In a striking response to the escalating frequency of extreme temperatures, the Japan Meteorological Agency (JMA) has introduced a new term to describe days when thermometers soar above 40°C. The designation, “kokushobi,” meaning “severely hot day,” is part of an expanded heat classification system that underscores the significant changes in Japan’s climate over recent years. This decision highlights the urgency with which the nation is confronting the realities of climate change.
A Shift in Temperature Terminology
Traditionally, the JMA utilised terms such as “natsubi” for summer days exceeding 25°C, “manatsubi” for those above 30°C, and “moshobi” for extremely hot days surpassing 35°C. The introduction of kokushobi reflects a growing recognition of the severity of extreme heat events, particularly as they become more common. This change was influenced by an online survey conducted in February and March, which presented 13 potential terms to the public, alongside expert evaluations.
Record-Breaking Heatwaves
The urgency of this new classification system is underscored by alarming statistics. Since the beginning of reliable temperature records in 1872, Japan has documented 108 days where temperatures exceeded 40°C. Remarkably, 41 of these days occurred within just the last three years, illustrating that over one-third of such extreme heat events have transpired in a mere snapshot of time. The summer of 2025 was particularly dire, with average temperatures soaring 2.36°C above the 30-year norm. An astonishing 132 of the 153 meteorological stations across Japan reported unprecedented temperature highs that summer, culminating in a staggering 41.8°C recorded in Isesaki, Gunma prefecture.
The Climate Crisis and Its Consequences
Japan’s temperature rise is accelerating at a rate that outstrips the global average. While global temperatures have increased by approximately 0.73°C over the past century, Japan has experienced a rise of 1.13°C. Climate scientists attribute much of this increase to the ongoing climate crisis, particularly the warming of ocean waters surrounding the archipelago. This phenomenon not only traps heat but also contributes to intensified rainfall and more potent typhoons, exacerbating the nation’s climate challenges.
The repercussions of such extreme heat are dire. Hospital admissions for heatstroke spike dramatically when temperatures exceed 35°C, and vulnerable populations, particularly the elderly, face an increased risk of mortality during heatwaves. The catastrophic July 2018 heatwave, which reached a then-record temperature of 41.1°C in Kumagaya, Saitama, resulted in 96 suspected heatstroke fatalities in Tokyo alone within a single month.
Looking Ahead: More Heat on the Horizon
Experts are already predicting another record-breaking summer for 2026, suggesting that the trend of rising temperatures is set to continue. If left unchecked, the ongoing climate crisis may lead Japan to lose its beloved four distinct seasons, transforming it into a nation characterised by two prolonged seasons of extreme heat. This troubling shift echoes patterns observed across Asia, where the warming climate is squeezing out spring and autumn and extending summer heatwaves.
Why it Matters
The introduction of kokushobi is more than just a linguistic adaptation; it is a stark acknowledgment of the escalating climate crisis that Japan—and indeed, the world—faces. As temperatures climb and the consequences of extreme heat become increasingly severe, it is imperative that citizens, policymakers, and scientists unite in their efforts to combat climate change. The time for action is now, as the health and wellbeing of future generations depend on our response to this urgent challenge.
