In a significant response to the alarming rise in temperatures, the Japan Meteorological Agency (JMA) has unveiled a new term for days when temperatures reach or exceed 40 degrees Celsius. The introduction of “kokushobi,” or “severely hot day,” is a clear acknowledgment of the increasing frequency of extreme heat events in the country. This change is not merely a linguistic adjustment; it reflects a broader and troubling trend in climate patterns that has been intensifying in Japan.
A New Lexicon for a Changing Climate
The JMA has decided to enhance its heat classification system, which already includes terms such as “natsubi” for summer days above 25 degrees, “manatsubi” for 30 degrees, and “moshobi” for extremely hot days at 35 degrees. The new addition, kokushobi, was chosen following an online survey conducted in February and March, which presented 13 potential candidates for public input, alongside expert recommendations.
The urgency of this decision arises from the dramatic shifts in Japan’s summer climate in recent years. Historical records reveal that out of the 108 days exceeding 40 degrees since 1872, over a third—41 days—occurred between 2023 and 2025. These figures starkly illustrate how recent years have been marked by unprecedented heat, culminating in the summer of 2025, which was declared the hottest on record.
Record-Breaking Temperatures and Implications
From June to August 2025, average temperatures soared to 2.36 degrees Celsius above the 30-year norm, with 132 out of 153 meteorological stations in Japan recording new temperature highs. The pinnacle of this heatwave was reached in August 2025, when Isesaki in Gunma prefecture recorded an astonishing 41.8 degrees Celsius, while 30 other locations across 13 prefectures similarly surpassed the 40-degree mark.
Such extreme heat is not an isolated phenomenon; it is part of a broader trend that sees Japan’s temperature rise outpacing the global average. While global temperatures have increased by approximately 0.73 degrees Celsius over the past century, Japan’s rise is measured at 1.13 degrees Celsius. This discrepancy can largely be attributed to the effects of climate change, particularly the warming of ocean waters surrounding the Japanese archipelago. This phenomenon not only intensifies heat but also fuels heavier rainfall and more severe typhoons, exacerbating the climate challenges facing Japan.
Health Risks and Future Projections
The health ramifications of these soaring temperatures are dire. Instances of heatstroke spike significantly when temperatures surpass 35 degrees, posing substantial risks, especially to the elderly population. The summer of 2018 exemplified this grave reality: during a heatwave that recorded a then-high of 41.1 degrees Celsius in Kumagaya, Saitama, 96 residents of Tokyo died from suspected heat-related ailments within just one month.
Looking ahead, forecasts for 2026 predict another record-breaking summer. Experts warn that if the current trajectory of climate change continues, Japan could face a future where its beloved four seasons are drastically altered. The traditional spring and autumn periods may increasingly be overshadowed by an extended summer, a troubling pattern already seen in other parts of Asia.
Why it Matters
The introduction of kokushobi is more than just a new term; it encapsulates the pressing reality of climate change and its profound impact on everyday life in Japan. As extreme weather becomes the norm rather than the exception, the implications for public health, agriculture, and overall quality of life are significant. Japan stands at a critical juncture, where the need for urgent climate action and adaptation strategies is more crucial than ever to safeguard future generations from the escalating threats of a warming world.