversity of life, from microbes to whales, and an immense variety of birds, many of which are endemic. Losses of sea ice, especially multi-year ice and coastal sea ice, are endangering polar life. Sea ice transports matter, builds a barrier against glacial ice loss, and protects coastlines against waves and erosion. It supports human mobilities and has been part of hominid expansion and culture even before the emergence of Homo sapiens, and remains a key element of human cultures and livelihoods today.
Polar oceans drive the ocean’s conveyor belt. The Southern Ocean is especially critical as a global sink for heat and CO2. When cold polar water sinks, it brings oxygen and dissolved organic matter, but also pollutants to the deep-sea. Fluxes of radiative heat and freshwater from enhanced precipitation, runoff and melting sea ice and ice sheets are increasing, thereby enhancing surface stratification. The ocean warming has far-reaching climatic implications for melt of Greenlandic marine-terminating glaciers and Antarctic ice shelves and ice sheet, thereby enhancing sea level rise and possibly ocean overturning at lower latitudes. Complex and poorly understood processes include oceanic eddies, tropical-polar interactions, interior warming and stratification changes, and future ability to take up CO2 by the physical and biological carbon pump.
At times present at much lower latitudes over Earth history, sea ice is conceivably a hotspot for evolution of microorganisms on Earth, and potentially harbours life on extraterrestrial ocean worlds. The role of sea ice as a major biome on Earth, and an integral part of polar ecosystem functioning remains to be understood. Establishing a baseline in the state of sea-ice related ecosystems constitutes a race against time in the face of the speed of transformations affecting the polar sea ice, particularly the oldest one. In losing old sea ice in the Arctic, we may lose our best analogue of conditions prevailing during past ice ages and in other ocean worlds.
As to polar ocean productivity, the lack of time series with adequate seasonal and regional resolution has limited our understanding as to the effect of warming. Satellite observations show mainly increases in phytoplankton biomass in the Arctic and Southern Ocean over the last 20 years, because the shrinking sea ice enhances light penetration. However, concurrent stratification changes, including increased air-sea momentum transfer enhancing ocean mixing, and it impacts nutrient supplies, thus future projections are highly uncertain.
The key drivers of polar productivity, the diatoms, get replaced by smaller algae as a result of ocean warming, changing the food web and potentially reducing the carbon pump. Loss of sea ice endangers krill, fish, marine mammals and seabirds as they find their prey there and use it as a platform to reproduce, leading to alarming massive breeding failure in emperor and Adélie penguin colonies. Loss of sea ice also affects microbial life and thereby the entire food web. While northward expansion of new species into the Arctic Ocean from the Pacific and Atlantic has been documented at all trophic levels, the actual causes (e.g. changes in water mass distribution, currents, seawater properties or sea ice dynamics) remain unclear, and therefore our capacity to make any prediction about marine ecosystems are limited.
At the current level of global warming of 0.2°C per decade, sea ice loss in the Arctic observed since 1978 is 3% per decade in wintertime, and 13% in summertime. The majority of multiyear sea ice has been replaced with annual, thinner and more dynamic sea ice. There is a close relationship between changes in Arctic summer sea ice, cumulative CO2 emissions, and global surface temperature - translating into around 3 square metre loss of
Arctic sea ice for each ton of CO2 emitted. In Antarctica, no significant sea ice trend was discernible from 1979 to 2020 (summer or winter), due to large internal variability and contrasting regional trends. However, we have now seen two years of an exceptionally low cover of sea ice, (in June 2023 -17% compared to average)..
Human influence is the main driver of Arctic sea ice retreat. As greenhouse gas emissions and their atmospheric concentrations have continued to increase, the Earth warms. 91% of this excess heat accumulates in the ocean, of which half sinks into the Southern Ocean. 3% leads to cryosphere melt, 5% to land warming and 1% to lower atmosphere warming. Thus, the warming ocean will be an increasing threat to the future of sea ice. Several additional amplifying feedback loops act on sea ice and involve surface albedo, water vapor, clouds, snow precipitation, air-sea fluxes, altogether causing polar amplification. The extent to which stabilizing feedback loops driven by physical and biological processes may dampen amplification is however not well constrained, adding to uncertainties. For instance, the production of climate active gases such as di-methyl sulfide, or aerosolized microbial ice nucleating particles, both favoring cloud production during newly ice-free areas, have been put forward. Especially in the Arctic, long-range contaminants, notably black carbon transported from intensifying industrial regions at lower latitude, do contribute to reduction in sea ice albedo and, therefore, to melt.
In the past 40 years, Arctic warming was 4 times faster than the global average. In scenarios and pathways assessed in the IPCC AR6 2021 report, there is a likelyhood already by 2030-2035 for a first ice-free (<15% ice-cover) summer for the Arctic Ocean, and at least once prior to 2050, linked to cumulative CO2 emissions reaching 1000 gigatons. Recent variations are related to atmospheric circulation patterns, with persistent consequences due to a warmer ocean and possibly a new Antarctic sea ice state. Heat uptake in the Southern Ocean accounts for >50% of the global ocean heat uptake.
