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The really scary thing about wildfires is how they can worsen climate change

This photo obtained August 23, 2013 courtesy of the US Forest Service, shows the Rim Fire as it burns near Yosemite National Park, California. AFP PHOTO / US FOREST SERVICE

The state of California, always bullish on trying to cut back on its greenhouse gas emissions, received a bit of a shock recently. It learned that the expanse of its drought-parched forested lands, when they go up in flames — and experts forecast a very bad fire season this year — may actually be contributing to global warming.

A 2006 law mandated that California would bring its emissions back down to 1990 levels by the year 2020. The target, though, assumed that the state’s forests — packed with carbon, stored in its biological form — wouldn’t contribute any net emissions.

But a study released last month by researchers from the National Park Service, the University of California, Berkeley, and several other institutions suggests that’s not a safe assumption. From 2001 to 2010, it found, the state had seen “a carbon stock decrease occurring largely in areas burned by wildfire.” And that helped drive the state’s ecosystems to contribute “as much as five to seven percent of state carbon emissions,” having lost some 69 million metric tons of carbon over the time period.

“In California, two-thirds of the carbon loss came from the six percent of the land that had burned in our nine-year period,” says Patrick Gonzalez, a researcher with the National Park Service and lead author of the study. Since the study period ended (in 2010), meanwhile, the state has seen several more megafires, including the gigantic 2013 Rim Fire, the third biggest ever in California’s history, consuming over 257,000 acres.

It’s not just California. Many forest scientists today are signaling a disturbing idea: That forests across the world could, like California’s, start to burn more, or burn in more devastating fires — ultimately contributing a volume of greenhouse gases that could be large enough to further stoke climate change. Which would be sadly ironic, in that these same scientists think climate change is making fires worse to begin with.

A worsening fire climate?

Here’s how it works: Just as growing plant life pulls carbon out of the atmosphere through the process of photosynthesis, so decomposing — or burning — plant life releases it back again. In the meantime, the carbon is stored in the plant, or in the case of forests, the trees.

In a climate in which wildfires are a steady, regular occurrence — but don’t change much in intensity or number from year to year — they will still release carbon, but the regrowth of forests and other plant life will also pull much of it back in again. “If climate and fire regimes equilibrate, then fire-induced atmospheric CO2 emissions are balanced by uptake from surviving vegetation or via regeneration,” noted a major 2009 study on the relationship between fires and the climate system.

But in a climate where there’s a change to the size, number, or intensity of wildfires, it’s possible that forests could burn and release carbon considerably faster than regrowth allows it to be replaced. Fire “has a substantial positive feedback on the climate system,” the 2009 study concluded.

“Fires have been part of ecosystems for 420 million years or more, and they’ve been a principal mechanism for recycling carbon,” explains William Sommers, a wildfire researcher at George Mason University. “But like everything else, with anthropogenic climate change, the acceleration of natural processes makes it a very difficult situation.”

One piece of the puzzle is already clearly in place: Scientists now say that wildfires are indeed worsening, and one of the reasons is climate change.

“We certainly know and have seen the wildfire extent — in other words, the numbers of acres of wildfire — increase dramatically since the late 1980s,”  says David Cleaves, the recently retired climate science adviser to the U.S. Forest Service. “The proportion of the area burned is increasingly in larger fires,” Cleaves continues. “So it’s not just the number of fires, as a matter of fact, they’re not actually increasing that much…it’s just that the distribution of fires in larger, higher severity events is increasing. That gives us the larger acreage.”

And it’s not only that. The bigger fires combust so much, scorch so much earth, that “there is, at least in a portion of these large fires…an area that is more difficult to regenerate,” says Cleaves. “And so if the area cannot regenerate, or we cannot get to it and reforest it, then it doesn’t get back into the carbon sequestration and storage game as quickly as it would otherwise.”

One region where wildfires could have a large climate impact is in the forest of the Arctic. The fear is of a future featuring more fires like the gigantic, “unprecedented2007 Anaktuvuk River fire that consumed 1,039 square kilometers of North Slope tundra, single-handedly giving off 2.1 million metric tons of carbon. And 60 percent of that, a team of scientists found in 2011, came not from vegetation but from the Arctic’s carbon-rich permafrost soils, which the fire had burned away.

