The uneven change of global expanding summer over the past 50 years

Climate change has fundamentally altered global seasonal patterns, yet quantifying these shifts remains challenging due to heterogeneous definitions of seasons. Here, we develop a standardized 5d temperature-based classification system for seasons from 1971 to 2021 using the fifth-generation ECMWF atmospheric reanalysis (ERA5). Our analysis reveals a pronounced redistribution of seasonal durations. Specifically, summer expanded by 3.2 % grid globally, while spring, autumn, and winter decreased by −2.8 %, −12.1 %, and − 0.9 % in 2021, respectively, compared to 1971. The prolongation of summer in mid-latitude regions is due to an earlier onset and delayed termination, as well as the poleward migration of thermal transition zones (1.1° latitude over 1971–2021). Additionally, hemispheric asymmetry is quantified by the poleward migration of warm-side boundaries: 0.22°N decade⁻¹ vs. 0.20°S decade⁻¹, i.e., 10.3 % faster in the North Hemispheric, attributed to its greater landmass and thermal sensitivity. This study provides a unified framework for understanding global seasonal dynamics, which is critical for predicting climate-ecosystem interactions.