The heatwaves moving in recent days across Canada, the northeastern United States and northern Europe have one thing in common: They are occurring quite early in the season.
That timing speaks to a broader trend, connected to human-caused climate change, in which summerlike weather is creeping earlier into June and lingering deeper into September - elongating the period of potentially extreme weather and amplifying heat risks.
“There is definitely an extension of the summer period into the shoulder months,” said Carlo Buontempo, director of Europe’s Copernicus Climate Change Service.
This shift is occurring remarkably quickly.
In a typical year between 1979 and 2000, the average Northern Hemisphere temperature would break the 21C threshold - indicative of the hottest period - starting at around July 10 and continuing for about five weeks, according to data from the University of Maine’s Climate Change Institute.
But last year, the hottest on record, the Northern Hemisphere’s average temperature held above 21 Celsius from June 13 until September 5.
This year is not quite on the same historic pace - but it is still weeks ahead of the normal summer from a few decades ago.
As of last week, the latest for which data was available, the average Northern Hemisphere temperature already stood at 20.91C.
“If I was to compare this with what happened in the 20th century, it would be very unusual,” said Sonia Seneviratne, a Swiss climate scientist at the Institute for Atmospheric and Climate Science of the ETH Zurich.
While the prolonged summer can have its perks - who doesn’t like a day at the beach? - it also causes greater health risks, particularly for the elderly and those with pre-existing frailties or little means to cope and stay cool.
In Europe, for instance, heat-related mortality has increased by 30% over the past 20 years.
Research indicates that early season heatwaves can pose greater health danger than those later in the summer, after the body has acclimatised and people are more prepared to cope.
Data from the US Environmental Protection Agency shows that heatwaves have grown longer, more frequent and more intense over the past seven decades.
The potential season for experiencing heatwaves has expanded too - from fewer than 50 days in the 1990s to 70 days in the 2020s.
High temperatures at or above 32.2C have covered wider swathes of land than normal so far this year.
The amount of land area in the Northern Hemisphere covered by high temperatures of this magnitude was above average on 163 out of 171 days from January to mid-June.
During periods in mid-January, mid-March, late April, mid-May and early June, the area affected by these levels of high heat was greater than any previous year since 1980, coinciding with prolonged early season heatwaves in South Asia.
The earlier summerlike heat has widespread implications, changing growing patterns, exacerbating water stress and extending the wildfire season.
This week, on the Greek island of Chios, officials declared a state of emergency and evacuated residents as blazes ripped through forest in one of the first wildfires of the year.
After several years of devastatingly hot and dry conditions, Greece has prepared for the season with a record number of firefighters.
The abnormally warm temperatures have also extended through to the Swiss Alps, where glaciers have started to melt and lose mass several weeks earlier than usual, said Matthias Huss, a glaciologist and the head of Glacier Monitoring in Switzerland.
“It’s definitely not looking good for Swiss glaciers,” Huss wrote on social media. “Oh no, where are you heading to, 2025?”
The heatwaves aren’t just confined to North America and Europe.
In Japan, numerous daily heat records have fallen in recent days. And temperatures have far exceeded the norm across Mongolia, Russia and western China - where in some cities it was 15 degrees warmer than usual.
Scientists draw a straightforward connection between rising heat across the world and the greenhouse gasses that humans have pumped into the atmosphere, mostly by burning fossil fuels, over the past two centuries.
Emissions continue to rise, and the pace of warming has accelerated: The 10 hottest years on record have all come during the past decade.
Earth is also absorbing more heat in its oceans, raising water temperatures. Increasing moisture is in turn a fuel for stronger hurricanes and cyclones and more damaging floods.
As heatwave conditions took hold several days ago in Britain, scientists from the group World Weather Attribution said that such an event could be expected once every five years.
Before human activity warmed the atmosphere, southeast England would have experienced a similar heatwave once every 50 years.
“All datasets show strong and statistically significant trends in June heatwaves as the world has warmed,” the scientists wrote.
