Climate science: Northern Hemisphere compound hot extremes on the rise

Compound hot extremes, in which both day and night temperatures for a specific date in summer are in the top 10% of historical temperatures for that day, will increase significantly under anthropogenic climate change. The study is published in Nature Communications. By the end of the century, population exposure in the Northern Hemisphere to these events may be four to eight times higher than levels in the 2010s.

Yang Chen and colleagues analysed temperature data from 1960 to 2012 for the Northern Hemisphere and show that there has been about one additional day of compound extreme heat per decade on average over this period. These events have also become warmer on average by 0.28°C per decade. A statistical analysis confirmed that this increase is largely caused by anthropogenic greenhouse gas emissions.

The authors also present results from an ensemble of climate models investigating future changes in compound extreme events for two emissions scenarios. They suggest that the frequency of compound hot extremes could increase by a factor of 4 to 32 days per summer under moderate greenhouse gas emissions (RCP4.5), and by a factor of 8 to around 69 days per summer for a high-end emissions scenario (RCP8.5) by the end of the century. A 2°C warmer world could lead to an extra 5 days of compound hot extremes per year and an additional 0.5°C increase in their intensity compared to conditions if global warming is limited to 1.5°C.