Scientists confirm fears that Alaska wildfires could make global warming worse

In not much more than a month, leaders from around the world will assemble in Paris in order to -- hopefully -- find a way to cap the world's greenhouse gas emissions and bring them down to safe levels.

But there's a problem. There are some greenhouse gas sources that these leaders can't fully control -- and in some cases, reasons to think that these sources may grow in the future. The point is being driven home this year by raging peat fires in Indonesia, which have already contributed more than a billion tons of carbon dioxide equivalent emissions to the atmosphere -- as much as Japan produces in a year from fossil fuels. And the blazes still appear to be on the rise, meaning that the net contribution this year could ultimately be considerably higher than that.

Indonesia isn't the only part of the world where fires -- which in many areas are expected to be worsened by climate change -- could provide a new net source of emissions to the atmosphere. Another region of major worry is the world's so-called "boreal" or northern forests, which store a gigantic amount of carbon in trees as well as in soils and frozen permafrost layers beneath the surface. Permafrost in this region is in many ways analogous to peatlands in Indonesia -- it's a repository of carbon that has accumulated over thousands of years but could now be released back to the atmosphere on a much shorter time scale.

Alaska's dramatic wildfire season this year -- where more than 5 million acres of largely black spruce forests burned -- raised great concerns about how events such as this could make global warming worse. The fear here is of a sort of triple whammy -- forests release the carbon stored in trees back to the atmosphere when they burn; the forests contain a deep upper soil layer that also burns off, releasing more carbon; and finally, beneath all of that is the carbon-rich permafrost, which becomes exposed after fires and can then thaw and start to emit.

And now, a study in Nature Climate Change reaffirms these concerns about the emissions of northern fires. The study, led by Ryan Kelly of the University of Illinois at Urbana, looked at a particular Alaska region that has seen intensive burning of late -- the remote Yukon Flats. The researchers used an ecosystem model to examine changes in the amount of carbon stored in the Yukon Flats going all the way back to the year 850 and carrying forward through 2006 -- a more than thousand-year period. The data used in the model came from "paleo-climate" reconstructions of what burn conditions were like in this area over large periods of time, based on charcoal layers found in cores of sediments extracted from the region.

This approach allowed the researchers to confirm that the recent burning in this area is dramatic when compared with its past history -- and thus that recent fires have been releasing much of the carbon that has been stored over hundreds of years. For the Yukon Flats, "this rapid increase in fire activity has led to pretty major losses of carbon from the ecosystem, on the order of 10 to 12 percent of total carbon stocks in a matter of 5 decades or so," said Kelly.

In addition, the researchers also determined that over time, change in fire patterns were by far the largest factor in how much carbon the ecosystem stored. In fact, the study noted, "long-term C [carbon] dynamics of the past millennium were almost entirely dictated by patterns of fire-regime variability." In other words, in more fire-intense periods, the forests lost a great deal of carbon to the atmosphere, whereas in less fire friendly periods, they stored it instead.

This, in turn, leads to the inference that with more global warming, more forest burning could worsen a process that's already underway. "Our study reveals that increased burning of boreal forests will probably cause massive losses of stored C, with the potential to amplify climate warming," the researchers concluded.

"I would definitely guess, and we speculate this in the paper, that the direct role of warming and rising CO2 will be more important in the future than they were over the 1,000 year time period we studied," said Kelly.

Granted, there is one limitation -- the research only concerns the Yukon Flats, and this area has seen quite dramatic burning, at levels that are not consistent with all burning across the forests of the global north. So it's an outlier in some ways, but also suggestive of how things could be heading for northern forests.

To be sure, there is one possible mitigating factor -- global warming could also even cause more plant growth in northern and Arctic regions, leading to an increase in how much carbon they're pulling out of the atmosphere. More northern trees, bushes, and even shrubs as tundras warm up could mean more carbon storage, even as fires may also rage at more powerful and extreme rates.

Climate researchers are still trying to figure out how all of these factors will interrelate. But the new research -- along with developments in Indonesia -- reaffirms that fire is a powerful determinant of how much carbon resides in land, rather than in the air, across our globe. Thus, even as we keep burning fossil fuels, fires may continually combust additional planetary carbon reserves -- pushing the globe that much closer to busting its carbon budget.

Chris Mooney

Chris Mooney covers climate change, energy, and the environment for The Washington Post. He has reported from the 2015 Paris climate negotiations, the Northwest Passage, and the Greenland ice sheet, among other locations, and has written four books about science, politics and climate change.