I focused extensively on this “wildfire-permafrost” feedback in a 2013 Mother Jones story — but the problem of worsening fires, and their resultant impacts on how forests store carbon, extends far beyond the Arctic.

U.S. forests — from sink to source?

In the U.S. overall, the 2014 National Climate Assessment suggested that currently, 16 percent of our total greenhouse gas emissions from fossil fuels are in effect offset by carbon storage in forests and wood products each year. However, global warming and the loss of forested land is “projected to reduce this rate of forest CO2 uptake” in the future.

Depending on assumptions about future levels of emissions, the report found that by 2030 or even earlier, the nation’s forests could flip from being a net carbon “sink” (storing more carbon than they lose each year) to being a “source” of emissions. Most vulnerable, in general, are forests in the west, followed in turn by drier eastern forests and then finally wetter ones, which will likely be the least impacted.

“Right now we’re a significant carbon sink, and emissions from forest fires right now are relatively small,” says Robert Bonnie, under secretary for natural resources and environment at the Agriculture Department. “I think the more compelling thing to think about is the role of fire and more broadly disturbance going forward. Forest Service scientists predict that we could see a doubling of forest fire by mid century.”

To be clear — it’s not that fires, alone, could flip the U.S.’s forests into a net emitter. Various other types of land use changes are also at play. But worsening fires and other disturbances, such as those related to pests like bark beetles, are also an important side of the story.

The global picture is similar — but the magnitude is potentially greater.

According to one 2010 study, between the years 1997 and 2009 , half a billion metric tons worth of net carbon per year came from global fires (these were emissions that, the study said, “may not be balanced by regrowth following fire”).

The future could portend even more. In 2014, the U.N.’s Intergovernmental Panel on Climate Change noted that a “reduction in terrestrial carbon sink” could be a medium to high range risk over the course of this century, with an “increase in fire frequency due to climate change” as one key cause.  Fires are projected to increase in frequency on a third of the Earth’s land area, says Gonzalez, in regions ranging from the Amazon to Australia to northern India.

The fire future

So in sum, the problem of wildfires driving more and more carbon emissions in the future could be substantial. In this, it is similar to the related problem of thawing Arctic permafrost, which is also expected to begin to emit carbon dioxide and methane to the atmosphere and thus make global warming worse.

[The Arctic climate threat that nobody’s even talking about yet]

Fortunately, the size of the wildfire-climate feedback — in which wildfires drive warming, which drives more wildfires, and so on — is not considered to be of the same magnitude as the permafrost-climate feedback (in which warming drives permafrost emissions, which drives more warming, and so on). But as the numbers above suggest, it’s likely to be meaningful.

And in a world where we’re already not on course to keep emissions low enough to prevent a dangerous 2 degrees Celsius of warming above pre-industrial levels — and in which, accordingly, emissions policies will have to get tighter — the last thing we need is any new emissions sources.

Moreover, there are things we don’t know about other effects from changes to global fires. For instance, glaciologist and Greenland expert Jason Box has launched the Dark Snow project to study how the long traveling smoke plumes from fires may depose soot atop the Greenland ice sheet, darkening its surface and thus leading it to absorb more sunlight — and melt more.

There’s also the interaction with other kinds of forest disturbances, like bark beetle outbreaks, which can also have devastating impacts on trees and, thus, additional carbon consequences. “I think one of the phenomena that we’ll see more of in the future is the interaction between fire, and insect and disease, drought, and invasive species,” says David Cleaves. “Climate is kind of pushing along these different stressors of the forest and in some ways coupling them even tighter.”

There is, at least, one partial bit of good news here. Forest managers are not powerless in the face of stronger wildfires — there are things they can do to partly curb intense fires and, thus, their carbon emissions. It is generally considered, for instance, that many years of ill-advised “fire suppression” tactics have actually worsened the U.S. wildfire situation, by contributing even more to the volume of fuels that can combust in forests.

“We want to go back to the more natural, low intensity fires, that act to reduce fuels, and actually, for a lot of these dry pine types, help to regenerate and renew those forests,” says the USDA’s Robert Bonnie.

In the meantime, though, the U.S. stares down another fire year that is expected to be intense — not only in its toll on Forest Service budgets and potentially some communities, but also, perhaps, when it comes to its emissions.