The timing and length of summer weather conditions in the midlatitudes matter due to connections with extreme weather events, plant and animal phenology, economic productivity, human and ecosystem health, drought and wildfire, and energy demand. Here, we show that midlatitude summers are growing longer and hotter, and that seasonal transitions are becoming more abrupt, relative to the 1961–1990 period. From 1990–2023, mean summer length has increased by 5–7 d decade−1 across inland areas, and similarly for coastal margins and oceans in the midlatitudes, with length generally expanding symmetrically. This rate is faster than the ∼4 d decade−1 reported in prior works for midlatitude land through 2012. The speed of summer seasonal transitions is also increasing, with temperatures changing more rapidly at both the onset and withdrawal of summer. Accumulated heat, or cumulative summer heat stress, is growing at 44 °C d decade−1 since 1990 for Northern Hemisphere land, more than three times as fast as the 14 °C d decade−1 increase from 1961 to 1990. This increase in accumulated summer heat may challenge the ability of humans in the midlatitudes to physiologically adapt and will likely increase the energy expended for daytime and nighttime cooling. We provide theoretical explanations for the increase in seasonal transition speed and non-linear growth of accumulated heat in response to warming. Finally, we highlight changes for ten urban areas around the globe, with summer lengthening in some, such as Sydney and Minneapolis, by more than one day per year.

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