Why can we blame the deadly heat dome and fire in Lytton, B.C., on climate change now?

The deadly heat wave that hit Western Canada in 2021 would have been virtually impossible without human-induced global warming, a new study has found.

Additionally, the researchers predict that if warming continues at even a moderate rate, Canadians in the region can expect similar heat waves roughly every 10 years by 2050.

The “heat dome” that lingered over western Canada and the northwestern United States in late June and early July – “a record-breaking and unprecedented historic heat wave” in the words of expert – has been held responsible for 619 deaths in British Columbia alone, and 1,400 heat-related deaths have been reported in the region.

Temperatures in the region have reached extraordinary heights. In Lytton, British Columbia, after three consecutive record days, the mercury peaked at 49.6°C on June 29, the highest temperature ever recorded in Canada.

The following day, a wildfire, fueled by strong winds and encouraged by dry, dry conditions, swept through Lytton with ferocious speed. By the time he disappeared, two people had died and 200 properties had been destroyed in and around Lytton.

Almost all the houses in the center of the village have disappeared,” Lytton staff wrote in a statement a few days later. “Where many buildings used to be, there is only charred earth.”

In the aftermath, many pointed to climate change as the main culprit.

Although the existence of human-induced climate change is well established, it is difficult to identify a particular weather event and definitively attribute it to climate change.

But researchers at Columbia University’s Columbia Climate School were able to blame the 2021 thermal dome on climate change.

Their research, published Thursday in the journal Natural climate change, gathered climate data from as far back as the 1950s as well as daily weather data from the weeks before, during and after the heat wave.

They concluded that although this particular heat wave was the result of several mechanisms – some related to climate change, others coincidental – it would not have been possible had certain weather patterns not been altered by warming. man-induced.

« It was so extreme that it’s tempting to apply the label of a ‘black swan’ event, an event that cannot be predicted, » says lead author Samuel Bartusek, a PhD student at the Columbia Climate School’s Lamont-Doherty Earth Observatory. . “But there is a boundary between the totally unpredictable, the plausible and the totally expected that is difficult to categorize. I would call it more gray swan.

It has been well established by climatologists that there has been an upward trend in temperature – human-induced global warming – in recent decades. But that, by itself, is not enough to explain the extreme heat events of 2021.

The heat wave, the researchers found, was caused by two major factors of climate change – disruption of a jet stream that typically carries air from west to east across the Pacific, and carries with it peaks and troughs of high and low pressure, and extremely dry ground in the region.

Largely due to high ocean temperatures, the jet stream crossing the Pacific slowed, stopped and distorted, creating concentrated regions of high pressure. In these regions of high pressure, air is compressed as it approaches the earth’s surface, creating heat. One of these systems stopped over western Canada, creating what we call a “heat dome”.

This has happened in various places around the world. Not only have there been heat waves in western Canada and the Pacific Northwest, but there have been heat waves related to the same mechanism in Scandinavia, Eastern Europe, the western Russia and northwestern Siberia at the same time.

What made the North American phenomenon different, however, was the fortuitous presence of another series of smaller atmospheric waves crossing the Pacific that merged with and amplified the high pressure systems caused by the blocked jet stream. .

This stalled system remained over western Canada and the northwestern United States for several days.

What has aggravated the situation is the aridity of the soil, which has dried up in recent decades due to the trend of rising temperatures caused by climate change.

Moisture, when present, serves to cool the soil surface through evaporation.

If you are in a desert with cans of hot beer, and you put a can in a wet sock and hang it on a tree, in no time the evaporation of the water in the sock will have cooled the beer.

In the same way, the evaporation of water from the ground cools the ground. In the absence of this moisture, the ground warms up, stays warmer, and adds heat to the air above it.

« It’s definitely part of the climate change signal, » says Mingfang Ting, a professor at Columbia University’s Columbia Climate School and co-author of the research. « And because of that, once you have the right conditions, the onset of an extreme heat event like this, then it starts to self-amplify through this earth-atmosphere feedback. »

« That’s where climate change comes in, because if the earth isn’t that dry, you won’t get an event of that magnitude. It’s a magnitude that’s completely unthinkable.

And unless there are major changes, these weather events are going to become more frequent.

Where in the 1950s the probability of all of these factors coming together could be characterized as a 10,000-year event, it now looks more like a 200-year event, Ting says.

And if global warming continues even at a moderate rate, the researchers conclude that by 2050, heat domes like the one that scorched Western Canada in 2021 and contributed to the wildfires that destroyed Lytton could occur. almost every 10 years.

“To tell the truth, we as scientists knew things were going to happen,” Ting says. “But how fast does it happen? How fast does this happen and show itself in front of us like this?

« It’s actually really scary, even for climatologists. »

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