In a significant move reflecting the alarming rise in extreme temperatures, the Japan Meteorological Agency (JMA) has introduced a new term to its heat classification system: “kokushobi,” or “severely hot day.” This decision comes as the nation grapples with increasingly severe summer heat, marked by a worrying trend of rising temperatures that has resulted in an unprecedented number of days exceeding 40°C in recent years.
A New Lexicon for Extreme Heat
The introduction of kokushobi is part of a broader effort to communicate the escalating severity of weather phenomena in Japan. This new classification joins an existing framework that includes “natsubi” for summer days over 25°C, “manatsubi” for midsummer days exceeding 30°C, and “moshobi” for extremely hot days above 35°C. The term was selected following a public survey conducted earlier this year, which presented 13 potential candidates to the public alongside expert recommendations.
The urgency for this new terminology is underscored by alarming statistics: since records began in 1872, a staggering 41 out of the 108 days recorded above 40°C have occurred in just the past three years. This accounts for over one-third of all such extreme heat days documented in the last century and a half.
Record-Breaking Heatwaves
The summer of 2025 was particularly catastrophic, marking the hottest season on record for Japan. Average temperatures from June to August soared to 2.36°C above the 30-year historical average, with 132 of the country’s 153 meteorological stations recording new temperature highs. Notably, Isesaki in Gunma prefecture witnessed the highest temperature in Japanese history at 41.8°C, with numerous other locations also surpassing the 40°C mark that day.
This relentless rise in temperature is not isolated; Japan’s warming trajectory is outpacing the global average. While global temperatures have climbed by approximately 0.73°C over the past century, Japan’s temperature increase has reached 1.13°C. Scientists attribute this alarming trend primarily to the climate crisis, particularly the warming of surrounding ocean waters, which exacerbates heat retention into the autumn months.
Health Risks and Societal Impact
The implications of escalating temperatures extend far beyond discomfort. The health risks associated with extreme heat are severe, with heatstroke hospitalisations surging during heatwaves. The summer of 2018 serves as a stark reminder; during a period when temperatures reached 41.1°C in Kumagaya, Saitama, 96 individuals in Tokyo lost their lives to suspected heat-related illnesses in just one month.
As Japan braces for another record-breaking summer in 2026, experts warn that the pattern of extreme heat is likely to persist. If current trends continue unabated, Japan faces the alarming prospect of losing its beloved four seasons, transitioning instead to a climate characterised by only two: a prolonged summer and a shortened spring and autumn. This shift mirrors patterns observed across other parts of Asia, where climate change is reshaping traditional weather cycles.
The Future of Japan’s Climate
The increasing frequency and intensity of heatwaves necessitate a reevaluation of how societies respond to climate change. As the JMA’s new classifications highlight the urgent need for awareness and action, the implications of these changes will reverberate through every aspect of Japanese life, from public health to agriculture and urban planning.
Why it Matters
Japan’s introduction of the kokushobi classification serves as a critical reminder of the climate crisis’s immediate and tangible effects. With extreme weather becoming a stark reality, it is imperative for nations worldwide to take decisive action to mitigate climate change. Failure to do so not only threatens the environment but also endangers the health and well-being of countless individuals. As Japan adapts to its new climate reality, the world must heed these warnings and strive for sustainable solutions to avert further catastrophe.