Unbearable Summers: How Extreme Downpours and Record Heatwaves Are Redefining Japan’s Climate

Japan’s summers are getting hotter and longer. This year, the heat wave hit as early as June, with high-pressure systems trapping warmth over the country. Experts warn that climate change is making Japan more vulnerable to extreme heat and intense rains.

Rising Ocean Temperatures

July 2025 was a record-breaking month. Average temperatures soared to 2.89°C above normal, making it the hottest July on record. On July 30, Tanba in Hyōgo reached a scorching 41.2°C, followed just days later by Isesaki in Gunma, which hit 41.8°C.

These blistering temperatures are linked to two factors: the North Pacific High, a system that brings sunny and dry weather, and the Foehn effect, which warms air as it descends mountains. Adding to this, ocean temperatures for July exceeded 25°C, a full degree warmer than in recent years, creating a heat dome over the islands.

Recently, researchers from Mie University and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) recorded a sharp rise in sea temperatures off the Tōhoku coast. In just a week, ocean temperatures climbed 3°C to 4°C, pushing far above typical levels. This phenomenon contributes to unusual weather patterns, including a dense fog that has plagued local farmers, but this year was worsened by warmer air temperatures.

Understanding the Weather Patterns

The unusual heat was also due to a combination of factors. The Japan Meteorological Agency reported that June temperatures were 2.34°C above normal, the highest on record. Different high-pressure zones funneled warm air into the region, disrupting typical weather patterns and ending the rainy season early.

A study from May had initially predicted a mild summer, but rapidly rising ocean temperatures forced experts to revise their forecasts. They now expect these extreme conditions to continue into the fall, raising concerns about extended heat and storms.

Increased Risk of Heavy Rainstorms

As oceans warm, they release more moisture into the atmosphere. This leads to a higher risk of torrential rains when low-pressure systems sweep across the ocean. In Japan, various types of rainstorms—ranging from brief afternoon showers to prolonged downpours—can cause severe flooding.

Recent events highlight these dangers. For example, in September 2024, heavy rain exacerbated problems for earthquake victims on the Noto Peninsula. Similarly, in July 2018, a series of heavy rainstorms triggered devastating floods in western Japan.

Worldwide, rising ocean temperatures are fueling extreme weather events. In Spain, record rainfall in 2024 resulted in deadly floods, showcasing that Japan is not alone in facing these challenges.

Changing Paths of Typhoons

Climate change is also altering the paths of typhoons. My book discusses how storms are deviating from their usual courses, making their movements erratic. This unpredictability makes forecasting more complicated and increases the chance of storms lingering over areas, as seen with Typhoon Shanshan in 2024, which spent a week making loops around Japan, causing extensive disruptions.

As the Kuroshio Current warms, it can intensify storms, making them linger longer and potentially grow stronger as they travel across the North Pacific.

In conclusion, climate change impacts more than just temperatures. Its effects ripple through various weather patterns, increasing the risks of heat waves, heavy rains, and unpredictable storms. To mitigate these risks, it’s essential to understand and combat climate change now.

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