Summer weather is arriving earlier, lasting longer and packing more heat than it used to—and it’s happening faster than scientists had previously measured. A new study by UBC researchers has found that between 1990 and 2023, the average summer between the tropics and the polar circles grew about six days longer per decade. That’s up from roughly four days per decade found in past research investigations up until the early 2010s.

For many cities, the numbers are even more striking. In Sydney, Australia, summer temperatures now last about 130 days, up from 80 days in 1990, adding 15 days per decade. Toronto summers are expanding by eight days per decade.

The researchers didn’t use the calendar definition of summer (June through August in the Northern Hemisphere and December through February in the Southern Hemisphere). Instead, they defined summer based on the weather: the stretch of days each year when temperatures rise above what was historically typical for a given location during the warmest part of the year—a threshold set using climate data from 1961 to 1990.