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6 hours ago
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Scientist Ted Scott could feel that summers in his home state of Minnesota were not what they used to be.
With the climate crisis accelerating, Scott could feel and see the seasons changing from their usual patterns – especially summer – and he wanted to know what the data said.
Working with researchers from the University of British Columbia, the PhD candidate set out to model how the average summer had changed in 10 global cities.
The research, published in the journal Environmental Research Letters, found what most people can already feel: summer conditions, driven largely by human-induced global heating, were arriving earlier, lasting longer and felt more intense than they used to.
They found that the length of summer – defined by the length of time summer conditions were experienced rather than calendar dates – was increasing on average by six days every decade.
In Minneapolis, Minnesota, it was nine days every decade. In Sydney, Australia, it was about 15 days.
“I grew up in Minnesota, kind of the north midwest of the United States, and I remember seasons being very different than how they are now, especially summer,” said Scott.
“I had the sense that they feel longer and they also feel like the transitions are much more abrupt. It feels like we had spring weather and then suddenly, boom, it’s completely warm.
“And the last thing was that summers feel more intense. It’s not just warmer, it feels like you get much less relief when summer arrives.”
The researchers analysed the stretch of days every year when temperatures were above a threshold that was historically typical for a specific city during the warmest part of the year. That threshold was set using data from 1961-1990, with the researchers examining the trends in each of those decades and the decades since.
Their analysis found Toronto in Canada was adding a little over eight days to its summer every decade. France’s capital Paris and Iceland’s capital Reykjavik were adding 7.2 days.
In the most extreme example, Sydney – where the threshold temperature was set at 21C – had a summer period that was growing at two-and-a-half times the global average.
Scott said data from 1961-70 showed Sydney’s summer tended to start on 6 January and end on 9 March. By 1991-2000 he said the researchers found a Sydney summer was starting on average by 21 December and ending on 12 March.
In the most recent decade analysed, 2014 to 2023, Sydney’s summer was starting almost a full month earlier on 27 November and ending on 28 March.
“So recent summers in Sydney are much closer to 125 to 130 days long whereas that time in the ‘60s was half as long, 65 days,” Scott said.
The research also found that the shift from one season to another was becoming more abrupt, with summer-like conditions arriving more suddenly rather than gradually warming up.
Scott said people’s perceptions of what was happening to the seasons “match what the data is telling us” and this had implications for every facet of life, from school terms to sporting seasons to the planting of crops.
Dr Andrew Watkins, an adjunct professor at Monash University’s School of Earth, Atmosphere and Environment, cautioned that the research had relied on globally aggregated datasets rather than focusing on datasets developed by local meteorological services. Watkins, who is also a councillor with Australia’s Climate Council, said the results for Sydney would represent the greater region, including western Sydney, where many people live and which is known for its increasingly extreme temperatures as the climate crisis accelerates.
“It doesn’t change the story, which is absolutely right and is what we all, as scientists, have seen: summers are getting longer, winters are getting shorter,” said Watkins.
He said it was important to consider all of the effects that flowed from that including longer fire seasons, more heatwaves and other extremes that affected people’s lives and health.
“It all boils down again to our continued use of fossil fuels and continued emissions of carbon dioxide,” he said.
“This is exactly what we expect with climate change. We need to adapt for what’s coming and mitigate and reduce fossil fuel usage.”
Prof Sarah Perkins-Kirkpatrick, of the Australian National University, said “the uptick in Sydney summers is not surprising”.
“We’ve seen quite drastic changes in the past few decades,” she said.
“If we can replicate this result with the locally based observation products we have here it would verify the specifics of that result.”
