Summers are getting longer, hotter and the trend is speeding up

The old idea of summer, a fixed season that arrives on cue and leaves on time, is becoming out of date.

Across the midlatitudes, the stretch of the year with historically summer-like temperatures is now arriving earlier, ending later, and building more heat than it used to. A new study led by researchers at the University of British Columbia found that from 1990 to 2023, average summer length between the tropics and the polar circles grew by about six days per decade. Earlier work had put that figure closer to four days per decade.

In some cities, the change is much sharper. Sydney’s summer-like period now lasts about 130 days, up from 80 days in 1990. Toronto’s is expanding by about eight days per decade. For places where people once counted on a slower seasonal shift, the new pattern is harder to miss.

“These findings challenge what we believe to be the normal cycle of the seasons,” said lead author Ted Scott, a PhD student in UBC’s department of geography. “When summer happens and how quickly it arrives impact patterns and behaviors in plant and animal life, and human society.”

The study, published Tuesday in Environmental Research Letters, did not define summer by calendar months. Instead, the researchers asked a different question: when do temperatures at a given place rise above what used to be typical during the warmest part of the year?

To answer it, the team used a local threshold based on the 75th percentile of daily mean temperatures from 1961 to 1990. They then tracked when each year’s smoothed temperature curve crossed above that line and when it dropped below it again.

That approach matters because the human experience of summer does not always line up neatly with June through August in the Northern Hemisphere or December through February in the Southern Hemisphere.

Scott, Rachel White, a professor in UBC’s department of earth, ocean and atmospheric sciences, and Simon Donner, a professor in the department of geography and the Institute for Resources, Environment and Sustainability, analyzed temperature data from 1961 to 2023 across inland areas, coastal margins and oceans in both hemispheres. They excluded the tropics and polar regions because the classic four-season pattern is less clear there.

Their broad finding was plain: the warm season has expanded across land, coasts and oceans.

The lengthening was not the only change.

The study found that the move into summer and the retreat into autumn are also becoming more abrupt. In other words, temperatures are not just warming over a longer span of the year. In many places, they are jumping into summer conditions more quickly.

Instead of a long ramp from spring into heat, the slope is steeper.

“The changes may be very disruptive to a wide range of systems,” Scott said. “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.”

That matters well beyond comfort. Plants and animals often rely on seasonal timing. If flowers bloom earlier but pollinators are not active yet, important biological relationships can fall out of sync. Faster spring warming can also speed up snowmelt, raising flood risk. Crops may need to be planted earlier, even though daylight patterns, another major driver of plant growth, have not changed.

One season can start to pull away from the rest.

The study also points to coastal areas as places to watch closely. In the Northern Hemisphere, coasts are seeing some of the fastest growth in summer length and accumulated heat.

That could carry real consequences. Many people have moved to coastal cities partly because they are seen as milder than inland regions. Yet the researchers found that coastal margins in the Northern Hemisphere are accumulating heat nearly as fast as inland areas, even if their overall heat totals remain lower.

Over oceans, summer-like conditions tend to begin and end about 30 days later than inland. Along coasts, the timing is also delayed compared with inland areas, by roughly 15 days in the Northern Hemisphere and 10 days in the Southern Hemisphere. But delay does not mean protection. Ocean and coastal summers are still growing.

The city-level examples help show how uneven the changes can be. Among the 10 cities examined with observational data, Sydney had the fastest summer-length trend at 14.8 days per decade. Toronto’s trend was 8.4 days per decade. Minneapolis reached 9.3 days per decade, while Tokyo came in at 2.1.

The paper suggests that part of this difference comes from the shape of a place’s yearly temperature curve. A city with a narrower annual temperature range, such as Sydney, can see a bigger jump in summer length from the same amount of warming than a city with a broader range, such as Tokyo.

The researchers also introduced a second measure, accumulated heat, to capture not just how long summer lasts but how much heat builds up during it.

That metric combines temperature and time. A place can accumulate more summer heat because it spends more days above its local summer threshold, because those days are hotter, or because both are happening together.

Across all surfaces and in both hemispheres, accumulated heat increased. The sharpest rise over land came in the Northern Hemisphere, where summer accumulated heat increased by 44.3 degree-days per decade from 1990 to 2023. The study says that if you spread that increase across a three-month summer, it is roughly equal to each summer month warming by 0.5 degrees Celsius per decade.

The pace has picked up. Over Northern Hemisphere land and coasts, accumulated heat has been rising more than three times faster since 1990 than it did from 1961 to 1990.

That has practical weight. Higher accumulated heat can mean more cooling demand, more strain on people facing heat stress, and more pressure on systems built around older seasonal expectations.

The study also tested how it defined the seasonal cycle. The researchers compared a Fourier smoothing method with a third-degree polynomial, a method used in some earlier studies. They found the Fourier approach fit the annual temperature cycle more accurately, especially at higher latitudes, and produced a more balanced picture in which summer onset and withdrawal changed at roughly similar rates across most regions.

This study points to a problem that touches daily life as much as climate science. If summer begins earlier and intensifies faster, public health plans, crop calendars, water systems, flood preparation and energy use may all need to shift with it.

The findings also suggest that people living in coastal areas may face rising heat burdens sooner than expected. And because the changes do not unfold evenly from one region to another, local planning may matter as much as global averages.

The calendar has not changed. But the temperatures people organize around already have.

Research findings are available online in the journal Environmental Research Letters.

The original story "Summers are getting longer, hotter and the trend is speeding up" is published in The Brighter Side of News.

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