The trend is worst in the tropics, even though overall warming is stronger at higher latitudes. The researchers say this is because temperature variability increases in the tropics, making extremes more frequent.

Hot nights

The number of hot hours also increases more at night than it does during the day. This is where the hourly analysis can start to seem more relatable than metrics of temperature change or counts of days. Days that fail to qualify as “hot days” still see an increase of several hot hours—mostly at night.  This wouldn’t show up if you just counted the number of hot days. And these shorter hot periods often precede or follow hot days, extending the stress a heatwave puts on your body.

Obviously, those hot days also see an increase in hot hours. Right now, the average hot day contains about 16 hot hours, but that increases to 19 hours in the medium scenario and 21 hours in the high scenario. So the miserable part of the day gets longer, reducing the hours that grant some relief.

The researchers also looked at differences in these trends for countries at different income levels and compared the lifetime total exposure to hot hours for people born in 1981 and 2023. Low-income countries see the strongest increase in hot hours. But even at the global average, those born in 2023 would experience about 1.4 to 1.8 times as many hot hours, depending on the greenhouse gas emissions scenario.

While the overall trend in hot weather is well known, the hourly details matter. They highlight that the number of hot hours is increasing even faster than the number of hot days, for example. But the details also help us better represent what this means for human health.

The researchers write, “As heat risks continue to intensify, hourly-scale assessments will become essential for anticipating health impacts, informing equitable adaptation strategies and designing responsive early warning systems that reflect the new reality of round-the-clock heat exposure.”

Nature Climate Chane, 2026. DOI: 10.1038/s41558-026-02724-8 (About DOIs).