Global Warming and Rising Humidity: A Scientific Examination of the Impact on Japan

1. Introduction

Global warming is not merely a phenomenon of rising temperatures. In Japan, many people feel its impact specifically through an increase in "sultriness." This paper explains, in a way accessible at the high school earth science level, how global warming affects humidity (the amount of water vapor in the atmosphere) and what changes this is bringing to Japan's climate.

2. Increase in Water Vapor Due to Global Warming

As temperatures rise due to global warming, the amount of water vapor the atmosphere can hold increases. This is a phenomenon based on the Clausius-Clapeyron relation and is explained by

it. From this relation, it can be seen that for every 1°C rise in temperature, the saturation water vapor pressure increases by approximately 7%. In other words, considering that Japan's average temperature has risen by about 1.2°C over the past 100 years, it can be inferred that the amount of water vapor in the atmosphere has increased at the same rate.

3. Measured Data on Rising Humidity in Japan

According to data from the Japan Meteorological Agency, Japan's annual average relative humidity has risen by approximately 2–3% over the past 50 years. In particular, the increase in humidity during summer is significant, and combined with the heat island effect, it is intensifying the "sultriness."

As specific data,

• the average relative humidity (in July) in Tokyo was approximately 75% in the 1970s, but it rose to approximately 78% in the 2020s.

• As a result, the Discomfort Index (DI) has also risen, and the perceived temperature on extremely hot days (maximum temperature of 35°C or higher) has come to feel several degrees higher than the actual temperature.

4. Impacts Caused by Rising Humidity

The rise in humidity in Japan is causing the following impacts:

1. Increased risk of heatstroke

   • High humidity hinders the evaporation of sweat, making it difficult to regulate body temperature.

   • Even at the same temperature, the risk of heatstroke increases as humidity rises.

2. Increase in localized torrential rain

   • High humidity promotes the development of cumulonimbus clouds, leading to more guerrilla rainstorms.

   • According to Japan Meteorological Agency statistics, the frequency of torrential rain of 50mm or more per hour has increased by approximately 1.4 times over the past 40 years.

3. Impact on agricultural crops and buildings

   • Increased humidity promotes the breeding of pests and diseases (mold and insects).

   • Deterioration due to humidity is also progressing in Japan's traditional wooden architecture.

5. Summary and Future Outlook

As described above, the impact of global warming is not limited to a simple rise in temperature, but is manifesting significantly in Japan as an increase in humidity. This rise in humidity has a wide-ranging impact on human health, meteorological disasters, and even cultural assets, making 'humidity management' as well as 'temperature control' an important theme for future global warming countermeasures.

Future research will require scientific and technical approaches to minimize the effects of global warming by analyzing more detailed humidity change patterns and implementing appropriate countermeasures.