Greenhouse gases are heating up nights faster than days, an analysis of the past 70 years has revealed. This trend is set to continue unless we cut emissions drastically, and that is worrying because sweltering nights are particularly dangerous to health.
Last week, the UK set a new record for its hottest night ever, with temperatures at Emley Moor in West Yorkshire not dropping below 25.9掳C 鈥� a full 2掳C higher than the previous record 鈥� and several locations not falling below 25掳C. Such record-breaking hot nights are becoming a familiar pattern the world over, but until now we haven鈥檛 been able to pin down why nights are heating faster than days.
To learn more, at the China University of Geosciences in Wuhan, China, and his colleagues ran climate simulations of temperature differences between day and night since 1951, which have shrunk by 0.41掳C in that time period. They looked at a number of scenarios, including a world without human influence, one with greenhouse gases from human activities and another with air pollution.
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Comparing the model runs with real-world observations made it clear that our greenhouse gas emissions are the main driver of the decreasing day-night temperature gap, closing it by 0.32掳C. This is probably because warmer air holds more moisture, increasing the chance of cloudy skies, which reflect the sun鈥檚 rays during the day, but trap heat at night.
Despite the UK鈥檚 record-breaking night, Europe as a whole has bucked the trend and seen daytime temperatures rise faster than night-time ones. 鈥淥wing to clear air laws, aerosol emissions have significantly decreased over Europe since the 1980s, which caused a reversal from solar dimming [pollution particles reflecting sunlight back to space] to brightening,鈥� says team member , also at the China University of Geosciences.
Humid nights can be dangerous because sweat doesn鈥檛 evaporate as easily, making it harder to stay cool than on fresher nights, putting a strain on organs like the heart. This could become a growing issue, as when the researchers ran the simulations to the end of this century, they found that the acceleration of hot and sticky nights will continue, and can only be stabilised if we reduce greenhouse emissions enough to keep global warming well below 2掳C.
Geophysical Research Letters
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