In a significant response to escalating temperatures, the Japan Meteorological Agency (JMA) has officially added a new term to its meteorological vocabulary, aimed at categorising days when the mercury exceeds 40°C. The introduction of the term “kokushobi,” or “severely hot day,” underscores the alarming frequency of extreme heat events in Japan, which have become increasingly common in recent years.
New Classification Reflects Climatic Changes
The term kokushobi is part of an expanded heat classification system that already includes existing terms such as natsubi (summer day) for temperatures over 25°C, manatsubi (midsummer day) for those exceeding 30°C, and moshobi (extremely hot day) for temperatures above 35°C. The selection of this new term followed an online survey conducted between February and March, where the JMA considered input from the public and consulted with experts.
This decision is particularly poignant given the dramatic changes observed in Japan’s climate. Since record-keeping began in 1872, there have been 108 days where temperatures exceeded 40°C, with 41 of those days occurring in just the last three years. This alarming statistic highlights that over one third of all such occurrences in the past century and a half have taken place recently.
Record-Breaking Heat Trends
The summer of 2025 marked a historic benchmark in Japan’s meteorological history, recording the highest average temperatures between June and August—2.36°C above the 30-year average. An overwhelming 132 out of 153 meteorological stations across the nation registered new temperature highs. Notably, Isesaki in Gunma prefecture reached an unprecedented 41.8°C in August 2025, with 30 locations across 13 prefectures also reporting temperatures above 40°C.
These figures are alarming not only for their immediate implications but also for what they signify about the long-term trajectory of Japan’s climate. Over the past century, global temperatures have risen by approximately 0.73°C, while Japan’s temperatures have surged by 1.13°C, significantly outpacing the global average. This discrepancy is largely attributed to the climate crisis, exacerbated by the warming of surrounding waters that create a sustained heat well, impacting the climate well into the autumn months.
Health Impacts of Extreme Heat
The health repercussions of such extreme temperatures are profound. Incidents of heatstroke frequently escalate during heatwaves, particularly on days with temperatures exceeding 35°C. The elderly are particularly vulnerable; for instance, during the heatwave of July 2018, when temperatures reached a previous record of 41.1°C in Kumagaya, Saitama, 96 residents of Tokyo succumbed to suspected heatstroke within just one month.
As forecasts for 2026 predict yet another record-breaking summer, experts warn that the trend of rising temperatures is likely to persist. The continuous warming raises serious concerns about the future of Japan’s climate patterns. If current trajectories remain unchanged, the nation risks losing its distinctive four seasons, with spring and autumn increasingly being overshadowed by a prolonged summer heat season—a phenomenon already observed in various parts of Asia.
Why it Matters
The introduction of kokushobi is more than a mere addition to Japan’s weather lexicon; it serves as a stark reminder of an urgent and escalating climate crisis. The shift towards more extreme weather patterns poses not only a direct threat to public health but also to the very fabric of Japan’s seasonal identity. As climate change continues to reshape weather patterns, it is imperative that both the government and society at large take proactive measures to mitigate its impacts, ensuring a sustainable future for generations to come.
