NEW YORK • A relatively dry El Nino winter, a warm spring and years of policies that left forests ripe for burning have contributed to the destructive wildfires that forced the evacuation of Fort McMurray in Alberta province, Canada, scientists say.

Global warming may have played a role too. There is little doubt that it has affected the frequency and intensity of fires, and lengthened the fire season in Alberta as it has elsewhere in North America.

"The warmer it is, the more fire there is," said wildland fire expert Mike Flannigan from the University of Alberta, adding that in Alberta, the fire season now starts a month earlier than in the past.

In Fort McMurray, the number of heatwave days, when temperatures are higher than normal, has tripled since 1950, said hydrologist Stefan Kienzle at the University of Lethbridge in Alberta.

Higher temperatures dry out the underbrush and ground litter that can fuel a fire. Warm weather in the region this spring made the situation worse by melting snow earlier and making the underbrush easier to ignite, said Professor David Martell, a forestry professor at the University of Toronto.

Alberta had already been affected by the powerful El Nino that developed last year.

It had a drier autumn - a period when it normally gets a lot of rainfall - and was drier than usual this winter, said Mr Anthony Barnston, chief forecaster at the International Research Institute for Climate and Society. This meant it was already experiencing a water deficit when El Nino-related dryness set in.

Some experts also say fire-suppression policies have helped make fires in Canada more destructive.

Like many forest agencies in the US, those in Canada have long had policies to fight every fire, no matter how small. This increases the amount of fuel available so that out-of-control fires can become hugely destructive.

Within the past decade though, some agencies have adopted a policy of letting smaller fires burn which leaves a mosaic of thinned-out patches that helps halt the spread of larger fires.

But Mr Flannigan said this is difficult in Alberta as "there's almost no place that doesn't have significant development".

That applies to Fort McMurray, where the fire can be seen as another example of how development into what had been largely wilderness makes certain fires so destructive.

NEW YORK TIMES