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Briefing Comments on IPCC AR6 SPM

— Messages from IPCC AR6 SPM – Changes in the Arctic —

Hiroyuki Enomoto 
NIPR Vice Director-General 

© 2021 Intergovernmental Panel on Climate Change (IPCC)

The Intergovernmental Panel on Climate Change (IPCC) published a summary for policymakers (SPM) of the Working Group I (WG I) report (“Climate Change 2021: The Physical Science Basis”) on August 9, 2021, as a part of the sixth Assessment Report (AR6). The IPCC has been publishing reports since 1990. This is the first release in eight years since the WG I report of the Fifth Assessment Report (AR5) unveiled in 2013.

The new report explicitly says that there is no doubt that human activities do have a great impact on climate change, emphasizing that the scale of its influence is unprecedented. The report has made headlines in media reports both in Japan and worldwide.

The full report will be made available the final edit.

Key points of this SPM

IPCC special report “Global Warming of 1.5℃” released in October 2018 emphasized that many climate change impacts could be avoided by limiting global warming to an average of 1.5℃ above pre-industrial levels. IPCC Sixth Assessment Report (AR6) predicts the temperature rise of more than 1.5℃ by 2040 even with greenhouse gas emissions under control, and in the worst-case scenario, up to 4.4℃ over the next century. On contrast, in scenarios where greenhouse gas emissions could stay sufficiently controlled, the report also predicts that the temperature will drop back down below by the end of this century after the temperature rise reaches over 1.5℃.

The report examines environmental changes of the ocean, land, and ice sheet each separately, explaining that there are serious effects such as air and sea temperatures increase and melting of snow and ice, and that an estimate of the equilibrium climate sensitivity (i.e., the value of global mean surface temperature in case atmospheric CO2 concentrations to be doubled from pre-industrial levels) are higher than in previous reports, where it narrows the range of uncertainty. Paleoclimate information contributed to the estimation.
Anthropogenic climate change has already been affecting many extreme events all over the world. The report also points out that such change caused by “human-induced activities” increases the frequency and intensity of extreme events such as heatwaves, heavy rains, droughts, and tropical cyclones, which are defined as the more conclusive evidence for causal identification compared to AR5.

In particular, the effects on sea ice, ice sheets, and permafrost are visible in the Arctic, and this is considered to become a factor that amplifies global warming. The speed of warming in the Arctic is reported to be 2 to 3 times faster than it is in the global mean, and the temperature rise is predicted to be 1.5 to 2 times faster than it is in the global mean, and 3 times faster for the coldest day. Arctic sea ice extent is currently at its minimum level since the 1850s, and its condition captured last summer (2020) has not been seen in the past 1,000 years. As for the future, the report predicts that the Arctic Ocean could be practically ice-free in September by 2050 under all scenarios without lowered CO2 emissions. However, the report states that there is no tipping point at which the trend of sea ice decline will continue irreversibly.

The rate of ice sheet loss has quadrupled in the last 20 years, and the report estimates that glaciers and the Greenland ice sheet will not stop melting for a long time as well as carbon leakage from thawing permafrost and that the sea level will most likely to continue to rise on the scale of hundreds of years, leading to an irreversible change (sea-level rising will not stop). Many of the changes caused by past and future greenhouse gas emissions, especially in the oceans, ice sheets, and sea levels, are now commonly stated with the century-long time scales and defined as “irreversible.” Extreme sea-level rise and ocean heatwaves both in the Arctic and Tropics are also discussed in this report.

At the North Pole in August 2020. Melt ponds (puddles of water on the sea ice) are spreading all over the area. (Photo by Daiki Nomura, Hokkaido University)

The report also mentions the relationship between Antarctic sea ice and climate change, and the possibility of a significant increase in melting of the Antarctic ice sheet. The sea ice extent in the Antarctic is showing a different trend than the Arctic. Antarctic sea ice extent has been increasing for a long time, however, recently it has been decreasing. The link between this event and climate change is still unclear. As for ice sheet melting, Greenland will continue to experience the two major factors, surface melting and iceberg separation, while any reliable prediction has yet to be available due to lack of knowledge in regards to ice shelf depletion and its impact on the ocean, and the instability of the ice buttress where the ice sheet meets the ocean. The SPM, on the contrary, notes that the melting of the West Antarctic ice sheet will have a significant impact on sea-level rise when ice sheet instability becomes active. This is a new statement that has never appeared in previous reports.

The report concludes that “It is unequivocal that human influence has warmed the atmosphere, ocean, and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere, and biosphere have occurred” and that “The scale of recent changes across the climate system as a whole – and the present state of many aspects of the climate system – are unprecedented over many centuries to many thousands of years.”
The challenge for us is to brace for possible future climates, risk assessment, and local adaptation, as well as how we limit future climate change. Although what the scientists have been calling is unchanged, more serious causes and effects elaborately identified in this report call for the urgent need to take action. The report shows that immediate action against greenhouse gas and aerosol emissions will have the prospect of faster effects, however, will still take over decades to change global mean temperature. We are confronted with the way we handle human activities ourselves in the future.

Domestic Ministries and Agencies

This article, originally written in Japanese, refers to “An Overview on IPCC AR6/WG1 SPM” published by following ministries on each website.