TORONTO -- Scientists say the extreme heat that resulted in the highest temperature ever recorded in Canada and is believed to be responsible for in British Columbia could become a once-a-decade event if global warming continues at its current pace.

by World Weather Attribution (WWA) concludes that last week’s heat wave is a one-in-1,000-years-level event under current conditions. It would have been a one-in-150,000-years event if not for global warming, and will be "a lot less rare" in the future if the global warming limit targeted by the Paris Agreement is not achieved.

"Without climate change, this event would not have happened," Friederike Otto, associate director of the Environmental Change Institute at the University of Oxford, said Wednesday at a press conference.

Unusually high temperatures baked Western Canada and the U.S. Pacific Northwest in the final few days of June. Lytton, B.C. broke the record for the highest temperature ever detected in Canada three days straight; its on June 29 was nearly 5 C higher than any temperature ever recorded anywhere in the country prior to last week.

The heat also created the perfect conditions for wildfires to spread – which is exactly what happened after , cooling the air but sparking hundreds of fires, many of which are . Almost all of Lytton itself was destroyed by a fire that .

"What is special about this one is by how much the record was broken," Otto said.

BAD LUCK, OR SOMETHING MORE?

To reach their conclusions, the 27 scientists involved in the project used computer simulations to compare the weather of last week to conditions that would have been experienced without global warming.

WWA researchers have used the same method to determine that climate change increased the likelihood of dozens of other extreme weather events, including the 2019-20 Australian bushfires and the record-breaking heat that hit Siberia last summer.

They say their analysis shows that the heat wave of last week would have been "virtually impossible" without climate change – but because it was so far beyond anything B.C. had experienced before, it was difficult for them to determine exactly to what extent climate change affected it, or what mechanisms were at play. They have two leading theories.

One theory holds that the heat wave was simply "really bad luck" – a weather event brought about by intense drought and unusual atmospheric conditions. While climate change likely increased the peak temperatures by about 2 C, in this theory, there would have been extreme heat even if the world was still at pre-industrial temperatures.

The other possibility is that global warming is causing peak temperatures to accelerate faster than has ever been observed before, which could call into question how prepared we are for the near-term effects of a warmer world.

Whichever theory is true, the scientists say, the extreme heat wave of last week should serve as a warning that countries and communities should be prepared for the possibility of temperatures significantly warmer than they have ever experienced before.

"Heat waves [are] how climate change kills us today. This is how climate change manifests most strongly," Otto said.

AN UNPRECEDENTED DISCOVERY

Climate and statistical models of the sort used in this analysis are relatively new, having gained acceptance in the mainstream climate science community during the past decade.

Geert Jan van Oldenborgh, senior researcher at the Royal Netherlands Meteorological Institute, said at the press conference that the models used by WWA initially considered it impossible for temperatures in B.C. to be as high as they were, and had to be manually overridden to accept them.

"We've never seen temperatures that broke that upper bound – until this heat wave," he said.

"This is something that nobody saw coming, that nobody thought possible."

As a result, van Oldenborgh said, it is possible that scientists are "overconfident" in their knowledge of heat wave causes and severity, and that further analysis of what happened in B.C. could help them better understand the interplay between climate change and periods of extreme heat.

The WWA findings will be submitted to a peer-reviewed journal for publication, Otto said.

World leaders have committed to limiting global warming to less than 2 C above pre-industrial temperatures under the Paris Agreement, and to make best efforts toward a lower limit of 1.5 C.

Since the agreement was signed in 2015, however, climate experts have repeatedly warned that the planet does not appear to be on pace to reach either goal. The World Meteorological Organization estimated in May that there is a 40 per cent chance the 1.5 C limit could temporarily be breached within the next five years, and the latest estimate from the UN Environment Programme puts warming at 3 C by the end of the century.

If the world does reach even 2 C of warming, the WWA scientists said, the sort of extreme heat that baked B.C. last week will return every five to 10 years.

"Our rapidly warming climate is bringing us into uncharted territory that has significant consequences for health, well-being, and livelihoods," they wrote in a summary of their findings.

"Adaptation and mitigation are urgently needed to prepare societies for a very different future."