Thermal migration and climatic de-territorialisation of North American professional sports leagues, 1950–2025

Dimitri Defrance 

Over the past seven decades, the four major North American professional sports leagues (NHL, MLB, NBA, NFL) have expanded and relocated across the continent, shifting their collective geography towards the Sun Belt. We quantify the climatic consequences of this movement by combining the complete franchise location history of the four leagues from 1950 to 2025 with ERA5-Land reanalysis data and the Koppen-Geiger classification. We construct a franchise-city-year panel, derive thermal and degree-day indicators, and introduce a normalised Climate Departure Index (CDI). Mean annual temperature across franchise cities increased by 2.5-3.7°C between 1951-1970 and 2006--2025. A formal decomposition attributes 63-73% of this thermal drift to the changing geographical composition of the leagues, with the remainder arising from in situ warming of their initial franchise geography. Treating each of 59 franchise relocations as a vector in climatic space shows that 63\% moved towards warmer cities, generally reducing heating-degree
demand while increasing cooling-degree demand. The National Hockey League follows a distinctive trajectory: its weak mean relocation vector masks extreme movements towards both warmer and colder cities, demonstrating that climatic de-territorialisation can be non-monotonic. The NHL also exhibits the largest departure from its league-specific historical thermal reference. We interpret these results as evidence of climatic de-territorialisation: a measurable weakening of the historical association between professional sport and the climatic environments in which it developed.