A new study from researchers at the University of British Columbia, delivers some of the clearest evidence yet that the rhythm of the seasons is fundamentally changing — and faster than scientists previously measured.
The study analyzed global temperature data from 1961 through 2023, tracking not the calendar definition of summer, but the actual number of days each year when temperatures reach typical peak summer warmth. The findings are striking: between 1990 and 2023, the average summer between the tropics and the polar circles grew about six days longer per decade — up from roughly four days per decade found in past research up until the early 2010s. That 50% acceleration means summers across the midlatitudes — a belt encompassing most of North America, Europe, and large parts of Asia — have gained roughly 30 more days of summer-like conditions compared to the 1960s. Some cities are seeing the shift even more dramatically. For example, in Sydney, Australia, summer temperatures now last about 130 days, up from 80 days in 1990, adding 15 days per decade.
It isn't just the length of summer that's changing — it's the intensity of heat accumulation. The total warmth building above summer temperature thresholds is growing more than three times faster than it was during the 1961–1990 baseline period.
The research points to urgent questions: how will longer, faster-transitioning summers affect the timing of extreme weather events? What will earlier spring heat mean for food supply, given that growing seasons are shifting but daylight — a key driver of plant growth — is not? And do today's climate models that inform planning and policy fully capture these trends, or do they need updating?
Access the complete study here to review the full climate data analysis
