In a significant response to the escalating temperatures gripping the nation, the Japan Meteorological Agency (JMA) has unveiled a new term for days when the mercury exceeds 40 degrees Celsius. This move reflects the alarming frequency of extreme weather events that have marked recent summers in Japan, signalling a worrying trend in the country’s climate patterns.
A New Term for a New Reality
The newly coined term, “Kokushobi,” translates to “severely hot day” and expands upon an existing heat classification system. This system already includes “natsubi” for summer days exceeding 25C, “manatsubi” for 30C, and “moshobi” for 35C. The introduction of Kokushobi comes after an extensive online survey conducted between February and March, where the public was invited to select from 13 proposed candidates, bolstered by input from climate experts.
The JMA’s decision underscores the dramatic climatic shifts Japan has experienced in recent years. Alarmingly, of the 108 recorded days where temperatures surpassed 40C since records began in 1872, a staggering 41 of these occurred within just the last three years. This means that over a third of all such extreme heat days in the past 150 years are attributed to a mere fraction of that time.
Record-Breaking Temperatures
The summer of 2025 set a new benchmark as Japan’s hottest on record, with average temperatures soaring 2.36C above the 30-year average. Data from 153 meteorological stations revealed that 132 of these locations logged unprecedented highs. The nation’s highest temperature ever recorded, a blistering 41.8C, was documented in Isesaki, Gunma Prefecture, in August 2025. On the same day, 30 locations across 13 prefectures also reported temperatures exceeding 40C.
Japan’s warming trend is outstripping the global average, with the country experiencing a temperature increase of 1.13C over the past century, compared to the global rise of approximately 0.73C. Scientists attribute this rapid warming to the climate crisis, particularly the rising sea temperatures surrounding the Japanese archipelago, which creates a heat-trapping effect that extends well into the autumn months. Moreover, warmer oceans contribute to heavier rainfall and more intense typhoons, compounding the myriad climate challenges facing the nation.
Health Risks Associated with Extreme Heat
The implications of extreme heat on public health are grave. There is a notable surge in hospitalisations for heatstroke when temperatures exceed 35C, and the elderly population is particularly vulnerable. During the record-breaking heatwave in July 2018, when temperatures hit 41.1C in Kumagaya, Saitama, tragically, 96 residents of Tokyo succumbed to suspected heatstroke within just one month.
Experts predict that 2026 is likely to bring another record-breaking summer, with the trend of rising temperatures expected to persist. If unchecked, the ongoing climate crisis could fundamentally alter Japan’s seasonal landscape, potentially reducing the country to a climate characterised by only two seasons. This pattern mirrors broader trends observed across Asia, where spring and autumn are increasingly being overshadowed by prolonged summer heat.
Why it Matters
The introduction of Kokushobi is a stark reminder of the urgent climate crisis that is reshaping not just the environment but the very fabric of daily life in Japan. As extreme heat becomes the new norm, the health risks and socio-economic impacts will likely intensify, demanding immediate action from both policymakers and citizens alike. Failure to address these changes could irreversibly alter Japan’s climate and cultural identity, calling into question the future of its cherished four seasons.