North America’s record heatwave ‘virtually impossible without climate change’

The scientists also warned of the possibility 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.

In the heatwave, parts of the Pacific north-west saw temperatures that broke records by several degrees Celsius.

The village of Lytton in British Columbia saw a new Canadian record high of 49.6C (121.3F), well above the country’s previous national record of 45C (113F), and was shortly afterwards largely destroyed by wildfire.

And hundreds of deaths have been attributed to the sweltering temperatures which soared above 40C in many cities in Oregon and Washington states in the US and the western provinces of Canada.

Scientists 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.

But statistical analysis suggests the temperature highs to be a one in a 1,000-year event in today’s climate, which has seen 1.2C of human-induced global warming since pre-industrial times, making it still a very rare event.

Without warming driven by greenhouse gas emissions from sources such as burning fossil fuels, it would have been at least 150 times rarer, or virtually impossible, the scientists said.

The researchers also had a stark warning about the consequences of future warming, highlighting the impact of 2C of global warming which could be reached as early as the 2040s on current levels of emissions.

At 2C of warming above pre-industrial levels, last week’s heatwave would have been another degree hotter, potentially pushing record temperatures up to more than 50C.

And at 2C of global warming, heatwaves with the kind of temperatures seen in parts of the US and Canada in recent days could occur every five to 10 years.

The scientists from the World Weather Attribution initiative reached their findings by using climate models to compare what would have happened without climate change and with the current level of warming.

And they said there were two explanations for how climate change made the extreme heat more likely.

One explanation is that while every heatwave occurring today is made more likely and more intense by climate change, the extremes seen in the US and Canada are still a very rare event.

Under this explanation the event 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 the very high temperatures, the researchers said.

As a result, the heatwave was about 2C hotter than it would have been if it took place at the beginning of the industrial revolution when global temperatures were 1.2C lower.

But the other 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 has been seen so far – potentially making heatwaves like last week’s already more likely to happen than climate models predict.

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 that extreme temperatures well outside of the expected range can happen far up into the northern hemisphere, such as in Europe, with impacts on human lives and health which must be planned for.

One of the leaders of the initiative, Friederike Otto, from the Environmental Change Institute, Oxford University, said: “What we are seeing is unprecedented.

“You’re not supposed to break records by four or five degrees Celsius (seven to nine degrees Fahrenheit).

“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.”

Geert Jan van Oldenborgh, from the Royal Netherlands Meteorological Institute, said: “While we expect heatwaves to become more frequent and intense, it was unexpected to see such levels of heat in this region.

“It raises serious questions whether we really understand how climate change is making heat waves hotter and more deadly.”

And his colleague Sjoukje Philip said: “Although it was a really rare event, an event of this extremity would have been virtually impossible in the past and we are going to be seeing more intense and more frequent heatwaves in the future as global warming continues.

“That’s something we must take seriously because of all the impacts.”