Will Climate Change Lead to More Boreal Fires?   Page 4

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 world’s 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 planet’s recent history, going back millennia.


"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 environment—BOREAS 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 strikes—one of the foremost causes of boreal fires—would 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 shouldn’t both the fires and the warmer climate stimulate increased plant growth, thereby enhancing the boreal forest’s 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.

Fire Plumes
Fires are already widespread throughout the boreal forests. Will warming temperatures increase the number and severity of these fires, potentially accelerating the rate of change? Researchers at NASA and elsewhere are using satellite data, computer models, and field work to find out. (Photograph Courtesy Forrest Hall, NASA Goddard Space Flight Center/University of Maryland)




Several times more carbon is stored in the organic soils of the boreal forest than in living plants. More fires will likely reduce the amount of carbon being stored by the boreal forests, especially if they are intense enough to burn the ground layer. This graph shows the amount of carbon stored in a typical region of the Alaskan forest relative to its age. (Graph by Kasischke, Christensen, and Stocks)


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 year—a role it has played for millennia—to 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 NASA’s 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 Earth’s 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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