Hotter does mean wetter

Kyoto, Japan -- Around the world, we are already witnessing the detrimental effects of climate change, which we know will only become more severe. Extreme weather events such as heavy rainfall, tropical cyclones, and heat waves are projected to intensify, and this will negatively impact both human society and natural ecosystems.

Assessing how climate change affects extreme weather is important not only from a scientific point of view, but also from a practical perspective. It is critical that we start adapting to climate change and mitigating the effects of potential disasters.

This situation has motivated a team of researchers at Kyoto University to investigate how climate change -- in particular, rising temperatures -- affects precipitation in Japan. The team has focused on heavy rainfall patterns and what kind of atmospheric conditions influence their characteristics.

"We know that the saturation of water vapor increases with temperature at a rate of about 7% per degree Celsius, but not all that water vapor is converted to rainfall, so we weren't sure this temperature scaling applies to the rainfall amount," says first author Sridhara Nayak.

Using climate change data generated by climate models from the Japan Meteorological Agency's Meteorological Research Institute, the team examined precipitation data for Japan, dividing the Japanese islands into seven regions to account for regional variations. They then analyzed the precipitation intensity and atmospheric conditions over Japan both in the present climate and in the projected future climate with a global average temperature increase of 4 degrees Celsius.

The findings revealed that a 4 degree global temperature increase will not only intensify extreme precipitation, but that it will indeed become stronger at a rate of 7% per degree of warming.

However, the atmospheric humidity, or amount of water vapor, plays a vital role in characterizing favorable conditions for an extreme precipitation occurrence. The heat to precipitation connection is applicable not on the hottest days, but at times when atmospheric moisture is sufficient. When the temperatures are at their hottest, the atmosphere becomes drier, and rainfall weakens. Therefore, extreme precipitation occurs not on the hottest days, but on days within the second highest temperature range of the year.

"With this in mind, we need to be prepared for the severe impacts of heavy rainfall in the future by creating a plan for climate change adaptation," says team leader Tetsuya Takemi.

Of course, there is more investigating to be done. This study utilized climate prediction data with a spatial resolution of 20 km, which is not sufficient to capture clouds that produce extreme precipitation. The researchers say that future studies should investigate the mechanisms for the occurrence of extreme precipitation using higher-resolution datasets.

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The paper "Regional and vertical scaling of water vapor with temperature over Japan during extreme precipitation in a changing climate" appeared on 15 October 2025 in Scientific Reports, with doi: 10.1038/s41598-025-22287-6

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en