In the fight against climate change, forests play a critical role — drawing more greenhouse gases out of the atmosphere than they emit.

But when they burn, much of those stored gases are released back into the atmosphere.

So far, the fires in Fort McMurray have released the equivalent of roughly five per cent of Canada’s annual greenhouse gas emissions, said Werner Kurz, a senior research scientist with the Canadian Forest Service in charge of Canada’s National Forest Carbon Accounting System.

The average emissions from forest fires in the boreal plains, where the northern Alberta fires are burning, are about 170 tonnes of carbon dioxide equivalents per hectare, Kurz said.

Multiply that by 239, 390 hectares, the size of the Fort McMurray fire on May 11, and the fire has already released about 41 megatonnes of CO2 equivalents in the form of carbon dioxide, methane, and nitrous oxide.

In 2014 Canada’s total greenhouse gas emissions were 732 megatonnes, excluding emissions from wildfires and other land use, land-use changes, and forestry activities.

The fires in Fort McMurray have already covered about 10 per cent of the average territory burned by wildfires every year in Canada — a significant feat for one fire so early in the season, said Bill de Groot, a research scientist at the Canadian Forest Service’s Great Lakes Forestry Centre.

On average, wildfires burn two million to 2.5 million hectares a year in Canada, he said.

“So it’s a really large amount for the very start of the fire season and there’s still a full fire season ahead of us, so it could potentially be a bad year if we don’t get some rain,” de Groot said.

But to really understand the climate impact of the fires you have to take a longer-term view de Groot said.

For 6,000 years boreal forests have operated on a cycle of growth, fire, and re-growth, said Merritt Turetsky, an ecosystem ecologist at the University of Guelph and a Canada Research Chair.

“Basically, what we know is that black spruce proliferated into the boreal forests around that time and fire came with it,” she said.

Over a 100-year period, forests store about the same amount of carbon, de Groot said, with slight fluctuations in the intervening years due to the fire-growth cycle.

But climate change is upsetting this millennia-old balance.

“Because of climate change we will be getting more fire and more fire means more carbon going into the atmosphere. So it never quite reaches that steady state balance, there’s always a little bit more going into the atmosphere,” de Groot said.

But, fire also causes the ecosystem to “reorganize” and “that reorganization post fire can actually … tip the balance toward net cooling,” Turetsky said.

While most of the boreal plains is made of dark coloured conifer trees, that absorb more energy from the sun, severe fires can allow lighter coloured deciduous trees to sprout up instead, which “bounces more energy back to the atmosphere and causes less ecosystem warming,” Turetsky said.

While this may be good news for the climate, Turetsky said it could have significant consequences for wildlife that rely on coniferous forests.

Regardless of which tree comes out on top, it will take at least a decade before the forests destroyed by the fires in Fort McMurray are storing more carbon than they’re releasing.

Both Kurz and de Groot said tree planting could help speed up forest re-growth, helping them reach carbon-sink status more quickly, especially if faster growing species like aspen are planted.

In a statement, Agriculture and Forestry Minister Oneil Carlier’s press secretary Renato Gandia said: “While reforestation is obviously an important issue to address, we are currently still fighting the largest forest fire in the province. We’re also still trying to help the evacuees. Once we are through this crisis period we can turn our attention to other matters.”