Summer weather is arriving earlier, lasting much longer, and packing significantly more heat—and the shift is happening far quicker than scientists previously realised.
A new study by researchers at the University of British Columbia (UBC) has revealed that between 1990 and 2023, the average summer between the tropics and the polar circles grew by about six days per decade. This marks a massive 50 per cent increase from previous research, which reported an expansion of just four days per decade up until the early 2010s.
For major coastal and metropolitan areas, the numbers are even more severe. In Sydney, Australia, summer conditions now last for roughly 130 days, up from just 80 days in 1990, effectively adding 15 days to the season every single decade. Meanwhile, summers in Toronto are expanding by eight days per decade.
Redefining the seasons
The researchers did not rely on traditional calendar definitions to reach these conclusions. Instead, they defined “summer” based on sustained weather conditions, specifically tracking the stretch of days each year when temperatures rise above historical norms set between 1961 and 1990.
Lead author Ted Scott, a PhD student in UBC’s Department of Geography, said: “These findings challenge what we believe to be the normal cycle of the seasons. When summer happens and how quickly it arrives impacts patterns and behaviours in plant and animal life, and human society.”
Abrupt transitions
Published in the journal Environmental Research Letters, the study also found that the gradual transitions between spring, summer, and autumn are vanishing.
Instead of a steady warm-up, intense summer-like temperatures are arriving suddenly, threatening to disrupt delicate ecological and agricultural systems. For example, flowers may bloom before their pollinators are active, crops may need to be planted significantly earlier, and rapid spring warming could trigger faster snowmelt and severe flooding.
Scott warned: “The changes may be very disruptive to a wide range of systems. An expectation in the Northern Hemisphere that June is when summer starts may be ingrained in planning and policy, meaning we could be ill-prepared for earlier heat.”
Metrics show heat is on
The cumulative heat building up over the warmest months is intensifying. By tracking a new metric that combines temperature and time, the authors discovered that accumulated summer heat over Northern Hemisphere land has risen more than three times faster since 1990 than it did in the previous three decades.
The study found that coastal areas in the Northern Hemisphere are seeing some of the absolute fastest growth in both summer length and accumulated heat, directly threatening millions of people who historically moved to these regions for their perceived moderate climates.
Co-authored by Dr Rachel White and Dr Simon Donner, the research raises urgent questions about the future of global food supplies, infrastructure planning, and extreme weather management as the fundamental rhythm of the year continues to warp.