Gaps in scientific knowledge concern especially land-ocean, ice-atmosphere-ocean, ocean-glacier and sea-ice-life interactions. It is important to better understand future changes in precipitation in a warming world associated with an intensification of the water cycle and its variability, including snow, and changes in the dynamics in atmospheric circulation and winds. Changes in large scale water mass distribution, notably between Atlantic and
Arctic Oceans, in the connection between fresh and cold surface waters and warm and salty deeper ones, including ocean mixing, and in sea ice growth/melt cycle and dynamics (advection, ridging, leads, melt ponds) further complexify the impact of global warming on polar ice. The lack of long-term time series with adequate regional resolution to achieve circumpolar assessments of change is critical to advance confidence in projections. Key needs are forecasts of the role of decadal dynamics and teleconnections of phenomena like El Nino, North Atlantic Oscillation and of the stability of the Atlantic Meridional Overturning Circulation, the Antarctic Circumpolar
Current and their links to sea ice. The fate of sea ice directly and indirectly affects food-webs and biological interactions from microbial life to krill, fish, whales and birds, as well as to benthic life including the deep-sea. Gaps in knowledge remain large as to the total diversity of polar life and its dynamics, its interactions with ocean chemistry, i.e., aerosol formation, dissolved matter, particle fluxes, and its implications for economic development, human health and well-being.
IMPACTS AND RISKS
- Polar oceans are experiencing climate change impacts with large magnitudes and will be profoundly different by 2050.
- The impacts of shrinking polar sea ice may be felt globally, including in weather patterns.
- Sea-ice loss, warming and acidifying oceans reduce available habitats for many polar marine species, including many shellfishes, fish species and marine mammals, while opening the way to invasive boreal species. Exposure to old (e.g. fisheries) and new stressors such as plastics and persistent chemicals and noise could worsen the negative effects for polar marine species.
- Sea ice decline leads to more opportunities but also growing risks from expanding and intensifying shipping, fisheries, tourism, resource development, and other industries - including intensification of geopolitical tensions.
- Sea-ice loss and extreme events arising from climate warming affect the lives and livelihoods of Arctic communities. Amplified climate changes increasingly threatens many aspects of Arctic lifestyles (culture, heritage, identity, health, including mental health, safety), especially for indigenous peoples.
- Science calls for a new international ambition in reducing greenhouse gas to limit global warming as close as possible to 1.5°C, and reach net zero CO2 emissions as soon as possible and with the smallest cumulative emissions until this is reached. This is an essential goal to avoid further large declines of sea ice cover, and to reduce weather extremes globally, thereby avoiding the escalation of losses and damages, and costs of adaptation and adaptation limits.
- International collaboration for the protection of polar oceans (for example as voiced in the Helsinki Declaration on Climate Change at the Antarctic Treaty Consultative Meeting (2023)) is essential to reach the goals of the Paris Agreement and Kunming Montreal frameworks.
- To reduce the pressure on polar life due to intensifying and expanding human activities, marine and terrestrial protected areas are key actions and need a new international ambition supported by science. In their planning and maintenance collaboration and support for self-determination among indigenous communities is important.
- Closing gaps in cryosphere education and literacy, information and forecasting is critical for society. It needs support for better process understanding, modeling tools and frontier research, including implications for habitability, water, food, and human security, unique ecosystems and biodiversity, equity, economy including eg. insurability.
- The cryosphere perspective should be included in estimates of the cost of carbon emissions, costs of adaptation, adaptation limits, and the planning of loss and damage funds.
- Local infrastructure to enhance resilience to climate change / sea ice loss, especially including self-determination of indigenous peoples.
- Long-term support of international sea-ice, ocean, biosphere and atmosphere programs in the Arctic and Antarctic, considering infrastructure (e.g. international sea ice buoy and Argo float programs, polar research vessel fleet, research aircrafts and stations) as well as large international missions (e.g. Tara Polar Station, Antarctica InSync, International Polar Year), nested into and coordinated with international observing networks and initiatives.
- Strongly reinforced cooperation among space, ocean and polar agencies to initiate polar orbiting satellites, consistently distribute Earth observation data for polar regions, develop new missions, new products and algorithms, and conduct dedicated field expeditions for ground truthing. Improve FAIR data accessibility.
- Regional cooperation, multi-scale biodiversity & marine protection networks (monitoring and governance), increasing science-based resources to reinforce resilient and sustainable development trajectories.
- More research capacity, with new cutting-edge scientific expertises for exploring the sea ice biodiversity.
- Funding of capacity building developed by, and co-developed with local communities, to serve both local and global science needs, and maximize regional collaborations and activities for more sustainable, year-round polar science.
- Transformation of polar infrastructure to develop lower carbon and ecological footprint solutions for research, and allow for near real-time data transmission.
- Accessible, context-specific sea-ice information products and tools, regional forecasts and projections to support decision-making.
- Complex interactive, data – enabled model (Digital twin) of polar seas, including risk management modules (adequate and appropriate emergency responses and adaptation) and guiding observation strategies.