Global Warming’s Dual Extremes: More Wildfires and Heavier Rain

This year, a series of large-scale forest fires have already caused tremendous damage across Japan. Due to increased global warming, the air is becoming drier, raising the likelihood of fires. At the same time, heavy rains are also predicted to increase. These phenomena may appear contradictory, but both are linked to fluctuations in atmospheric water vapor caused by rising temperatures. It this article, I will elucidate how it is possible to simultaneously have more rain and more fires.

In Japan, the number of forest fires has been decreasing when viewed over a medium- to long-term span (Figure 1). The main causes of forest fires are human activities such as campfires, the controlled burning of forests, and arson . During the Showa era (1926–1989), forestry was more prevalent as an industry in Japan. Mountain areas had more foot traffic, which likely contributed to a higher incidence of forest fires than we see today.
Recently, around 1,000 forest fires occur each year.

Figure 4 shows the difference between the relative humidity (an indicator of the degree of atmospheric dryness) at an altitude of approximately 750m (925hPa) in February 2025 and the average value for the reference period (1991–2020). The red area indicates that relative humidity was lower than normal, meaning the air was drier. Arrows indicate in which direction the wind was blowing stronger than average.
February 2025 saw stronger westerly winds than usual, so the air colliding with the Ou Mountains brought snow and rain to the Sea of Japan side, while dry air flowed to the Pacific side. This effect led to the air being particularly dry in the Sanriku coastal area around Ofunato City. The combined effects depicted in both figures below created weather conditions conducive to forest fires.

As global warming progresses, atmospheric temperatures rise, causing, the total amount of water vapor that the air can retain (saturated water vapor amount) to increase^*4. At first, you might think that if the amount of water vapor in the air increases the air shouldn’t dry out as easily. However, in reality, water vapor does not increase uniformly. Its accumulation varies depending on the region and season, leading to uneven increases. As a consequence, as global warming progresses, the gap between places that become drier and places that experience more heavy rainfall will widen.
The key indicator here is relative humidity: the ratio of the actual amount of water vapor contained in the air to the maximum amount of water vapor that air can hold at that temperature. When the actual amount of water vapor is less than the total amount of water vapor that can be retained, the air is considered to be dry.

Therefore, in places where the temperature rises but the supply of water vapor cannot keep up (where moist air cannot flow in easily), the maximum amount of water vapor that can be retained increases, but the actual amount of water vapor does not, so relative humidity decreases (Figure 5-1). This drier air intensifies the risk of forest fires, making it easier for even small fires to spread and cause damage.
On the other hand, in places where water vapor easily accumulates (where moist air tends to flow in), when the temperature rises, not only does the amount of water vapor that can be retained increase, but the actual water vapor volume contained in the air also tends to increase (Figure 5-2). When the amount of water vapor increases to the limit at which air can retain it (relative humidity of 100%), the water vapor eventually falls as rain. The greater the actual water vapor volume, the greater the amount of rainfall is likely to be. In this way, the increase in water vapor in the air accompanying rising temperatures is thought to be one of the factors contributing to the occurrence of extreme heavy rainfall, such as in linear rainbands.
This phenomenon is mentioned in the report "Climate Change in Japan 2025,"^*5 which summarizes the results of global warming observations and future predictions compiled by the Ministry of Education, Culture, Sports, Science and Technology and the JMA. The same report predicts that as global warming progresses, the frequency and intensity of heavy rainfall will increase, while the number of days with less than 1.0mm of precipitation, known as "no precipitation days," will also increase.
The annual average temperature in Japan has risen by about 1.40°C in the last 100 years. We believe the state of water vapor in the air is already being affected. Since there is no observational data for Ofunato prior to 1962, consider data from Miyako City, which is nearby. Comparing a 30-year period 100 years ago (1895–1924) with the most recent 30-year period (1995–2024), the average annual temperature in Miyako has risen by approximately 0.96°C^*6. Figure 6 outlines the changes in relative humidity during this period.

Global warming causes two seemingly contradictory phenomena: low humidity and heavy rainfall. Both cases are mainly caused by an increase in the amount of water vapor that the atmosphere can retain (saturated water vapor) as temperatures rise. In other words, climate polarization is expected to continue, with some areas becoming drier and more prone to wildfires, while others experience more severe torrential rains.
Global warming is not just about rising temperatures. Understanding the bigger picture of how extreme weather events can become even more extreme is the first step in preparing for disasters.