Large uncertainty to persist in projected Southern Ocean sequestration of excess heat from greenhouse warming
IMAGE: THE CIRCUMPOLAR SOUTHERN OCEAN LIES BENEATH SEA ICE AND ICE SHELVES AROUND THE ANTARCTIC MARGINS. THE INTERACTIONS BETWEEN THE OCEAN AND SEA ICE AND ICE SHELVES MAKE THE ANTARCTIC CRYOSPHERE VULNERABLE TO A WARMING SOUTHERN OCEAN. view more
CREDIT: CREDIT: JINGWEI ZHANG, TAKEN AT PLENEAU ISLAND ON 9 JANUARY 2023.
This study is led by Dr. Wenju Cai (Environment, Commonwealth Scientific Industrial Research Organization, Australia) and involves scientists from China, Australia, and United States. The mid-to-high latitude Southern Ocean (30°S southwards) features prevailing westerly winds, the strongest mean sea-surface winds on Earth, which draws up ocean water from below 2–3 km in a wide circumpolar ring. This circulation system exerts a huge influence on climate under greenhouse warming, because the upwelled water was last in contact with the atmosphere hundreds of years earlier and once brought to the surface, absorbs a vast amount of anthropogenic heat and carbon from the atmosphere. However, based on the latest climate models, even under same emission scenario, inter-model differences in simulated amount of heat absorbed by the Southern Ocean are large, with a ratio of the highest over the lowest as high as three times. “The large spread is a concern, and has huge implications, including for melt of Antarctic Sea ice, ice sheets and ice shelves (center image below), radiative budget of the climate system, hemisphere rainfall distribution, and global sea level rise,” says Dr. Cai.
Dr. Cai and his team sought to determine what causes the large inter-model differences by synthesizing recent advances, and through examination of available outputs from latest models participating in the Phase 6 of Coupled Model Intercomparison Project. The team surveyed a large body of literatures and performed extensive analysis of data from some 30 participating models.
The team found that the large inter-model spread is not simply due to differences in the climate sensitivity, which measures the amount of radiative heating required to raise the Earth surface temperature by 1.0 oC. Previous studies suggest sequestration of heat by the Southern Ocean is mainly through mean circulation that facilitates uptake of additional heating, and that circulation changes because of greenhouse warming play a relatively small role. The new analysis instead shows that circulation changes contribute to much of the inter-model differences beyond that are attributable to climate sensitivity. For example, under greenhouse warming, the prevailing Southern Ocean westerly intensifies toward the Antarctic, but the wind changes are vastly different across models.
The wind intensification induces changes in the intensity and distribution of upwelling, with serious consequences. An increase in upwelling accelerates melt of Antarctic ice sheets and ice shelves, and the associated meltwater flux into the ocean leads to a more stratified upper ocean, which in turn slows heat and carbon uptake by the Southern Ocean. “There are many such complex interactions at work, some of which are poorly understood and not represented in models, contributing to the large uncertainty.” Cai says.
The researchers also found that the uncertainty can arise from a response of remote process to greenhouse warming, such as El Niño, the most consequential climate variability of the Earth climate. An El Niño is known to induce droughts over the western Pacific countries and floods over the eastern Pacific regions. Less known is that El Niño also leads to weakened prevailing Southern Ocean high-latitude westerlies, which over a warming period affecting shelf ocean warming, melt of sea ice, ice shelves and ice sheets. Surprisingly, inert-model differences in the response of El Niño to greenhouse warming systematically influenced the inter-model differences in Southern Ocean warming. But the changes in El Niño under greenhouse warming itself vastly differs across models. “New perspectives like this add to the growing realization that projecting Southern Ocean warming is one of the most complex issues in climate science.” Cai says.
The study suggests that the science of Southern Ocean warming involves appropriately representing interactions of ice shelves and ice sheets with the warming ocean, interactions of changing circulations with carbon cycles that force the circulation changes in the first place, and polar-tropical interactions, to name just a few. Because each of these areas is a challenge, uncertainty likely persists for some time to come. However, identification of such challenges is a start in any effort to move the science forward.
