The sprawling forests of the northern hemisphere which extend from China and Siberia to Canada and Alaska are in danger of becoming a gigantic source of carbon dioxide rather than being a major "sink" that helps to offset man-made emissions of the greenhouse gas. Studies show the risk of fires in the boreal forests of the north has increased in recent years because of climate change. It shows that the world's temperate woodlands are beginning to lose their ability to be an overall absorber of carbon dioxide.

Scientists fear there may soon come a point when the amount of carbon dioxide released from the northern forests as a result of forest fires and the drying out of the soil will exceed the amount that is absorbed during the annual growth of the trees. Such a prospect would make it more difficult to control global warming because northern forests are seen as a key element in the overall equations to mitigate the effect of man-made CO2 emissions.

Two studies published today show that the increase in forest fires in the boreal forests – the second largest forests after tropical rainforests – have weakened one of the earth's greatest terrestrial sinks of carbon dioxide. One of the studies showed that in some years, forest fires in the US result in more carbon dioxide being pumped into the atmosphere over the space of a couple of months than the entire annual emissions coming from cars and energy production of a typical US state.

A second study found that, over a 60-year period, the risk of forest fires in 1 million sq kms of Canadian wilderness had increased significantly, largely as a result of drier conditions caused by global warming and climate change. Tom Gower, professor of forest ecology at the University of Wisconsin-Madison, said his study showed that fires had a greater impact on overall carbon emissions from boreal forests during the 60-year period than other factors such as rainfall, yet climate was at the heart of the issue.

The intensity and frequency of forest fires are influenced by climate change because heatwaves and drier undergrowth trigger the fires. "Climate change is what's causing the fire changes. They're very tightly coupled systems," Professor Gower said. "All it takes is a low snowpack year and a dry summer. With a few lightning strikes, it's a tinderbox," he said.

Historically, the boreal forests have been a powerful carbon sink, with more carbon dioxide being absorbed by the forests than being released. However, the latest study, published in the journal Nature, suggests the sink has become smaller in recent decades, and it may actually be shifting towards becoming a carbon source, Professor Gower said. "The soil is the major source, the plants are the major sink, and how those two interplay over the life of a stand [of trees] really determines whether the boreal forest is a sink or a source of carbon," he said.

"Based on our current understanding, fire was a more important driver of the carbon balance than climate was in the past 50 years. But if carbon dioxide concentration really doubles in the next 50 years and the temperature increases 4C to 8C, all bets may be off."

The second study, published in Carbon Balance and Management, found carbon dioxide emissions from some forest fires exceeded the annual car and energy emissions from individual US states.

Christine Wiedinmyer of the US National Centre for Atmospheric Research in Boulder, Colorado, used satellite imaging datato estimate CO2 output based on the degree of forest cover in a particular area. In some years, the amount of CO2 released from forest fires was equivalent to about 5 per cent of the man-made total. But in other years, more widespread and intense forest fires resulted in massively increased emissions. "There is a significant potential for additional net release of carbon from forests of the United States due to changing fire dynamics in the coming decades," Dr Wiedinmyer said.

(By Steve Connor, The Independent, 01/11/2007)