Climate change made B.C. floods at least twice as likely, study suggests

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Catastrophic floods that swamped much of southern British Columbia last fall were at least twice as likely because of climate change, suggests new research from Environment Canada.

The study, now undergoing peer review, concludes that the likelihood of similar events in the future will only increase as global warming continues to upend normal weather patterns.

“We do find substantial ongoing increases in the probability of these kinds of events,” said Nathan Gillett, an atmospheric physicist and manager of the Canadian Centre for Climate Modelling and Analysis.

In November, B.C. saw three events come together to create unprecedented flooding.

A so-called “atmospheric river” brought two days of drenching rain. It fell on already-sodden soil that couldn’t absorb much more and was augmented by high temperatures that swelled bursting stream beds with snow melt.

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The result was almost 15,000 people forced from their homes, major roads and bridges washed away and farms flooded in up to two metres of water. Landslides killed at least five people.

Insured losses have been estimated at $450 million; 600,000 chickens and 12,000 hogs died.

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Gillett and his colleagues wanted to estimate the contribution of climate change to the disaster while it was still fresh in public memory. They turned to the science of climate attribution, which uses climate models to estimate the influence of one or more factors on weather events.

“We compared simulations with human influence and compared them with simulations without human influence,” he said.

The team worked with a group at Oxford University doing similar research. To ensure the results weren’t influenced by quirks in any one model, the team used more than two dozen different ones. The results were consistent across all of them.

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“All of the models show an increase in this kind of event in response to human-induced climate change.”

The team concluded that climate change had increased the odds of an atmospheric river like the one that swamped B.C. by at least 60 per cent.

“There’s higher odds of this kind of atmospheric river event occurring now than there was back in the 19th century,” Gillett said. “Where there were two events before, now there’s three.”

When the scientists factored in the other contributors to the disaster, they concluded that the odds of what happened to B.C. had been at least doubled by climate change, and may have been quadrupled.

And that’s for the current climate, already affected by climate change. The chance of another catastrophe continues to increase as greenhouse gases keep entering the atmosphere.

“We expect that to increase as the climate continues to warm,” Gillett said.

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Evidence keeps mounting that climate change is already causing damaging and extreme weather. Last summer, researchers at Oxford concluded the heat dome that brought unprecedented temperatures to B.C. and set the table for a wildfire that destroyed the village of Lytton would have been all but impossible without climate change.

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Gillett said, if nothing else, the conclusions point to the need to rebuild roads and buildings that are able to withstand more severe weather than in the past.

“It’s important to consider when we’re rebuilding infrastructure how the risks of these kinds of events are increasing, and taking that into account.”

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