The impact of the climate change Japan is experiencing in the rainy season

The rainy season has arrived in Japan. Meteorological Agency announced that it started in the Kanto Koshin region on June 6, and it will soon reach other areas.

The rainy season can be seen all over Japan except Hokkaido and the Ogasawara Islands and is usually from mid-June to mid-July in Tokyo. After the rainy season, it will be the true summer. The rain falling during the rainy season is indispensable for agriculture, especially for rice cultivation, and also plays a role in storing water in dams.

However, the climate in the rainy season becomes more extreme, and short-duration heavy rains have been increasing in recent years. In the summer of 2021, about half of the annual rainfall fell in one week at many locations. There is no doubt that climate change is behind this increase in extreme weather events.

Due to its topography with a short distance from river mouths relative to its elevation, rainfall flows down from the mountains to the sea at once in Japan. This type of land causes frequent floods and landslides due to heavy rainfall.

Increased short-duration heavy rainfall about 1.4 times in the last ten years

In fact, in the 2010s, torrential rains exceeding 100 mm per hour repeatedly occurred during the rainy season. This increase in short-duration heavy rainfall is believed to be caused by global warming, which accelerates the evaporation rate of water and activates the rainy season front. A comparison of short duration heavy rainfall shows that the number of occurrences has increased about 1.4 times in the last ten years, confirming a long-term increase.

The impact of Arctic warming

In the monsoon regions of Asia, including Japan, rainy season precipitation is on the increase compared to other regions. As one of the reasons for this, Hokkaido University has found that Arctic warming is affecting the increase in rainy season precipitation.

As a result of research focusing on the blocking high-pressure that caused the heat wave in Siberia in the summer of 2020, it became clear that the greater the degree of development of the blocking high-pressure, the stronger the precipitation in the rainy season. Furthermore, climate projection simulations based on the assumption that Arctic warming will progress in the future show that the frequency of blocking highs in Siberia will increase, and summer precipitation in East Asia will increase accordingly.

Advance season, prolonged rainy season

In 2021, the onset of the rainy season in many parts of Japan moved forward a few days. This is believed to be due to the Pacific High, which has gained strength due to global warming, pushing the rainy season front to the north.

In addition, rising sea surface temperatures due to global warming tend to delay the northward movement of the prevailing westerly winds and prolong the rainy season. In addition, the rise in sea surface temperatures increases the amount of water vapor in the atmosphere that flows into the rainy season front, which in turn increases the amount of precipitation, another factor that prolongs the end of the rainy season.

Now is the time to raise your voice

IPCC warns that temperature rise will reach at least 1.5°C within the next 20 years, but Japan’s climate change measures are far too inadequate.

To change inadequate policies, various large-scale measures are necessary, including raising the Japanese public’s awareness of the urgency of global warming and increasing media coverage of climate change. Otherwise, the rainy season will be a season to bring a disaster rather than a rain of grace. Now is the time to start the climate action when we are experiencing the effects of climate change.