Rising heat is stretching summer and changing how people live

Climate change is leaving an increasingly visible mark on the planet, with temperatures continuing to rise year after year. The latest study shows that summer is now arriving earlier and lasting longer, with growing consequences for daily life, urban planning, and infrastructure.

Research from the University of British Columbia (UBC) found that between 1990 and 2023, summer length across the world’s mid-latitudes increased by an average of around six days per decade. That marks a notably faster rate of change than earlier research, which found that summer had lengthened by 4.8 days per decade before 2012.

Compared with the 1960s, summer today is about 30 days longer overall. The trend has been observed across all environments, including land, coastal areas, and the oceans. In the most recent decade, summer length over the oceans has expanded even faster than over land, largely because seasonal temperature ranges have narrowed.

One of the clearest examples is Sydney, where summer-like conditions now last around 130 days, up sharply from just 80 days in 1990. In the Northern Hemisphere, Toronto and Minneapolis have also seen summer length increase by around eight and 9.3 days per decade, respectively.

Even a rise of just 1 to 2 degrees Celsius over the course of a season can quadruple the number of extremely hot days. In Perth, Australia, the number of days above 41.3 degrees Celsius has nearly tripled compared with the period before 1975. In Melbourne, the number of days above 42 degrees Celsius since 2000 equals the total recorded during the previous 90 years combined.

Dr Linden Ashcroft, a climate scientist at the University of Melbourne, said these heat events are not only becoming more frequent but also more intense. She stressed that the changes are being driven by the continued rise in greenhouse gas emissions caused by human activity.

The study also introduced the idea of “cumulative heat”, which measures both the intensity and duration of heat above a local threshold. It found that the rate of increase in cumulative heat in the Northern Hemisphere since 1990 has been 300% higher than during the 1961–1990 period.

Beyond summer becoming longer, the shift between seasons is also happening more abruptly than in the past. Instead of following a gradual warming pattern, summer is now beginning earlier and ending later, leaving seasonal systems increasingly out of sync.

That disruption can have serious ecological consequences. Flowers may bloom before pollinating insects become active, while snowmelt that happens too quickly can raise the risk of flash floods. Farmers are also facing new challenges in crop planning, as the growing season expands while daylight hours remain unchanged.

Ted Scott, the study’s lead researcher from UBC’s Department of Geography, said many cities are still unprepared for heat that arrives earlier than expected. Measures such as cooling centres and public heat warnings are often not ready in time.

Geography also plays a major role. Coastal areas may benefit from sea breezes that help ease extreme heat more than inland areas do. At the same time, dense urban development dominated by concrete and high-rise buildings can push felt temperatures far above those in greener areas, with unavoidable consequences for health and productivity.

Night-time temperatures have also risen steadily in cities since the 1950s, though they have received far less attention than daytime heat. Yet this is a crucial issue because night is when the body should be recovering from the heat absorbed during the day.

Prof Ollie Jay from the Heat and Health Research Centre at the University of Sydney said that night-time temperatures of 29 degrees Celsius or higher begin to pose health risks. At that level, the body can lose significant amounts of sweat during sleep, causing dehydration and increasing the danger of facing another hot day without adequate recovery.

Hotter conditions also directly affect sleep quality. As temperatures rise, sleep becomes less effective, reducing work performance. During periods of severe heatwaves, the strain can also damage physical health and intensify stress.

Prof Ben Newell, a behavioural psychologist, said people are remarkably quick to adapt to new norms. Because memory is often shaped by emotional experience, people may forget how much cooler the weather once was and begin to regard today’s heat as normal.

This psychological adjustment resembles the “boiling frog” effect, in which conditions worsen so gradually that people fail to sense the urgency. If society becomes accustomed to abnormal heat, the need to cut greenhouse gas emissions and tackle the root cause of global warming may no longer receive the attention it deserves.