Limit global warming below 1.5°C to prevent the new normal climate beyond the record sea surface temperatures we have experienced – ScienceDaily

Over the past decade, the fringe seas of Japan have frequently experienced extremely high sea surface temperatures (SST). A new study by researchers at the National Institute for Environmental Studies (NIES) has found that the increased frequency of extreme ocean warming events since the 2000s is attributable to global warming due to industrialization.

In August 2020, the southern area of ​​Japan and the northwest Pacific Ocean experienced unprecedented SSTs, according to the Japan Meteorological Agency (JMA). A recent study published in January 2021 found that the record-breaking Pacific Northwest SST observed in August 2020 could not occur without human-induced climate changes. Since then, the JMA has again announced that record SSTs were observed near Japan in July and October 2021 and from June to August 2022, but it is still unclear to what extent climate change has altered the likelihood of occurrence. of these regional extreme warming events.

“The impacts of global warming are not uniform, but rather show regional and seasonal differences,” said co-author Hideo Shiogama, head of the Earth System Risk Assessment Section at the Earth System Division, NIES. . “A comprehensive analysis of regional SSTs over a long period can provide a quantitative understanding of the state of the ocean near Japan that has been and will be affected by global warming. This better informs policy makers to plan mitigation strategies. mitigation and adaptation to climate change.

The article published in Geophysical Research Letters now calculates the contribution of global warming to discrete monthly extreme ocean warming events in Japan’s marginal seas, which could occur less than once every 20 years in the pre-industrial era. A NIES climate research group focused on ten monitoring areas used operationally by the JMA, including the Sea of ​​Japan, East China Sea, Okinawa Islands, east of Taiwan and the Pacific coasts of Japan. Scientists confirmed that observed SST changes from 1982 to 2021 were well reproduced by 24 climate models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6), except for the region east of ‘Hokkaido. Then, extreme ocean warming events were identified in nine monitoring areas to reveal the contribution of climate change in them.

Extreme ocean warming and climate change

“In today’s climate, every extreme ocean warming event is linked to global warming,” said corresponding lead author Michiya Hayashi, associate researcher at NIES. Scientists estimated the frequencies of occurrence of each event under current and pre-industrial climate conditions from January 1982 to July 2022 based on CMIP6 climate models. “We found that the probability of occurrence of almost all extreme ocean warming events has already at least doubled since the 2000s compared to the pre-industrial era. It has increased more than tenfold in important cases since the mid-2000s. 2010s, especially in southern Japan.”

For example, in July 2022, abnormally high SSTs observed in five monitoring areas, including the Sea of ​​Japan (areas 1, 3), the East China Sea (areas 5, 8) and southern Okinawa near Taiwan (zone 10), are identified as extreme ocean warming events. Updated results based on preliminary data extracted from the NEAR-GOOS RRTDB website on September 15, 2022 (not included in the published article) show that in August 2022, events are also identified in six monitoring areas in the south of 35°N: the East China Sea (zones 5, 8), south and east of Okinawa (zones 10, 9), southeastern Kanto (zone 7) and the seas off Shikoku and Tokai (area 6). “We estimate that, in all these events identified in July and August 2022, the frequencies of occurrence are increased at least doubled due to climate change, and more than tenfold for those south of 35°N except for the north of the ‘East China. Sea,’ Hayashi said.

“The impacts of climate change on extreme ocean warming events in northern Japan began to appear relatively late compared to southern Japan,” Shiogama noted. The increase in global aerosol emissions until the 1980s tends to cool the Earth’s surface, which is greatest in the North Pacific, especially near northern Japan, via large-scale atmospheric circulation changes . In addition, the natural year-to-year variability of SST is large in northern Japan, so the global warming signal was less detectable than in southern Japan. Since over the past few decades global aerosol emissions have been reduced, the cooling effect becomes less dominant for human-induced greenhouse gas warming. “Our study indicates,” continued Shiogama, “that the contribution of climate change to SST extremes has already been noticeable beyond natural variability, even in northern Japan under current climatic conditions.”

What about the expected ocean conditions in the future? The researchers further compared the probabilities of exceeding monthly SST records around Japan at different levels of global warming from 0°C to 2°C using the 24 outputs of the CMIP6 climate model from 1901 to 2100. “Once the global warming reaches 2°C, all of the nine monitoring areas are expected to experience SSTs warmer than past highs at least every two years,” said Tomoo Ogura, co-author and head of the modeling and modeling section. analysis at the Earth System Division, NIES. He added: “It is necessary to limit global warming below 1.5°C so as not to have the record warm conditions in the marginal seas of Japan as new normal climate”.

Quantitative analysis of SSTs around Japan implies that climate change has already become the major factor for most of the record SSTs in recent years. “Going forward, the dynamics of each extreme warming event will need to be examined taking into account long-term climate change and natural year-to-year variability,” Hayashi noted. “Nevertheless, we hope that our statistical results based on the latest climate models will help to implement climate change adaptation and mitigation measures.”

Teresa H. Sadler