Without a doubt the combination of unusually warm, dry and windy conditions in the region during May helped the fire to take hold and spread fast. But questions have also been asked about the role that global warming and landscape change due to exploration for oil and gas have played. Wildfires have always shaped the North American landscape, but some argue that human-influenced fires are now dominating in some areas.

To better understand the main triggers for modern wildfires, Marc-André Parisien from the Canadian Forest Service and his colleagues modelled fire probability between 1984 and 2014 across the US and Canada. Dividing the region into 16 identical-sized hexagonal polygons they investigated the probability of fire as a function of climate, landscape features (topography and amount of fuel cover), lightning strikes, population density, human activity and remoteness.

Their results uncovered a complex relationship between humans and wildfire. In general, the denser the population, the fewer the large wildfires. Partly this is because urban and agricultural areas have heavily modified landscapes and less fuel for wildfires to burn. But human vigilance also plays a role.

"The more people there are, the faster the fire ignitions are detected and acted upon," said Parisien, who published his findings in Environmental Research Letters (ERL). Intriguingly, some of the most remote and unpopulated areas bucked this trend, exhibiting some of the lowest probabilities of wildfire. However, these remote regions also tend to be less likely to have thunderstorms, reducing the chances of lightning strike, which is the most common ignition source for natural wildfires.

Parisien and colleagues also observed that some regions, including Alberta, are going through an "ecological frontier" transition, where industrial expansion (of oil and gas in the case of Alberta) into relatively flammable forested wild-lands has raised the risk of wildfire.

"A similar peak in wildfire activity followed the construction of major railways in North America in the late 18th and early 19th centuries," said Parisien. "These are 'transition' states, in that once humans are well established – or passed through – there is a decrease in area burned." Parisien and his PhD student François-Nicolas Robinne explore this in a forthcoming paper in the International Journal of Wildland Fire.

Climate change is also playing a role, with the results showing the probability of wildfire becoming greater in high latitude areas. "These areas have a greater frequency of hot, dry and windy days, which increases the potential for fires to ignite and spread," said Parisien.

Overall, the study concludes that there are few purely natural fire regimes in North America today, and that human impacts, whether landscape change, fire-suppression activities or climate change, heavily influence fire risk. Better understanding what influences wildfires in different regions, and the likely triggers, could help to reduce the risk of fire, via prevention programmes, fire suppression activities and modifying natural vegetation. The information could even be used to "restore" fire to some regions via controlled burning, in order to rebalance ecosystems that depend on periodic fires.

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