According to Forrest Hall, boreal fires can affect climate in two broad ways: (1) by changing the carbon balance, and (2) by changing Earth’s radiant energy balance. The boreal carbon cycle is regulated by four processes: (1) the rate of plant growth, which determines how much and how fast plants absorb carbon dioxide from the air during photosynthesis; (2) the rate of decomposition of dead biomass, which releases carbon dioxide back into the atmosphere; (3) the rate of formation of frozen soil (called "permafrost"), which prevents the organic matter in the soil from decomposing; and (4) the frequency and intensity of fires, which release carbon, methane, and aerosol particles into the atmosphere. (Kasischke et al 1995)

Tipping the Carbon Balance
Over geologic time, as tree litter fell to the forest floor, boreal soils became a carbon-rich sink. An estimated 231 petagrams (231 billion metric tons, or 43 percent of the world’s total) of carbon is stored in boreal soils, while another 58 petagrams (58 billion metric tons, or 13 percent of the world’s total) of carbon is stored in the live vegetation (Kasischke et al 1995). (For more details, please see "The Mystery of the Missing Carbon.")

"For the past 7,000 years, the boreal forest floor has been accumulating carbon at a rate of about 30 grams (or roughly 1 ounce) per square meter per year," observes Hall. "When you walk through the boreal forest, you can literally go from ankle deep to in over your head in carbon litter."
 

Fires in the boreal forest have a complex effect on climate. For example, they release carbon dioxide into the atmosphere, which has a warming effect, but also contribute aerosols, which reflect sunlight and cool the surface beneath. (Photograph Courtesy Forrest Hall, NASA Goddard Space Flight Center/University of Maryland)

Hall shares the concern of many Earth scientists that if the frequency and areal extent of wildfire in the boreal forests should increase, then that ecosystem could shift from being a net sink to a net source of carbon. In turn, this new net source of carbon released into the atmosphere could significantly contribute to global warming. Of particular concern to scientists are the fires that get so hot they burn deeply into the soil, releasing the carbon stored there. According to Hall, it only takes a change in the storage of roughly 50 grams of carbon per square meter per year to release a billion tons of carbon.

"All long-term carbon is stored in the ground moss, duff (decayed organic matter in the soil), and root production," Hall explains. "When the moss and duff are wet, they act as a surface fire retardant. But when they are dry, they catch fire easily; the moss acting much like a mattress fire, smoking and smoldering as it spreads and then flaming up when it hits a tree. In a drought year, the boreal forest floor dries quickly—within a few weeks—down to the water table (about 1 meter deep). When the moss and duff burns, it produces this unbearably thick, dingy smoke."

In addition to fire’s direct effects of releasing carbon dioxide, Kasischke points to another, longer term effect fire has on the carbon cycle. "Organic soils and mosses conduct heat five to ten times less efficiently than mineral soils," he explains. "When those layers are consumed by fire it’s like removing an insulating blanket, so the efficiency of heat transfer improves tremendously."

In the wake of fire there is no longer a forest canopy to shade and cool the surface. Uninsulated by moss and organic layers of topsoil, the boreal forest floor warms and dries more readily in the arid summer air, thus accelerating the rate of decomposition. The boreal soil is converted into a longer-term (beyond the immediate effects of the fire) source of carbon until, decades later, the forest regenerates there and begins to revert back into a carbon sink.

Some scientists counter that although increased fire activity would release more carbon into the atmosphere, the affect on climate would only be a short-term one. They argue that the presence of fire stimulates vigorous new plant growth and, therefore, in the longer term the boreal forest would become a more efficient carbon sink than it is now. (This debate is further addressed on the next page.)
 

Crown fires like this one are the most destructive. They are often hot enough to burn the soil (peat), releasing stored carbon dioxide into the atmosphere. It takes hundreds or thousands of years of growth to replenish the soil after a severe fire. (Photograph courtesy Brian Stocks, Canadian Forest Service)

Unbalancing the Radiation Budget
In addition to affecting the carbon cycle, wildfire also impacts the boreal region’s radiant energy budget. Clearing away the forest canopy dramatically changes the albedo (amount of reflected sunlight) for a region, particularly in the fall, winter, and spring when there is snow on the ground. (For more details, please see "Should We Talk About the Weather?") The BOREAS team demonstrated that changing albedo over such a large region has a noticeable effect on weather.

Snow on the ground reflects about 80 percent of the sun’s light and absorbs 20 percent (Betts 1999). The more sunlight that is reflected back up into the atmosphere, the cooler the surface temperatures. In contrast, conifer trees (e.g. spruce and pine) reflect only about 10 percent of the sun’s light and absorb the rest, which warms the surface and, in turn, the lower atmosphere (Betts 1999). These changes also influence wind currents, the formation of clouds, and precipitation patterns across the boreal region.

Moreover, due to the smoke particles released, fire has an added cooling effect. Aerosol particles tend to cool the area beneath them by reflecting and scattering incoming sunlight, as well as by promoting increased cloud formation. Overall, however, because greenhouse gases remain in the atmosphere much longer (years to decades) than aerosols (days to weeks), scientists think that the warming effect of fires will have a greater effect over the long term.

 Will Climate Change Lead to More Boreal Fires?
 A Widespread Problem

Burn scars change the local energy balance, especially when there is snow cover. Energy from sunlight, which would be absorbed by the dark trees in a mature forest, is reflected by the bare snow that lies atop burn scars. (Photograph Courtesy Forrest Hall, NASA Goddard Space Flight Center/University of Maryland)