Hourly heat extremes to triple by century’s end, study finds
New analysis in Nature Climate Change shows low-income nations and younger generations face disproportionate exposure to intensifying summer heat stress.
A new study led by Weilin Liao and Xuanzong Zhang at Sun Yat-sen University indicates that the frequency of hourly extreme heat events is accelerating at a rate that traditional daily metrics often obscure. Published in Nature Climate Change, the research utilises an hourly analysis to demonstrate that the global average of 270 “hourly heat extremes” per summer has been increasing by 62 hours each decade.
The researchers defined extreme heat as temperatures exceeding the 90th percentile of recorded temperatures at that specific time of day, based on baseline data from 1981 to 2010. This granular approach reveals that hot hours are increasing more rapidly at night than during the day. The trend is most pronounced in the tropics, where increased temperature variability makes extremes more frequent, despite higher latitudes experiencing stronger overall warming.
Projections for the period between 2070 and 2099 suggest a significant escalation in heat stress. Under a medium emissions scenario leading to 3°C of warming, the total number of hourly heat extremes is expected to triple. In a high emissions scenario resulting in 5°C of warming, these extremes could increase by 4.5 times. The study notes that for every 0.5°C increase in global average temperature, there is an associated rise of approximately 200 hot hours per summer.
The analysis highlights that the duration of heat stress within individual days is also lengthening. Currently, the average hot day contains about 16 hot hours. Under the medium emissions scenario, this figure rises to 19 hours, while the high emissions scenario projects 21 hot hours per day. This extension reduces the hours available for relief, with shorter hot periods often preceding or following hot days, thereby compounding the physiological stress on individuals.
Low-income countries are projected to face the strongest increases in hot hours, raising concerns regarding equitable adaptation strategies. The study also quantifies the generational impact of these trends, finding that individuals born in 2023 are expected to experience 1.4 to 1.8 times more hot hours over their lifetime compared to those born in 1981. The authors argue that hourly-scale assessments are essential for anticipating health impacts and designing early warning systems that reflect the reality of round-the-clock heat exposure.
