Japan’s summer is becoming hotter than ever due to global warming, but heatstroke is not the only concern from the rising temperatures. It is also triggering more rainfall — or sudden torrential rain that is hard to predict — causing significant damage in a short period of time.
The torrential rain earlier this month in Chiba Prefecture was one such example.
The city of Chiba’s Chuo Ward received 367 millimeters of rain in the 12 hours through the morning of Aug. 14, three times the city’s average monthly rainfall for August.
Researchers in Japan say rising temperatures and increasing rainfall are intertwined. A paper published in 2025 by researchers from Kyoto University and private weather forecaster Japan Meteorological Corp. (JMC) revealed that temperatures rising by 1 degree Celsius will result in a corresponding 7% increase in precipitation.
“If temperature increases, it will fuel the atmosphere to produce more rainfall because temperature increase means more moisture will be in the atmosphere,” said Sridhara Nayak, the chief scientist at JMC who co-authored the report. “That’s why we’re getting more rainfall.”
The average temperature in Japan has risen 1.44 C in the past century, with the increase accelerating since the 1990s. Japanese cities have had a record 34 instances in which they registered temperatures of 40 C or higher this summer as of Tuesday, already topping 2025’s 30 times.
Nayak noted that while many countries have seen more rain due to global warming, Japan is particularly susceptible to greater precipitation.
“It is surrounded by the ocean, so it’s obvious more moisture from the ocean will come to the land, and it will build up and it will bring more rainfall,” said Nayak. “Also, there are many typhoons (which) carry a lot of rainfall to Japan.”
What is making the rainfall more damaging is when a huge amount of rain falls suddenly in a short amount of time, giving people little time to take refuge.
When Chiba was hit by heavy rain earlier this month, the city of Chiba experienced 107.5 mm of rain in an hour from 7 p.m. on Aug. 13. That’s equivalent to about 100 kilograms of rain in 1 square meter, which the weather agency describes as raining so hard people would feel frightened.
The huge amount of rain caused flash floods on many streets. Drivers were stuck in flooded underpasses, and, in some cases, drowned because they could not get out of their vehicles due to the water pressure.
Such localized torrential rain has increased in the past few decades.
A Meteorological Research Institute report released in 2022 showed that annual cases of torrential rain — which it defines as rainfall that exceeds 130 mm in a three-hour period — increased about 2.2 times in the 45 years from 1976 to 2020.
With extreme precipitation events inevitable in the years to come, effective drainage systems — such as the underwater tunnel system in Saitama Prefecture that can store up to 670,000 cubic meters of water — are key to limiting their impact.
JMC’s Nayak said that he thinks Japan is ahead of other countries, such as India and the U.S., in building such systems, but more is needed.
“We just need to adapt,” Nayak said. “We need to pass these kinds of (drainage infrastructure) laws so (extreme precipitation events) will not cause much damage.”