The marginal seas of Japan have repeatedly experienced unusually high sea surface temperatures (SSTs) over the last ten years. According to a recent study led by scientists at the National Institute for Environmental Studies (NIES), industrialization-related global warming is to blame for the increased frequency of extreme ocean warming events that have occurred since the 2000s.
According to the Japan Meteorological Agency (JMA), SSTs in August 2020 in the southern region of Japan and the northwest Pacific Ocean reached previously unheard-of heights. The record-high northwestern Pacific SST seen in August 2020 is not likely to occur without human-induced climate change, according to a recent study that was released in January 2021.
Since then, the JMA has stated again that record-breaking SSTs were seen close to Japan between July and October 2021, as well as from June to August 2022. However, it is still unknown how much climate change has affected the possibility that these regional extreme warming events would occur.
Impacts of global warming is not uniform, rather show regional and seasonal differences. A comprehensive analysis on regional SSTs for a long period may provide a quantitative understanding of how much ocean condition near Japan has been and will be affected by global warming. This better informs policymakers to plan climate change mitigation and adaptation strategies.
Hideo Shiogama, Study Co-Author and Head, Earth System Risk Assessment Section, Earth System Division, National Institute for Environmental Studies
The study, which was published in Geophysical Research Letters, examines the role that global warming plays in the distinct monthly intense ocean warming events that occur in Japan’s marginal seas.
These occurrences could occur less than once per 20 years in the preindustrial era. The Japan Sea, East China Sea, Okinawa Islands, Taiwan’s east coast, and the Pacific shores of Japan were among the ten zones that the JMA monitors in its business operations.
With the exception of the eastern portion of Hokkaido, the scientists confirmed that the observed SST variations between 1982 and 2021 were accurately simulated by 24 climate models taking part in the Coupled Model Intercomparison Project (CMIP6sixth)’s phase.
The impact of climate change there was then revealed by identifying the extreme ocean warming occurrences in nine monitoring regions.
Extreme Ocean Warming and Climate Change
In the present climate, every extreme ocean warming event is linked to global warming.
Michiya Hayashi, Study Corresponding Lead Author and Research Associate, National Institute for Environmental Studies
Based on the CMIP6 climate models, the researchers calculated the frequency of each event between January 1982 and July 2022 in both the current and preindustrial climates.
Hayashi added, “We found that the occurrence probability of almost all the extreme ocean warming events has already at least doubled since the 2000s than the preindustrial era. It is increased more than tenfold in sizeable cases since the mid-2010s, especially in southern Japan.”
For example, the extreme ocean warming episodes in July 2022 are identified as the anomalously high SSTs detected in five monitoring areas, including the Japan Sea (Areas 1, 3), East China Sea (Areas 5, 8), and south of Okinawa near Taiwan (Area 10).
The updated results, which are based on the preliminary data downloaded from the NEAR-GOOS RRTDB website on September 15th, 2022, reveal that the events are also established in six monitoring areas south of 35°N for August 2022: the East China Sea (Areas 5, 8), the south and east of Okinawa (Areas 10, 9), the southeast of Kanto (Area 7), and the seas off Shikoku and Tokai (Area 6).
“We estimate that, in all of these identified events in July and August 2022, the occurrence frequencies are increased at least doubled due to climate change, and more than tenfold for those in the south of 35°N except for the north of East China Sea,” stated Hayashi.
Shiogama remarked, “Climate change impacts on extreme ocean warming events in northern Japan began to emerge relatively late compared to southern Japan.”
The Earth’s surface tends to cool as a result of the increased global aerosol emissions up until the 1980s, which is particularly pronounced in the North Pacific, especially close to northern Japan. Additionally, the SST naturally varies from year to year in northern Japan, making it harder to discern the effects of global warming there than in southern Japan.
The cooling impact is less dominating than warming caused by human-induced greenhouse gas emissions since global aerosol emissions have decreased over the last few decades.
Shiogama further stated, “Our study indicates that the contribution of climate change to SST extremes has been already discernible beyond natural variability even in northern Japan under the present climate condition.”
Some Expected Changes in Ocean Conditions
Using the results from 24 CMIP6 climate models from 1901 to 2100, the researchers also examined the probabilities of exceeding the monthly record high SSTs around Japan at various levels of global warming from 0 °C to 2 °C.
Once global warming reaches 2 °C, all of nine monitoring areas are expected to experience SSTs warmer than the past highest levels at least every two years. Limiting global warming below 1.5 °C is necessary not to have the record warm conditions in Japan's marginal seas as the new normal climate.
Tomoo Ogura, Study Co-Author and Head, Climate Modeling and Analysis Section, Earth System Division, National Institute for Environmental Studies
According to a quantitative investigation of SSTs near Japan, climate change is already the predominant reason for the majority of the recent record-high SSTs.
Hayashi notes, “In the future, dynamics of each extreme warming event need to be examined by taking the long-term climate change and year-to-year natural variability into account. Nevertheless, we expect that our statistical results based on the latest climate models will help to implement adaptation and mitigation measures for climate change.”
Hayashi, M., et al. (2022) The Contribution of Climate Change to Increasing Extreme Ocean Warming Around Japan. Geophysical Research Letter. doi:10.1029/2022GL100785.