The heat wave scorching North America — Canada and the United States — at the end of June was “virtually impossible” without climate change, say scientists.
In their study, the team of researchers says that the deadly heatwave was a one-in-a-1,000-year event. But this is become more common as the world heats up due to climate change.
If humans hadn’t influenced the climate to the extent that they have, the event would have been 150 times less likely.
Scientists worry that global heating, largely as a result of burning fossil fuels, is now driving up temperatures faster than models predict.
Heatwaves are breaking records all over the world in recent years.
Canada’s previous national record for high temperature was 45°C but the recent heat in the village of Lytton in British Columbia saw a figure of 49.6°C recorded at the height of the event.
This was shortly before the village itself was largely destroyed by a wildfire.
All across the region, in the US states of Oregon and Washington and in the west of Canada, multiple cities hit new records far above 40°C.
These temperatures had deadly consequences for hundreds of people, with spikes in sudden deaths and big increases in hospital visits for heat-related illness.
An international team of 27 climate researchers who are part of the World Weather Attribution network managed to analyse the data in just eight days.
While the study is yet to be peer reviewed, it used 21 climate models to estimate how much climate change influenced the heat experienced in the area around the cities of Seattle, Portland and Vancouver.
They compared the climate as it is today, with the world as it would be without human-induced warming.
“We conclude that a one-in-1000-year event would have been at least 150 times rarer in the past,” said lead author Sjoukje Philip, from the Royal Netherlands Meteorological Institute.
“So that’s in a climate without human-induced climate change, when the climate was about 1.2°C cooler than it is now. The heatwave would also have been about two degrees cooler in the past.”
Co-author Dr Friederike Otto, from the University of Oxford, explained what the researchers meant when they said the extreme heat was “virtually impossible” without climate change.
“Without the additional greenhouse gases in the atmosphere, in the statistics that we have available with our models, and also the statistical models based on observations, such an event just does not occur,” she explained.
“Or if an event like this occurs, it occurs once in a million times, which is the statistical equivalent of never,” she told a news briefing.
This type of research, which seeks to determine the contribution of human-induced climate change to extreme weather events, is known as an attribution study.
According to the analysis, if the world warms by 2°C, which could happen in about 20 years’ time, then the chances of having a heatwave similar to last week’s drop from around once every 1,000 years to roughly once every 5-10 years.
The scientists say there are two possibilities for the extreme jump in peak temperatures seen in the region.
The first is that it is just an extremely rare event, made worse by climate change, “the statistical equivalent of really bad luck”, according to the paper.
The other possibility is that the climate may have crossed a “threshold,” that would make the kind of heatwaves witnessed recently much more likely.