As it drives tens of thousands from their homes and turns whole neighbourhoods to ash, the wildfire raging through Canada's oil-rich Alberta province is feeding on conditions that got their start thousands of kilometres away in the equatorial Pacific Ocean.

An El Nino in the Pacific disrupted weather patterns to bring northern Alberta a dry northern autumn last year and very little snow throughout the winter, said Daniel Thompson, a fire research scientist with Natural Resources Canada in Edmonton.

Then, in the last week, record temperatures with dry air and winds added to the tinderbox environment.

That's what you get with El Nino, and not just in Canada: the phenomenon's been blamed for blazes in Indonesia and high-risk conditions across the western US. "In 1997-98, we saw a similar sort of setup," Thompson said. "It's safe to say that weather played a major role in the fire here."

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ABOUT EL NINO
1 El Nino happens when the surface of the equatorial Pacific warms
2 That triggers a reaction in the atmosphere above the ocean, and disrupts weather patterns globally
3 It occurs at irregular intervals and with varying intensities.
4 The current one started in March 2015 and is unusually strong.


The area around the oil town of Fort McMurray, worst-hit by the fire, gets most of its rain in the summer months, so snow is important to give the ground a drink in spring when it melts off, Thompson said.

It's abnormally dry right now after various stages of drought since last year, according to the North American Drought Monitor, which is overseen by the US National Centres for Environmental Information in Asheville, North Carolina.

Weather often shapes the course of a fire, as well as providing the spark that starts it.

A combination of dry conditions and intense lightning storms in June 2008 started 1700 fires in California that cost the state more than US$100 million to extinguish.

The risks are high this year too, across much of the US Great Plains and further west. This time there's an additional culprit - the unusually wet weather last year, which allowed more plants to thrive.

Then came a mild winter and a rain-light spring, drying out the vegetation and leaving the region covered with potential fuel just waiting for a spark.

From January to April, about 607,029ha have burned across the US, according to the National Interagency Fire Centre in Boise, Idaho. California, Nevada and Idaho are all forecast to have higher than normal fire risks in the coming months.

In Canada, the fire risk in much of Alberta and neighbouring Saskatchewan is currently rated as extreme, the highest category a five-step danger scale, and the threat is forecast to remain high through August.

Alberta's weather conditions nudged the fire season two weeks earlier than usual, said Judith Kulig, a professor of health services at the University of Lethbridge in the province.

That's got people asking whether climate change is playing a role.

"There have been predictions that, as climate change progresses, there would be more disasters," said Kulig, who studies the impacts of such events on communities. "We know this past winter was dryer, we didn't have the snowpack, the conditions were ripe for this perfect storm."

Thompson said that scientists' models do predict climate change will alter the make-up of the forest and lengthen the fire season. But he said that attributing a specific fire like the one that's devastated Fort McMurray to climate change is "not really the scientific approach we would take."

Canadian Prime Minister Justin Trudeau made the same distinction between the general and the particular.

"It's well-known that one of the consequences of climate change will be a greater prevalence of extreme weather events around the planet," he said. At the same time, "There have always been fires. There have always been floods. Pointing at any one incident and saying, 'Well, this is because of that,' is neither helpful nor entirely accurate."

Thompson pointed to something else that's helped the spread of the fire - the amount of daylight. In Fort McMurray, more than 800km north of the US border, it starts getting light at around 4.45 am and doesn't get dark till 10 pm. With the sun in the sky longer than in, say, California, temperatures stay higher for more hours of the day and it keeps the air dry.

In terms of land area, the Fort McMurray fire isn't particularly large. In 2011, another fire raged about 50km north of the town that was more than 20 times larger. "It's by no means a record breaker," Thompson said. "But it doesn't take a lot of hectares burned to affect a lot of people."