Heat wave would have been 'virtually impossible' without climate change, report suggests

The recent heat wave that overwhelmed communities in both Canada and the Pacific Northwest in the U.S. was at least 150 times more likely to happen because of climate change, new data suggests.

A team of 27 researchers from the World Weather Attribution Initiative, including scientists from universities and meteorological agencies across North America and Europe, looked at observations and computer simulations to compare today's climate with that of the past.

They concluded the record-breaking heat wave would have been "virtually impossible" without human influence. 

Lytton, B.C., a small village in the Fraser Canyon, broke record for the hottest temperature ever recorded in Canada three days in a row during the heat wave, eventually hitting 49.6 C. That's up from 45 C, recorded in Saskatchewan in 1937.

"When we look at the records of temperature through time, there's a steady increase in the hottest temperatures of the year, but then this event came along and it just broke that record," said Faron Anslow, a climatologist at the University of Victoria's Pacific Climate Impacts Consortium and a co-author on the report, which was released Wednesday.

"Literally, the data points are plotting in a way that we had to circle them to draw attention to them, because the eye is not expecting them to be way up there," Anslow told CBC News. 

"What we are seeing is unprecedented," said co-author Friederike Otto, a climatologist at the University of Oxford, in a statement. "You're not supposed to break records by four or five degrees Celsius." 

The report has yet to be peer-reviewed, but the scientists plan to submit it to a journal in the near future.

While climate change made such an extreme heat wave more likely, the scientists say it was still very unusual.  

They estimate that such events in today's climate — which is about 1.2 C warmer than before the industrial revolution — should only occur once every 1,000 years. 

But if it gets 2 C warmer, which could happen by 2050, the researchers say heat waves like that could happen at least once or twice a decade.

The team came up with two possible explanations for how a heat wave of this magnitude happened.

First, they say a pre-existing drought and the unusual atmospheric conditions that created the "heat dome," combined with climate change, created very high temperatures. Without climate change, peak temperatures would have been about 2 C lower. 

The second explanation is that Earth's climate has already crossed a threshold where a relatively small temperature change like 1.2 C is causing a faster rise in extreme temperatures than models can predict.

Anslow says it's hard to pinpoint the exact cause. The record highs were set a whole month before what is typically the warmest time of the year. 

"What that implies is, had this event been later, it might have been a little bit hotter," said Anslow. "But also, this event happened really soon after the longest days of the year [...] you just have these extra hours and minutes of time for that sun's energy to come down and bake that landscape."

Last year, a heat wave in Siberia sparked wildfires and melted permafrost with temperatures as high as 38 C. The World Weather Attribution Initiative published a report saying climate change had made that event at least 600 times more likely.

Stewart Cohen, a retired climatologist who was not involved in the report, cautions that it came out "pretty quickly" and should be treated as a "quick exercise."

But, he says, its results support what other scientists have quickly been learning — that Canada's strategy for dealing with heat warrants another look.

"It's just one more piece of evidence that indicates the urgency of a re-evaluation of how we manage extremes," said Cohen.