See the article:
Wenju Cai, Libao Gao, Yiyong Luo, Xichen Li, Xiaotong Zheng, Xuebin Zhang, Xuhua Cheng, Fan Jia, Ariaan Purich, Agus Santoso, Yan Du, David M. Holland, Jia-Rui Shi, Baoqiang Xiang, Shang-Ping Xie. Southern Ocean warming and its climatic impacts. Science Bulletin, 2023, 68(13): 946-960
https://doi.org/10.1016/j.scib.2023.03.049
JOURNAL
Science Bulletin
DOI
10-year countdown to sea-ice-free Arctic
IMAGE: - BLACK LINE (UNCONSTRAINED): RAW MODEL PROJECTIONS - RED, BLUE, GREEN LINES: MODEL PREDICTIONS CONSTRAINED BY THREE OBSERVED DATASETS. → THE RESULTS PREDICT THAT ALL OF ARCTIC SEA ICE WILL DISAPPEAR IN THE 2030S IF INCREASES IN GREENHOUSE GAS EMISSIONS REMAIN AT CURRENT RATE, AND EVEN WITH EMISSION REDUCTION, IT WILL EVENTUALLY DISAPPEAR DURING THE 2050S. view more
CREDIT: POSTECH
If the world keeps increasing greenhouse gas emissions at its current speed, all sea ice in the Arctic will disappear in the 2030s, an event that could at best be postponed until the 2050s should emissions be somehow reduced. The prediction is a decade earlier than what the Intergovernmental Panel on Climate Change (IPCC) has projected: an ice-free Arctic by the 2040s.
A possible ice-free Arctic in the 2030-2050s was projected regardless of humanity’s efforts to reduce its greenhouse gas emissions by Professor Seung-Ki Min and Research Professor Yeon-Hee Kim from the Division of Environmental Science and Engineering at Pohang University of Science and Technology (POSTECH) and a joint team of researchers from the Environment Climate Change Canada and Universität Hamburg, Germany. The research was published in the international journal, Nature Communications.
The term global warming has become a household name since it was first used by a climate scientist at NASA in 1988. The Earth has seen a rapid decline in the Arctic sea ice area as its temperature has increased over the past several decades. This reduction in Arctic sea ice has induced the acceleration of Arctic warming, which is suggested to contribute to the increased frequency of extreme weather events in mid-latitude regions.
To predict the timing of Arctic sea ice depletion, the research team analyzed 41 years of data from 1979 to 2019. By comparing the results of multiple model simulations with three satellite observational datasets, it was confirmed that the primary cause of the decline is attributed to 'man-made greenhouse gas emissions’. Greenhouse gas emissions resulting from human fossil fuel combustion and deforestation have been the primary drivers of Arctic sea ice decline over the past 41 years, while the influence of aerosols, solar and volcanic activities has been found to be minimal. Monthly analysis found that increased greenhouse gas emissions were reducing Arctic sea ice all year round, regardless of season or timing, although September exhibited the smallest extent of sea ice reduction.
Furthermore, it was revealed that climate models used in previous IPCC predictions generally underestimated the declining trend of sea ice area, which was taken into account to adjust the simulation values for future predictions. The results showed accelerated decline rates across all scenarios, most importantly confirming that Arctic sea ice could completely disappear by the 2050s even with reductions in greenhouse gas emissions. This finding highlights for the first time that the extinction of Arctic sea ice is possible irrespective of achieving ‘carbon neutrality.’
The accelerated decline of Arctic sea ice, faster than previously anticipated, is expected to have significant impacts not only on the Arctic region but also on human societies and ecosystems worldwide. The reduction of sea ice can result in more frequent occurrences of extreme weather events such as severe cold waves, heat waves, and heavy rainfalls all across the globe, with the thawing of the Siberian permafrost in the Arctic region possibly intensifying global warming further. We may witness terrifying scenarios, which we have seen only in disaster movies, unfold right before our eyes.
Professor Seung-Ki Min, who led the study, explained, “We have confirmed an even faster timing of Arctic sea ice depletion than previous IPCC predictions after scaling model simulations based on observational data.” He added, “We need to be vigilant about the potential disappearance of Arctic sea ice, regardless of carbon neutrality policies.” He also expressed the importance of “evaluating the various climate change impacts resulting from the disappearance of Arctic sea ice and developing adaptation measures alongside carbon emission reduction policies.”
The study was funded by the National Research Foundation of Korea (Mid-Career Researcher program).
JOURNAL
Nature Communications
ARTICLE TITLE
Observationally-constrained projections of an ice-free Arctic even under a low emission scenario
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