Summer Is Stretching the Climate Calendar as Study Finds Earlier, Longer Seasons in Canada and Beyond
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A new study by researchers at the University of British Columbia suggests that summer is not only getting hotter, but also arriving earlier and lasting longer at a faster pace than earlier studies had estimated across Canada and broad parts of the midlatitudes. The researchers found that average summer length increased by about 5 to 7 days per decade between 1990 and 2023, compared with earlier estimates of roughly 4 days per decade.
The study defines summer using “climatic summer” rather than the calendar season. In other words, summer begins when temperatures rise above what was historically typical for the warmest part of the year, based on a 1961-1990 reference period. The researchers also found that the transitions from spring to summer and from summer to autumn are becoming sharper and more abrupt, potentially disrupting systems that rely on gradual seasonal change.
In Canada, Toronto stood out as one of the cities showing a clear shift, with UBC reporting that the city’s summer-like season is expanding by about 8 days per decade. The implications go beyond climate averages, touching agriculture, water management, public health, energy demand, planting schedules, snowmelt timing and even spring flood risk.
The study also found that accumulated summer heat is rising quickly. Over Northern Hemisphere land, cumulative summer heat stress has increased by 44 degree-days per decade since 1990, more than three times the pace recorded during the 1961-1990 baseline period. The researchers say the issue is no longer just about a longer warm season, but also about the heavier heat burden it carries, raising practical questions about whether infrastructure and public policy are keeping up with a faster-changing seasonal cycle.
