Global warming is leading to more extremely hot weather. There is perhaps no more obvious consequence of climate change. But changing how we analyze this problem can still help scientists think about it in a different way that may yield surprises.

Generally, studies on heat extremes work with some combination of daily minimum, maximum, and mean temperatures. Sometimes they focus on multi-day heatwaves using those metrics, since the discomfort and health impacts increase for longer events. There are different thresholds you can use to define extreme heat in these analyses, either based on past statistics or on the human body’s physiological limits.

A new study led by Weilin Liao and Xuanzong Zhang at Sun Yat-sen University tried breaking down heat extremes to the hourly level instead, which reveals some interesting nuances.

Hourly reports

The researchers calculated each hour of the day separately, comparing 8 am temperatures only to other 8 am temperatures. They defined extreme heat as anything above the 90th percentile of recorded temperatures at that time of day between 1981 and 2010. To put this in context, the global average over the last four decades has been that people experience about 270 hours of “hourly heat extremes” each summer. But that average obscures a very obvious trend: that number has been increasing by 62 hours each decade.

To project this into the future, the researchers used climate model results from two scenarios of future greenhouse gas emissions—one with medium emissions leading to 3°C (about 5.5°F) warming by 2100, and one with high emissions and 5°C (9°F) warming.

Comparing the past couple of decades to 2070–2099, the total number of hourly heat extremes triples in the medium scenario and increases by 4.5 times in the high scenario. For every half a degree C (almost 1°F) of global average temperature increase, the study shows an increase of about 200 hot hours per summer.