When the Ocean Changes - Global Warming and the Future of Ecosystems in the Seas Around Japan (May 2007 - July 2023)

When the Ocean Changes - Global Warming and the Future of Ecosystems in the Seas Around Japan (May 2007 - July 2023)

Sea surface temperatures in the ocean around Japan have risen markedly over the past approximately 100 years, with a rate of increase of +1.28°C per 100 years, according to data from the Japan Meteorological Agency. This is larger than the global average (+0.61°C/100 years) and comparable to the rate of temperature increase in Japan (+1.35°C/100 years). In particular, abnormal high temperatures have been observed in the Pacific Ocean off the coast of Tohoku and Hokkaido, with sea surface temperatures more than 5°C above normal in July 2023. In addition, in Tateyama, Chiba Prefecture, sea surface temperatures are estimated to rise by about +5°C on a 100-year basis based on data from 2003 to 2022. In the Nansei Islands, coral bleaching due to high water temperatures is becoming more serious, affecting the fishing industry.

This change is having a significant impact on fisheries and ecosystems. Along the coast of the Kinan region in Wakayama Prefecture, the catch of chub mackerel has decreased and the catch of sesame mackerel has increased sharply. The water temperature at a depth of 100 m was in the 15°C range until 1993, but rose to the 16°C range in 1995, and reached the 17°C range after 2002. As a result, the catch of sesame mackerel surpassed that of chub mackerel from 2003, and in 2005 it accounted for 80% of the total mackerel catch.

In Aomori Prefecture, the growing range of makombu has shrunk. According to a survey conducted by the Aomori Prefectural Fisheries Research Center's Aquaculture Research Institute, rising sea water temperatures during the winter (January to March) have had an impact. In the Tsugaru Straits, the growth of makombu has deteriorated, and the phenomenon of "isoyaki," a lack of seaweed growth on the seafloor, has been observed. Off the coast of Oma, warm-water seaweeds are replacing cold-water seaweeds such as makombu and gagome kelp. Kelp, which has high economic value, has decreased, seriously affecting the fishing industry.

In the Seto Inland Sea, rising sea water temperatures have also been observed, which has had a negative impact on nori-nori cultivation. According to fisheries officials in Okayama Prefecture, the rise in sea water temperatures during the winter has made it difficult to grow nori-growing. In addition, wintertime red tides, which used to rarely occur, have begun to occur, raising concerns among fishery officials.

Rising sea temperatures are also associated with increased frequency and intensity of typhoons and rising sea levels, affecting coastal ecosystems and people's livelihoods. In the future, projections based on climate change scenarios predict a further rise in sea surface temperatures near Japan by the end of the 21st century, with a rise of about 3.6°C under the high emissions scenario (RCP8.5). This calls for measures to reduce greenhouse gas emissions and monitor the marine environment.

Related Information
- Japan's sea surface temperature rise rate (per 100 years): +1.28°C
- Global average sea surface temperature rise: +0.61°C/100 years
- Estimated sea surface temperature rise in Tateyama, Chiba Prefecture: +5°C/100 years
- Anomalously high temperatures off the coast of Tohoku and Hokkaido (July 2023): +5°C above normal
- Future projection (RCP8.5): +3.6°C by the end of the 21st century
- Impacts: Coral bleaching, adverse effects on fisheries, increased typhoon intensity, sea level rise

Source: Japan Meteorological Agency
- Japan Meteorological Agency
- WWF Japan
- JAXA
- Niterra Eye of the Future
- Econews