Global Extreme Heat Seasons Are Lengthening Asymmetrically

As temperatures rise with ongoing anthropogenic climate change, extreme heat events are becoming more frequent and are starting to occur outside of the expected heat season. Unusually early or late extreme heat events can create outsized impacts as individuals may be less acclimated to intense heat or unready to employ cooling strategies. Here we characterize the historical baseline seasonality of extreme heat around the globe and quantify how this seasonality is shifting over time. We define heat seasons as the three months with the highest historical fraction of extreme heat events during a baseline period in the 1980s. We find that in this baseline period, the extreme heat season is distinct from meteorological summer throughout much of the world, but captures a high fraction of the total annual observed extreme heat days. This is true even in low latitudes where the amplitude of temperature seasonality is low. Heat seasons are also expanding asymmetrically: some regions now experience a higher fraction of extreme heat events in the months following the heat season, while others experience a higher fraction preceding the heat season. At most locations around the globe, this observed asymmetrical lengthening of the extreme heat season is not explained by annual mean warming shifting up preexisting seasonal mean temperatures. Regional trends in seasonal mean temperature and humidity underscore that additional local dynamics are altering extreme heat drivers differentially during the two shoulder seasons. These results highlight the need for heat alert systems to focus on new times of year and prompt further study of compound events where extreme heat seasons are encroaching on the peak seasons for hazards such as wildfire or hurricanes.