In response to the alarming rise in summer temperatures, the Japan Meteorological Agency (JMA) has announced the addition of a new term to its heat classification system. The introduction of “kokushobi,” translating to “severely hot day,” marks a significant shift in how Japan characterises its increasingly extreme weather patterns, particularly as the nation grapples with the consequences of climate change.
New Terminology for Rising Temperatures
The introduction of kokushobi is part of an expanded heat classification framework aimed at accurately reflecting the growing incidence of days exceeding 40 degrees Celsius. This system already includes terms such as natsubi (summer day) for temperatures above 25°C, manatsubi (midsummer day) for 30°C, and moshobi (extremely hot day) for 35°C. The term was selected following a public survey conducted in February and March, where 13 potential candidates were evaluated alongside expert insights.
The decision comes amidst a backdrop of dramatic temperature shifts, with over a third of all recorded days surpassing 40°C occurring between 2023 and 2025 alone. Since records began in 1872, Japan has documented 108 such days, underscoring the urgent need for a revised approach to weather reporting.
Record-Breaking Summers
The summer of 2025 has been recorded as the hottest on file, with average temperatures from June to August soaring to 2.36°C above the 30-year average. A staggering 132 of the 153 meteorological stations across the country reported new high temperature records that year. The peak temperature reached an unprecedented 41.8°C in Isesaki, Gunma prefecture, with 30 locations across 13 prefectures exceeding the 40°C threshold on that fateful day.
Such extreme heat not only disrupts daily life but also poses significant health risks. Historical data indicates a sharp increase in heatstroke-related hospitalisations when temperatures rise above 35°C. The 2018 heatwave, which saw temperatures reach 41.1°C in Kumagaya, resulted in 96 documented heatstroke-related fatalities in Tokyo within a single month, highlighting the perils of a warming climate.
The Impacts of Climate Change
Japan’s warming trend is notably outpacing the global average. While worldwide temperatures have increased by approximately 0.73°C over the past century, Japan’s temperatures have surged by 1.13°C. Scientists attribute this phenomenon largely to the climate crisis, particularly the elevated temperatures of surrounding waters, which trap heat and extend warmer conditions well into autumn. The resulting warmer oceans contribute to more intense rainfall and severe typhoons, further exacerbating the climate challenges facing the nation.
Looking forward, experts predict another record-breaking summer for 2026, indicating that the trend of rising temperatures is set to persist. Should the climate crisis continue unchecked, Japan could face the potential loss of its cherished four distinct seasons, with spring and autumn increasingly squeezed out by an extended summer heat season—an alarming pattern observed across various regions in Asia.
Why it Matters
The introduction of a new heat classification in Japan is more than just a linguistic update; it reflects a profound shift in the climate reality facing the country. As extreme weather events become more frequent and severe, the health implications for the population, particularly vulnerable groups such as the elderly, cannot be overstated. This change in terminology serves as a stark reminder of the urgent need for comprehensive climate action, as Japan—and the world—confronts the escalating impacts of a warming planet. The future of Japan’s climate, and indeed its society, hinges on the measures taken today to mitigate these alarming trends.
