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A wall of fire rages outside of Fort McMurray, Alta., Tuesday May 3, 2016. Raging forest fires whipped up by shifting winds sliced through the middle of the remote oilsands hub city of Fort McMurray Tuesday, sending tens of thousands fleeing in both directions and prompting the evacuation of the entire city.

The Canadian Press

Over the past 30 years, human-caused climate change has nearly doubled the amount of forest area lost to wildfires in the western United States, a new study has found.

The result puts hard numbers to a growing hazard that experts say both Canada and the U.S. must prepare for as western forests across North America grow warmer and drier and increasingly spawn wildfires that cannot be contained.

"Climate change is playing a substantial role in the variability of fire activity… and we expect that to continue into the future. The question is how are people going to respond to that," said John Abatzoglou, a climatologist at the University of Idaho and lead author of the study published Monday in the Proceedings of the National Academy of Sciences.

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Opinion: What 'Fort McMurray Strong' means to me

Related: Inside Fort McMurray: Helicopter crews battle wildfires to save the city's infrastructure

Related: Residents forced to flee Fort McMurray had to make a quick decision: what to bring

In their analysis, Dr. Abatzoglou and a collaborator aimed to tease out how much climate change, primarily due to fossil-fuel emissions, has contributed to the walloping ninefold increase in fire activity recorded in western states between 1987 and 2015. That meant they had to disentangle climate change from other effects such as the legacy of fire suppression that has left larger areas available for burning and the expansion of human settlement and presence in the west which has increased the number of fires started.

They also had to account for natural climate variability, which in some years would be expected to cause higher-than-average fire seasons even if the planet was not getting warmer on average.

Once they were able to combine all of the effects into a mathematical analysis that reproduced the amount of burning that has actually occurred they were able to remove climate change from the equation and look at the difference.

Overall, they found that the amount of forest area that was dry enough to pose a high risk of wildfire grew by 75 per cent because of climate change, while the length of time during which there was a potential for wildfires grew by nine days per year.

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A key factor was the dramatic increase in the average dryness of the forest material that fuels wildfires. The analysis showed that just over half of that increase can be chalked up human-caused climate change.

Mike Flannigan, a fire scientist with the University of Alberta who was not involved in the study, said that while U.S. and Canadian forests differ in their make-up and in the amount of opportunity for human-ignited wildfires, the analysis is relevant to Canada.

"If you pull back the curtain, it's really the extreme fire weather that's driving this change in area burned, and that's the common element between Canada and the U.S." he said.

In Idaho, the change has been more than academic, said Dr. Abatzoglou, who described the impact on air quality at the university's Moscow, Idaho, campus last year from fires raging upwind in Washington State.

"I can still taste the smoke," he said.

Yet, he added, asking westerners to endure more smoke-filled days might be one way to try to mitigate the growing climate-induced risk. If more wildfires are allowed to burn during summers that are relatively moist, it may reduce the likelihood of catastrophic fires springing up in summers that are extremely hot and dry.

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The study is the latest to show how climate change is amplifying the impact of wildfires and setting up conditions like those that destroyed homes and forced the evacuation of Fort McMurray, Alta., earlier this year.

In 2014, a team of U.S. researchers showed that for every one degree Celsius of climate warming, the number of lightning strikes that can ignite wildfires increases by about 12 per cent.

Dr. Flannigan, whose research focuses on the interplay of climate change and wildfire, says he is now studying how fire intensity has increased as the west grows warmer and drier. More intense fires pose a greater threat because their energy output can exceed the threshold beyond which fire-fighting methods, including water bombing, are ineffective. Such fires are almost impossible to contained and typically burn unchecked until there is a change in weather or in available fuel.

"We're finding that we're going to be exceeding these thresholds a lot more in the future, so fire management is going to become even more challenging that it is now," Dr. Flannigan said.

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