This summer was 2.36 degrees above average, updating the record for hottest summer — Temperature baseline up and natural disaster preparedness required

The Japan Meteorological Agency (JMA) announced on the 1st that the average temperature in Japan this summer (June-August) exceeded the norm by 2.36 degrees, becoming the hottest summer since statistics began in 1898. The previous record was 1.76 degrees above normal in 2023 and 2024, but this figure increased by 0.6 degrees. This marks the third consecutive year of record-breaking summer heat, with rising temperatures showing no signs of abating.

The JMA predicts that high temperatures may continue until November this year and continues to call for heat stroke prevention measures. It also stated that global warming has "raised the temperature baseline" and that extremely hot summers are likely to increase in the long term from next year onwards. September 1 is "Disaster Prevention Day," which was established to prepare for major earthquakes, tsunamis, and typhoons in remembrance of the Great Kanto Earthquake. Preparation for natural disasters such as record-breaking heat waves and heavy rains, which are expected to intensify due to the effects of global warming, has now become increasingly important.

Locations across Japan with 40℃+ temperatures reached a record 30 sites

According to the JMA, many locations nationwide set new maximum temperature records, including Isesaki City in Gunma Prefecture, which recorded 41.8℃ in August, increasing the national record. The total number of locations reaching 40℃ or higher was 30, the highest ever recorded. This summer's average temperature deviation from the norm was +3.4℃ in northern Japan, +2.3℃ in eastern Japan, and +1.7℃ in western Japan.

The cumulative number of observation points nationwide recording extremely hot days (maximum temperature of 35℃ or higher) at "AMeDAS (Automated Meteorological Data Acquisition System)" stations reached 9,385 points, exceeding 2024's record of 8,821 points, which had been the highest since 2010 when comparable data became available. Additionally, 132 out of 153 meteorological stations nationwide recorded their highest summer average temperatures on record. Hita City in Oita Prefecture had the most extremely hot days (55), followed by Kofu City in Yamanashi Prefecture and Kyotanabe City in Kyoto Prefecture, both with 53 days.

Furthermore, this summer's sunshine duration was considerably longer on both the Japan Sea and Pacific sides of northern, eastern, and western Japan, which were frequently covered by the Pacific high-pressure system. As a percentage of the normal amount, summer sunshine duration reached 140% on the Japan Sea side of eastern Japan and 137% on the Pacific side, suggesting the most sunshine in summer since statistics began in 1946. In contrast, summer precipitation was low on the Pacific side of northern and eastern Japan and on both the Japan Sea and Pacific sides of western Japan, due to reduced influence from weather fronts and low-pressure systems.

"Double high-pressure system" major factor in record heat

The JMA explains the causes of this unprecedented heat wave as follows.

Throughout this summer, westerly winds around Japan tended to flow further north than normal, covering the entire country with warm air. From June, the "Pacific high-pressure system" extended toward Japan. Although the rainy season typically brings much rain during normal years, this summer saw weak activity of the seasonal rain front, leading to continued clear days.

The seasonal progression from spring to high summer was considerably early, with the rainy season beginning in May and ending within June, except in the Tohoku region. Some areas experienced record-breaking early starts and ends to the rainy season. Simultaneously, the "Tibetan high-pressure system" also strengthened, pushed northward by westerly winds, and allowed warm air to flow in. This created a "double high-pressure system" state with two high-pressure systems overlapping in the skies around Japan.

Regarding the temperature and precipitation until late September, the JMA predicted that high temperatures would continue nationwide due to frequent coverage by warm air masses. Temperatures were expected to be particularly high during the first half of this period. Additionally, northern, eastern, and western Japan have been covered by high-pressure systems since late June, resulting in continued low precipitation. Precipitation was also expected to be frequently affected by high-pressure systems. This meant that the Pacific side of eastern Japan was expected to see normal or below-normal precipitation.

High temperatures may continue until November

The JMA released its weather outlook for September through to November on August 19. According to this forecast, warm air masses are likely to frequently cover the entire country until November, with high temperatures likely to continue. When it comes to atmospheric and oceanic characteristics in mid-latitude regions around the Japanese archipelago, overall atmospheric temperatures are high due to the effects of global warming and other factors. While sea surface temperatures are low in the central equatorial Pacific, they are high from the eastern Indian Ocean to waters east of the Philippines, with increased cumulonimbus cloud formation from the eastern Indian Ocean to waters east of the Philippines.

Due to these influences, upper-level westerly winds will continue to tend to flow further north than normal. The Pacific high-pressure system is expected to strengthen, centered southeast of Japan. Additionally, seasonal progression will continue to be slow, with the entire country likely to be frequently covered by warm air masses.

Unquestionable Influence of Global Warming

According to analysis by the JMA, Japan's average summer temperatures show a long-term rising trend despite fluctuations, increasing at a rate of 1.38 degrees per 100 years. Sea surface temperatures have risen due to the effects of global warming, and meteorological experts, including Professor Yoshihiro Tachibana of the Graduate School of Bioresources, Mie University, unanimously point out that "sea surface temperatures around Japan are remarkably high." They note that high tropical sea surface temperatures in areas such as the Indian Ocean have strengthened high-pressure systems affecting Japan's weather, while simultaneously, high sea surface temperatures near western Japan supplied abundant water vapor, leading to the heavy rains in the Kyushu region during August.

Tachibana has long pointed out that warming around the Arctic region causes meandering and positional changes in westerly winds, becoming a major factor in recent heavy rains, droughts, and record heat waves across the Northern Hemisphere, frequently emphasizing the importance of global warming countermeasures. Additionally, Professor Shoji Yokobori of Nippon Medical School, who specializes in emergency medicine, points out that "increasing heat stroke damage is now not just disaster-level but ultra-disaster-level" and calls for "measures to protect one's own life."

The United Nations Intergovernmental Panel on Climate Change (IPCC) published its "Sixth Assessment Report" in August 2021. This report pointed out that even if greenhouse gas emissions are kept low in the future, temperatures may exceed 1.5 degrees above pre-industrial levels by 2040. It further analyzed that a "1.5-degree rise" would increase "once-in-50-years heat waves" by 8.6 times compared with 1850-1900, while a "2-degree rise" would increase them by 13.9 times. It also stated that "once-in-a-decade heavy rainfall" would increase 1.5 times compared with 1850-1900 levels with a "1.5-degree rise."

In the Northern Hemisphere this summer, not only Japan but also European countries experienced record-breaking heat waves, and northwestern India suffered damage from record-breaking heavy rainfall. Extreme weather events caused by global warming, repeatedly pointed out and warned about by the IPCC, are becoming manifest. Meanwhile, countermeasures under the "Paris Agreement" international framework for global warming prevention are progressing slowly, and the sense of crisis continues to grow. If a certain degree of global warming progression cannot be avoided, there is an even stronger demand for implementing what the IPCC and the Ministry of the Environment term "adaptation measures"—policies to protect people's lives and national territories.