A group of researchers led by Weilin Liao, from Sun Yat-sen University in Guangzhou, has analyzed “hourly heat extremes” (HHE) on a global scale for the first time, crossing historical climate data from 1981 to 2023 with projections up to 2099. The study, published in Nature Climate Change, changes the standard unit of measurement of heat risk: no longer the day, classified as “hot” or “normal” based on a single maximum value, but the time. It is a difference that matters, because two days classified in the same way can hide a very different number of truly dangerous hours – a margin that current metrics do not record.
The picture that emerges is clear. In the high-emissions scenario SSP5-8.5, the global frequency of HHEs is expected to quadruple by the end of the century, while the global population’s overall exposure to these events will increase six-fold. Each day already classified as “hot” will gain an average of four more hours of extreme temperatures compared to today. But the most significant data concerns the days that traditional indices consider normal: here, however, another three-four hours of extreme heat will accumulate, a phenomenon that conventional daily metrics do not record at all.
The statistical breakdown of the factors confirms how climate warming, rather than population growth, is the driving force of this trend: climate change alone explains 92% of the increase in exposure expected by the end of the century in the intermediate SSP2-4.5 scenario, a percentage which rises to 96% in the more pessimistic SSP5-8.5 scenario.
The researchers also calculated the effect of different levels of global warming compared to the pre-industrial era: with every additional half degree, the average duration of HHEs per summer season grows by about 200 hours, going from 402 hours at one degree of warming to more than 1,061 hours at 2.5 degrees. At 1.5 degrees, less than 1% of the land surface would experience a doubling in the frequency of these events; at 2 degrees the share rises to 60%, at 2.5 degrees it exceeds 93%.
The geographic distribution of exposure is not uniform. Over three-quarters of the current and future burden of extreme heat falls on low- and middle-income countries, particularly South America, West and Central Africa, and Southeast Asia. In poorer regions, every half-degree of warming adds more than an hour of extreme heat per hot day, versus about half an hour in high-income regions — a gap the study calls intergenerational as well as geographic inequality.
In fact, those born today inherit a much greater exposure to heat than those born in 1981. In the SSP5-8.5 high emissions scenario, the 2023 cohort will accumulate exposure over their lifetime up to four times higher than the 1981 cohort. A second line of analysis, conducted by separating the results by level of global warming rather than by emission scenario, shows that the multiplier grows almost linearly with temperature: from approximately 2.2 times to 1.5°C heating up to 3.2 times at 2.5°C. Even in this second analysis, the gap by income remains large: in low-income countries the multiplier rises from 3.1 to 4.6 times between 1.5°C and 2.5°C of warming, compared to an increase of 1.6 to 2.3 times in high-income countries.
The authors also link hourly exposure to heat-related mortality observed in several countries between 2001 and 2020, finding solid statistical correlations, ranging between 0.63 and 0.95 depending on the country, a sign that the hourly measurement unit better captures the real risk to public health compared to daily indices.
The study proposes a paradigm shift in climate risk assessment, asking that adaptation policies and warning systems abandon the “hot day” logic in favor of hourly monitoring, capable of intercepting even the heat that accumulates silently in apparently ordinary nights and days.
