Robert Ziel has seen his share of wildfire seasons, but 2015 in Alaska was something different.

“The problem grew so rapidly,” he tells Inverse. “The ignitions, many of them came over basically a one-week period around the solstice. Lighting day after day after day ended up producing nearly 300 fires in that week of time.” Many of those fires grew and threatened towns, he says, adding, “Our ability to respond was overwhelmed almost immediately.”

Alaska called in crews from the Lower 48 and from Canada — hotshot crews, smokejumpers, helicopters, and air tankers.

Despite a NASA prediction, the 2015 season came just short of the 2004 record for number of acres burned. But for the sheer speed of forests lighting up, Ziel says, this season may be unprecedented.

The Alder Fire burns in Yellowstone National Park in 2013.

While wildfires threaten surrounding communities, they also pose a problem for the planet. The boreal forest, a vast ecosystem that covers much of Alaska, Russia, Scandinavia, and Northern Canada is one of the world’s great carbon sinks. The millions of plants pull carbon dioxide from the atmosphere through photosynthesis, and because the soil is boggy and cold, the organic matter accumulates as peat instead of decomposing and returning to the air.

But wildfires release this trapped carbon in large quantities. With climate change increasing the frequency and severity of fires in the boreal ecosystem in particular, the forest could one day switch from carbon sink to carbon emitter. Some scientists say it already has.

Areas in red have burned since 1950. The white rectangle indicates the location of the Yukon Flats.

A new study published in Nature Climate Change found that the Alaska’s Yukon Flats have released 12 percent of their stored carbon since 1950 thanks to a dramatic increase in wildfire frequency.

Wildfire records for Alaska only go back to 1939, making it tough to model their climate. With no earlier data, modelers are left to assume that fire seasons of recent decades are representative of how they have been in the past. Of course, they likely are not. Global warming has already significantly affected the planet, particularly at higher latitudes.

Lead author Ryan Kelly wanted to tackle the problem of how badly the models were getting things wrong, so he collected core samples at the Yukon Flats, an 11,000 square mile patch of protected wetland and forest in Eastern Alaska, to retrieve fire frequency data going back 10,000 years.

“We thought hey, we could finally put some numbers on how big of a deal this is, and it turned out kind of even more dramatic than we thought,” he tells Inverse.

When he ran a climate model without including the old fire data, it looked as if the area continued to be a carbon sink — sequestering more organic matter than it releases, on average. But when the changing fire regime was taken into account, the model suggested that the Yukon Flats have released a significant amount of stored carbon in recent fires.

The Michigan Army National Guard fights a forest fire by helicopter in 2007.

The Yukon Flats may be exceptional. Certainly the fires in that area over the last few decades have been dramatic. Kelly found that the region has not seen so many fires in at least 10,000 years.

But the Yukon Flats could also say something about the future. “We specifically wanted a place that was burning a lot already because basically any prediction you look at suggests that boreal forests throughout the biome are going to burn more in the future as the climate warms,” Kelly says.

His research did not attempt to determine whether the boreal forest today is a sink or a source of carbon. But it does suggest that earlier calculations may have overestimated how much carbon is being stored, since they mostly do not account for increasing fire regimes. It also suggests that as fires become more frequent and more severe in the boreal, the ecosystem may shift from sink to source.

A wildfire burns in Idaho.

Mike Flannigan, a researcher at Canada’s University of Alberta, thinks it already has. “I’d wager quite strongly that you punch in the numbers, and the papers that are out there, there’s more papers arguing for carbon source than carbon sink for our boreal forests,” he tells Inverse.

The carbon emissions from an intense wildfire season can be enormous. A single year of burning in Indonesia produced the equivalent of perhaps four months of the world’s fossil fuel emissions, according to a paper in Nature.

“The boreal has 30 times more peat than Indonesia has,” says Flannigan. “Our boreal forests dwarf Indonesia.”

A comprehensive review paper published by NRC Research Press found that between 1990 and 2008 the Canadian boreal forest was, on average, a carbon sink. But that wasn’t true in big fire years, like 1995, 1998, and 2002.

In years where the black square falls below the dotted line, the Canadian boreal forest released more carbon than it absorbed.

Lately, Canada has seen a lot of wildfire. “We had three really severe fire seasons in Canada — 2013 in Quebec, 2014 Northwest Territories, 2015 in Saskatchewan and BC,” Flannigan says.

There are three ways that a warming climate causes more fire, Flannigan explains. For one, it lengthens the fire season, meaning more chances for more fires. Secondly, warmer air produces more lightning. More lightning means more fires.

Finally, warm air holds more humidity, so it pulls more moisture from the forest, making it easier to burn. This could potentially be offset by increased rainfall, although according to one recent paper it would take a 15 percent increase in precipitation during the summer months to offset the drying effect a one degree Celsius rise in temperature, says Flannigan. Almost nowhere in the boreal will this be the case.

Determining exactly how much carbon is being sequestered or released from the boreal is a tricky proposition. But the trend is quite clear. Wildfires are going to get more ferocious. The boreal forest — the great carbon sink — will likely become an emitter of greenhouse gasses, if it is not already. More warming, more fires, more emissions, more warming.

Welcome to the future.