The impact of climate change on the extreme ocean warming events observed in Japan’s marginal seas for 1982-2022

Anthropogenic global warming has potentially caused regional record-high sea surface temperatures (SSTs) observed around the world, but it is not yet clarified to what extent climate change has increased the frequency of discrete extreme ocean warming (EOW) around Japan. In this study, the impact of climate change on EOW events in Japan’s marginal seas is examined by focusing on each calendar month from January 1982 to December 2022 and on multiple areas including the Japan Sea, East China Sea, Okinawa, Taiwan, and South Korea. We analyzed 24 CMIP6 climate models that have the equilibrium climate sensitivity between 2 K and 5 K to estimate the probability of occurrences of monthly SSTs in the present and preindustrial conditions. The EOW event is defined as high SSTs less frequent than once per 20 years at the preindustrial level. The fraction of attributable risks (FAR) is used to quantify the impact of climate change on SSTs.

In contrast to the gradually increasing occurrences of EOW events for the past 41 years, the most frequent EOW events occurred around 1998 and in the mid-2010s, especially in the southern side of Japan. The impact of climate change on EOW events is not yet dominant around 1998 but has multiplied the occurrence probability of most EOW events by at least twice since 2000. Based on our method, all the EOW events identified in 2022 are attributable to climate change. The regional and seasonal differences of the climate change impact on EOW events around Japan are discussed. In addition, this study suggests that the possibility of the future typical climate exceeding record high SSTs can be sharply reduced by limiting global warming levels from 2°C to 1.5°C, indicating rapid acceleration of mitigation efforts is critical.