Ryuki Iritani, Kohei Oshima, Takahiro Oda, Orie Sasaki, Kazuo Oki, Hitoshi Miyamoto, Yukiko Hirabayashi
Received 2 May, 2025
Accepted 29 August, 2025
Published online 2 December, 2025

Ryuki Iritani¹⁾, Kohei Oshima¹⁾, Takahiro Oda²⁾, Orie Sasaki³⁾, Kazuo Oki⁴⁾, Hitoshi Miyamoto¹⁾, Yukiko Hirabayashi¹⁾

1) Department of Civil Engineering, Shibaura Institute of Technology, Japan
2) aiESG, Inc., Japan
3) Department of Civil and Environmental Engineering, Institute of Science Tokyo, Japan
4) Faculty of Engineering, Kyoto University of Advanced Science, Japan

Increases in water temperature due to climate change have been reported in many parts of the world, raising concerns about potential impacts on ecosystems, human water use, and water quality. In Japan, it is difficult to obtain trends in water temperature changes over a wide area because only a limited number of locations have long-term water temperature observations. Here, we analyzed historical changes in water temperature in Japan using both in situ observation data and satellite-derived surface water temperature (SWT). The annual mean water temperature change in Japan from 1982 to 2022, based on in situ observations, was 0.27 ± 0.19°C decade^–1. This is slightly lower than the estimate for the shorter period from 1982 to 2016 (0.30 ± 0.22°C decade^–1), but the number of sites showing a significant increasing trend was higher (85 sites) compared to the shorter period (66 sites). Similarly, the number of basins with a significant increase in SWT rose from 447 to 520 out of 675 with the inclusion of recent years. The nationwide average satellite-derived annual SWT trend from 1985 to 2022 was 0.47 ± 0.25°C decade^–1, with a particularly strong increasing trend observed in central Japan.

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