Fires’ carbon contribution

Fires’ carbon contribution

12 November 2007

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USA/Canada — Researchers have long wondered what roleforests play in the carbon cycle — whether forest fires produce more carbondioxide than the trees capture. Now, two recent studies say that the matter isnot that straightforward: The relationship between forests and carbon dioxideemissions is complex, researchers say, and scientists must convey thatcomplexity to policymakers who are considering carbon dioxide mitigation andforest policies. Both studies suggest, however, that fires can release morecarbon into the atmosphere than the forest can capture and that this may be agrowing problem.

When much of Southern California was burning in late October, one of thequestions that arose was how much carbon dioxide those fires were sending intothe atmosphere. Every year, fires burn 3 million to 4 million square kilometersof Earth’s land surface area, and release more than a billion tons of carboninto the atmosphere in the form of carbon dioxide. However, massive old-growthnorthern latitude forests are also considered a carbon “sink,” becauseolder trees are repositories of decades or centuries of carbon and because theheavy canopy they produce blocks sunlight from reaching the forest floor anddecomposing the forest litter.

In the natural system, after a fire, new vegetation movesinto the burned land and over time reabsorbs much of the carbon dioxide that thefire had released. Such new vegetation stores the carbon as organic matter. Forthis reason, some people have suggested planting trees as a way to remove carbondioxide from the atmosphere. Eventually, however, those trees decompose or burn,thus returning carbon to the atmosphere. Left alone, terrestrial and atmosphericcarbon stay relatively in balance, researchers say. However, large or morefrequent fires, as can be exacerbated by warming temperatures and changingprecipitation levels, can change that carbon balance.

In a study published Nov. 1 in Nature, S. Tom Gowerand Ben Bond-Lamberty, forest ecologists at the University of Wisconsin atMadison, and colleagues examined fires in 1 million square kilometers of forestarea in central Canada that had occurred over the past 58 years. They modeledthe forest’s changing carbon balance using historical records of fires in thearea and records of changes in carbon dioxide and climate. They combined thatinformation with satellite imaging of the varying species and density of trees,and other details about the forest, such as soil composition, taken fromexisting databases.  

They found that of three variables they modeled — climatechange, change in overall atmospheric carbon dioxide and fire — the factorthat had the biggest impact on the overall carbon balance was fire. When firedestroys an area of forest, it forces the forest to start over. It sweeps awayeverything, including the old generation trees. The trees and plants that movein after a fire are often different species than the ones they are replacing,and the new, immature forest is not an immediate carbon sink. Furthermore,thinning the canopy can allow more sunlight to peak through, which speeds updecomposition and thus releases more carbon dioxide.

Gower and Bond-Lamberty also found that the incidence andseverity of the fires in the northern forest had increased over the period theywere studying. It increased most markedly in the final 10 years of their studyperiod, 1996 through 2005. The increasing frequency of fires was shrinking theold growth forest areas and replacing them with younger forests. “We foundthat parts of the forest had gone from a carbon sink to a carbon source,”Bond-Lamberty says. Younger forests do absorb carbon dioxide from the atmosphere,but they are not the same strongholds of carbon storage that older forests are,he says.

Gower and Bond-Lamberty added that climate change itselfdoes not directly affect a forest’s carbon balance, but continuing climatechange will ultimately lead to repercussions, they say. “[T]he fire regime isdriven by climate, and climate is affected by rising carbon dioxide, andthat’s affected by increases in fossil fuel combustion,” Bond-Lamberty says.

In another study, published in Carbon Balance andManagement Nov. 1, Jason Neff at the University of Colorado at Boulder andChristine Wiedinmyer at the National Center for Atmospheric Research in Boulderlooked closely at the amount of carbon dioxide emitted by forest fires in thelower 48 states. They wanted to estimate the contribution of forest fires to thetotal carbon dioxide emissions for any given state or region. These estimates,the researchers say, would be useful to monitors of carbon dioxide output and topolicymakers who need to know all of the different sources of carbon dioxide andwhich sources they must focus mitigation strategies on.

From 2002 to 2006, Neff and Wiedinmyer used two NASAsatellites to locate fires and gather data about the affected areas, such aswhat kinds of trees were burning, how much of the area was forest and how muchwas bare land. They found that in some regions, such as the western states,fires are becoming more frequent and severe. They attributed this trend in partto local year-to-year variations. But they say that the main reason fires areexpanding — and thus releasing more carbon dioxide into the atmosphere — isprobably fire suppression.

To suppress a forest fire, large crews of firefighterscreate long gaps in the trees (called firebreaks) to stop the fire fromprogressing. In the United States, fire suppression has been commonplace for thepast 100 years for reasons of self-preservation. More and more people have movedinto forested areas, putting yet more homes and lives at risk. Keeping a firesuppressed means keeping it away from those homes and lives.

Recently, however, some experts have begun advocating firesuppression as a way to reduce the carbon dioxide in the atmosphere. Firesuppression preserves trees, and trees absorb carbon dioxide. Protecting forestsfrom fire could thus be attractive to policymakers, Neff says. It is not thatdifferent from offset programs, many of which plant or regrow forests to helpabsorb that much more carbon dioxide. Policymakers have options in approachingcarbon dioxide emission reductions, but promoting and protecting forests wouldseem fairly inexpensive, he says. Policymakers might think, “We could alldrive Priuses, or we could try to increase forest growth to take carbon dioxideout of the atmosphere,” he says. But, he warns, protecting and growing forestsis a tradeoff. Fire suppression protects forests from burning, but at the sametime, increases the fuel supply for the next fire that will inevitably happen.

Although carbon dioxide from forest fires is a smallfraction of the carbon dioxide produced by burning fossils fuels, it’s still animportant issue to study, the researchers of both studies say. Fires do notcause or even significantly contribute to climate change, says Brian Amiro, headof the Department of Soil Sciences at the University of Manitoba, who designedmodels that influenced both studies. “Industrial emissions are the realproblem.” Still, he says, “analyses like these are important.” Theydemonstrate that the carbon balance cannot be reduced into a simple matter ofcause and effect.

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