The marginal seas of Japan have frequently experienced extremely high sea surface temperatures over the past ten years (SSTs). A recent study led by scientists at the National Institute for Environmental Studies (NIES) says that the rise in the number of extreme ocean warming events since the 2000s is due to global warming caused by industrialization.
According to the Japan Meteorological Agency, SSTs in August 2020 in thesouthern region of Japan and the northwest Pacific Ocean reached previously unheard-of heights (JMA). The record-high northwestern Pacific SST observed in August 2020 could not be anticipated to occur without human-induced climate change, according to a recent study that was published in January 2021. Since then, the JMA has once more reported that record-breaking SSTs were seen close to Japan in July and October 2021, as well as from June to August 2022. But no one knows yet how much climate change has changed the chances that these extreme warming events will happen in certain regions.
According to a co-author and head of the Earth System Risk Assessment Section at the Earth System Division, NIES, Hideo Shiogama, “Impacts of global warming are not uniform; rather, they show regional and seasonal differences.” “A thorough examination of regional SSTs over an extended period of time could offer a quantitative understanding of how much global warming has already and will continue to affect the ocean conditions close to Japan. Policymakers can better plan mitigation and adaptation measures for climate change as a result of this information.
The study, which was just published in Geophysical Research Letters, examines the role that global warming plays in the discrete monthly extreme ocean warming events that occur in Japan’s marginal seas. These events may have happened less frequently than once every 20 years in the preindustrial era. The Japan Sea, East China Sea, Okinawa Islands, Taiwan’s east coast, and the Pacific coast of Japan were among the ten monitoring areas that the JMA uses in its daily operations. With the exception of the area east of Hokkaido, the scientists confirmed that the observed SST changes between 1982 and 2021 were accurately simulated by 24 climate models taking part in the Coupled Model Intercomparison Project (CMIP6sixth ) phase. The impact of climate change there was then revealed by identifying extreme ocean warming events in nine monitoring areas.
Extreme ocean warming and climate change
According to Michiya Hayashi, a research associate at NIES and the study’s corresponding lead author, “In the current climate, every extreme ocean warming event is related to global warming.” Based on the CMIP6 climate models, the researchers calculated the frequency of each event between January 1982 and July 2022 under both the current and preindustrial climates. “We discovered that since the 2000s, compared to the preindustrial era, the probability of almost all extreme ocean warming events has already doubled. Since the middle of the 2010s, it has increased significantly in some cases—particularly in southern Japan—more than tenfold. ”
For instance, the extreme ocean warming events in July 2022 are identified as the anomalously high SSTs observed in five monitoring areas, including the Japan Sea (Areas 1, 3, and 5), the East China Sea (Areas 5, 8, and south of Okinawa near Taiwan (Area 10). The updated results based on the preliminary data downloaded from the NEAR-GOOS RRTDB website on September 15, 2022 (not included in the paper published) reveal that the events are also identified in six monitoring areas south of 35°N for August 2022: the East China Sea (Areas 5, 8, and the seas off Shikoku and Tokai (Area 6). Hayashi said, “We think that climate change will make all of these events more likely to happen in July and August 2022 by at least a factor of two, and by at least a factor of ten for those south of 35°N, except for those in the north of the East China Sea.”
In contrast to southern Japan, Shiogama observed that “climate change impacts on extreme ocean warming events in northern Japan began to emerge relatively late.” The Earth’s surface tends to cool due to the increased global aerosol emissions up until the 1980s; this cooling is more pronounced in the North Pacific, particularly close to northern Japan. Additionally, the SST naturally varies greatly from year to year in northern Japan, making it harder to detect the effects of global warming there than in southern Japan. The cooling effect is less dominant than the warming caused by human-induced greenhouse gas emissions because global aerosol emissions have decreased over the past few decades. Shiogama continued, “Our study suggests that, even in northern Japan under the current climate conditions, the contribution of climate change to SST extremes has already been discernible beyond natural variability.”
What about the predicted changes in ocean conditions? Using the outputs from 24 CMIP6 climate models from 1901 to 2100, the researchers further compared the likelihood of exceeding the monthly record high SSTs around Japan at various levels of global warming from 0°C to 2°C. Tomoo Ogura, a co-author and the head of the Climate Modeling and Analysis Section at the Earth System Division, NIES, stated, “Once global warming reaches 2 °C, all nine monitoring areas are expected to experience SSTs warmer than the past highest levels at least every two years.” He continued, “Limiting global warming below 1.5°C is necessary to prevent the new normal climate in Japan’s marginal seas becoming record warm conditions.”
According to a quantitative analysis of SSTs near Japan, climate change is already the main cause of the majority of the recent record-high SSTs. According to Hayashi, future analysis of the dynamics of each extreme warming event must take into account both long-term climate change and annual natural variability. However, we anticipate that our statistical findings based on the most recent climate models will contribute to the implementation of climate change adaptation and mitigation measures.
