THE boreal forests of Siberia, known as the taiga, account for about 30% of the world’s forests. Their role as a carbon dioxide sink is vital to the prevention of global warming. The taiga is the earth’s lungs. In recent years, however, frequent forest fires have been lowering the taiga’s capacity to absorb carbon dioxide from the atmosphere. Figure 1 shows how much forested land has been lost worldwide between 1964 and 1994. Although much is heard about deforestation in the Amazon region of South America, because forest regrowth is swift there, the cumulative loss of forests in the Amazon is no more than 10% of the loss in Russia (mainly Siberia). In the 30 years represented in Figure 1, forests covering an area 2.5 times that of Japan’s territory were destroyed in the northern regions of the Northern Hemisphere, and Siberia was the main site of forest loss. The main cause of deforestation was forest fires (Figure 2).
Forest fires in the Siberian taiga cause a whole range of environmental and ecological changes and the effects the fires will have on global warming are a great source of concern. Since 1998, a team of Japanese and Russian scientists including the author has been conducting research on the effects of Siberian forest fires on global warming.
Forest Fires and Carbon Dioxide
Because of the sheer vastness of the Siberian taiga, it is difficult to pinpoint the location of forest fires. Thus, we have developed a method of locating forest fires and measuring their expansion in real time using NOAA satellite image resolution technology. The AVHRR (Advanced Very High Resolution Radiometer) sensor detects unusual increases in the temperature of the earth’s surface and discriminates between clouds and forest fire smoke to detect and locate the outbreak of forest fires. Figure 3 shows the distribution of forest fires in far eastern Siberia in 1998 as identified by the NOAA AVHRR system.
Because the fires often break out simultaneously and many occur along roads and rivers, it is assumed that they are caused by human error or activity. According to Russian statistics, around 80% of forest fires are caused by man. The number of fires and the area burned varies year to year due to weather conditions, but it is estimated that the average annual loss over the past 20 years is around 3 million hectares.
Not only do forest fires release carbon dioxide directly into the atmosphere, but they also cause the functional loss of forest’s photosynthesis when the forests burn. Thus, forest fires decrease the amount of carbon dioxide that the taiga can absorb by both direct and indirect means. In order to quantify the decline in carbon dioxide absorption, our research team built an observation tower in the eastern Siberian taiga from which to record long-term changes in the carbon dioxide flux and the energy-mass flux (Figure 4).
We compared the carbon dioxide flux for forests before and after burning or cutting (Columns A and B in Figure 5) with undisturbed forests (Column C). Figure 5 shows quantitatively how disturbance of the forest ecosystem converts forests from net carbon dioxide sinks to net carbon dioxide releasers.
The upper layer of Siberia’s permafrost contains high concentrations of methane gas that was generated earlier in the earth’s history and locked in the frozen soil. Figure 6 shows the concentration of methane gas and carbon dioxide in air bubbles trapped in the upper part of the permafrost. While the concentration of methane gas in the atmosphere today is 1.8 ppm, methane concentration in air bubbles in the permafrost is several thousand times higher. Where this methane gas originated is not yet clearly understood, but scientists have recently discovered that methane-generating bacteria live in the 40,000-year-old permafrost and that the bacteria are capable of producing methane under the low temperature conditions found there.
It is possible that global warming will further activate these methane-producing bacteria and accelerate the rate at which methane is added to the atmosphere.
In the aftermath of a forest fire, the heat-balance at the ground surface is thrown into disarray, and large-scale thawing of the permafrost occurs. This releases large amounts of methane gas into the atmosphere. Methane has a much more powerful greenhouse effect than carbon dioxide.
The effects on global warming caused by forest fires in the taiga are cumulative and they continue long after the fires are extinguished.
Controlling Global Warming
The effects of Siberia’s frequent forest fires on global warming are revealed by Read More >>