Record-shattering temperatures in the US and Canada recently, which caused hundreds of deaths and extensive wildfires, would be "virtually impossible without climate change", according to analysis by leading climate scientists.
The evaluation of last week's heatwave in parts of North America found global warming caused by human activity made it at least 150 times more likely to happen.
The scientists from the World Weather Attribution (WWA) warned the climate system may have crossed a threshold where a small amount of warming was causing a faster rise in extreme temperatures, posing the risk of more deadly heatwaves.
The village of Lytton in British Columbia saw a new Canadian record high of 49.6 degrees, well above the country's previous national record of 45 degrees. It was largely destroyed by wildfires. Temperatures soared above 40 degrees in cities in Oregon and Washington states.
WWA scientists, who are part of an international collaboration, said the temperatures were so extreme they lie far outside the range of historically observed temperatures, making it hard to tell just how rare the event was.
Statistical analysis suggests, however, temperature highs were a “one in a 1,000-year event” in today’s climate, which has seen 1.2 degrees of human-induced global warming since pre-industrial times.
Predictions for 2040s
Without warming driven by carbon emissions from sources such as burning fossil fuels, it would have been at least 150 times rarer, or virtually impossible, they concluded in a study released on Wednesday.
The researchers warned of the consequences of future warming, highlighting the impact of two degrees of global warming which could be reached as early as the 2040s, based on current levels of emissions.
At two degrees of warming above pre-industrial levels, last week’s heatwave would have been another degree hotter, potentially pushing record temperatures up to more than 50 degrees.
They used climate models to compare what would have happened without climate change and with the current level of warming.
The WWA analyses and communicates the possible influence of climate change on extreme weather events, such as storms, extreme rainfall, heatwaves, cold spells and droughts.
One explanation for the recent extreme heat is that while every heatwave occurring today is made more likely and more intense by climate change, the extremes seen in North America are still a very rare event.
‘Really bad luck’
Under this explanation. it was the statistical equivalent of “really bad luck”, with pre-existing drought and unusual atmospheric conditions known as a “heat dome” combining with climate change to create very high temperatures, the researchers noted.
As a result, the heatwave was about two degrees hotter than it would have been if it took place at the beginning of the industrial revolution when global temperatures were 1.2 degrees lower.
The other possibility is that the climate system has crossed thresholds where a small amount of overall global warming is causing a faster rise in extreme temperatures than seen so far.
This could include droughts or heat domes being made more likely by climate change, and playing a role pushing up temperatures to such extremes, but the scientists said they did not yet know if this was the case.
They warned the heatwave showed extreme temperatures well outside of the expected range can happen far up into the northern hemisphere, such as in Europe.
Friederike Otto of the Environmental Change Institute at Oxford University said: "What we are seeing is unprecedented. You're not supposed to break records by four or five degrees. This is such an exceptional event that we can't rule out the possibility that we're experiencing heat extremes today that we only expected to come at higher levels of global warming."
