|How Boreal Fires Impact Global Climate|
According to Forrest Hall, boreal fires can affect climate in two broad ways: (1) by changing the carbon balance, and (2) by changing Earths 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
"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."
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 quicklywithin a few weeksdown to the water table (about 1 meter deep). When the moss and duff burns, it produces this unbearably thick, dingy smoke."
In addition to fires 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 its 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.)
Unbalancing the Radiation Budget
Snow on the ground reflects about 80 percent of the suns 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 suns 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?|
The Intergovernmental Panel on Climate Change (IPCC) concluded recently
that "the observed increase in global mean temperature over the last
century (0.3-0.6°C) is unlikely to be entirely due to natural causes, and
that a pattern of climate response to human activities is identifiable in the
climatological record" (IPCC 1995). The warming trend over the last 100
years has been most dramatic at high northern latitudes, resulting in
temperature increases of 2-3°C in the worlds boreal regions (Stocks
et al 1998). Moreover, many General Circulation Models predict that the rate of
global temperature increase will accelerate over the next century, by anywhere
from 0.8 to 3.5°C warmer than today (Stocks et al 1998). Such a rate of
change would be unprecedented in our planets recent history, going back
"One speculation is that as it gets warmer, the boreal forest will get drier," stated Hall. "If this happens, you could see an increase in fire frequency."
Another hypothesis suggests that as the atmosphere warms, it will hold more moisture and precipitation will increase, thereby reducing fire frequency. But to investigate this alternative hypothesis, scientists need better models of the interactions between the land surface and atmosphere. But it seems unlikely that the increase in precipitation would be enough to eliminate the problem. Kasischke points out that, despite appearances, the boreal ecosystem is actually a very dry environmentBOREAS investigators dubbed it the "green desert." He says the Alaskan boreal forest interior only receives about 25 cm (10 inches) of precipitation per year.
Some fire scientists use computer models to help them understand and predict how climate change is likely to affect the frequency and extent of wildfires in the next century. (For background information, please see "Modeling the Land Biosphere.") They constructed a hypothetical scenario in which the levels of carbon dioxide in the atmosphere doubled pre-industrial levels of the greenhouse gas over 100 years ago. The models produced some alarming results. In that scenario, the danger of wildfire outbreak across the North American boreal forest would increase by 50 percent, the length of the fire season would increase by 30 days, and the frequency of lightning strikesone of the foremost causes of boreal fireswould increase significantly (Stocks et al 1998).
According to AVHRR data, 1987 and 1998 were severe fire years in North America. Not coincidentally, those were also severe drought years. Scientists estimate that in the 1970s, roughly 1.5 million hectares of boreal forest burned annually in North America, while today an annual average of 3.2 million hectares burns. But in years like 1998, upwards of 15 to 20 million hectares (150,000 to 200,000 square km) of boreal forest may burn (Kasischke et al 1999). These statistics, together with the fact that the areal extent of burned regions has been steadily increasing in recent decades, paint an ominous picture for a future that could include global warming.
"When we compared AVHRR satellite images of Russian and North American boreal fires," Kasischke states, "we noticed they have an 'episodic nature'. They are not uniformly distributed (over time or space). Most of the burning tends to occur in severe years, like we saw last year.
"Looking at fire activity in the North American boreal forest over the last 30 years, most of the burning occurred during seven (of those) years. During the most severe years, 5.5 million hectares burned, while only 1 million hectares burned during the other 23 years."
Scientists are concerned that a continued warming trend would increase the frequency of severe boreal wildfire years and, due to the accelerating rate carbon of dioxide emissions, a "positive feedback loop" could occur in which each trend drives the other. But shouldnt both the fires and the warmer climate stimulate increased plant growth, thereby enhancing the boreal forests ability to act as a carbon sink?
Yes, says Hall and his colleagues, but the rate of carbon storage is likely
to be slower than the rate of its release. To date, both experimental and model
data suggest that there is no way the plants can absorb carbon as quickly as
fire and decomposition can release it from the soils where it has been stored
for thousands of years.
Moreover, in our lifetimes we might witness the transformation of the boreal ecosystem from a sink of 1-2 billion metric tons of carbon per yeara role it has played for millenniato a significant source of carbon. Models indicate that a global warming scenario, combined with the increased presence of fire, would yield a net transfer of 27 to 52 petagrams (27 to 52 billion metric tons) of carbon from the boreal ecosystem to the atmosphere over the next century (Kasischke et al 1995).
The BOREAS scientists are eagerly awaiting the launch of NASAs Terra spacecraft. They say that, in particular, the Moderate-resolution Imaging Spectroradiometer (MODIS) instrument provides them the multi-spectral and multi-temporal resolution they need to monitor the boreal ecosystem over the next decade. The new sensor is expected to not only dramatically improve scientists ability to measure the extent of fires globally, but it also help them quantify the effects fires have on the Earths carbon and radiation budgets. With MODIS data, they hope to begin answering some of the burning questions raised by an increased presence of wildfires in the boreal forest.
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