Early springs show Siberia is warming fast
by Catherine Brahic, New Scientist, August 1, 2007
Siberia is experiencing earlier springs, a study of satellite images has revealed. The trend is likely to be triggering more forest fires, say researchers, and to be linked to global warming.
In a study of a wide range of Siberian ecosystems, Heiko Baltzer of the University of Leicester, UK, and his colleagues found that from 1982 to 1999 spring began and peaked increasingly earlier for almost all the ecosystems.
The advance was greatest in urban environments, where the start of the growing season advanced by an average of 0.7 days per year -- a total of 12.6 days over the 18 years. The advance was also significant in non-evergreen broadleaf forests -- an average of 0.5 days every year.
The growing season is starting earlier because of warmer temperatures, which are causing the snow to melt earlier. "Global warming in Siberia is happening faster than the global average," says Baltzer. "This has been documented by the UN's Intergovernmental Panel on Climate Change."
Baltzer believes the early onset of spring is causing more and more forest fires in Siberia. "During the 19th century, a typical forest in Siberia had about 100 years after a fire to recover before it burned again. But new observations by Vjatcheslav Kharuk at the Russian Academy of Sciences have shown that fires now return more frequently -- about every 65 years during the 20th century," says Baltzer.
He says that a consequence of the snow melting earlier in the year -- triggering the start of the growing season -- is that the soil tends to be drier later on in the year. This effect has been shown to help fuel wildfires in the Rocky Mountains in the US in a 2006 study led by Anthony Westerling of Scripps Institution of Oceanography in California, US (Science, vol 313, p 888).
Baltzer found that early springs in Siberia were linked to the Arctic Oscillation, an atmospheric phenomenon similar to the El Niño Southern Oscillation in the Pacific Ocean.
He and his colleagues had previously shown that forest fires in Siberia were similarly linked to this phenomenon. His latest research suggests that years of high Arctic Oscillation activity tend to be characterised by early springs, as well high forest fire activity.
The Arctic Oscillation governs how fast storms circle around the North Pole, but Baltzer cautions that this is simply a correlation -- the vortex does not necessarily cause the fires and the snowmelt.
"What is likely is that the Arctic Oscillation Index is an indicator of large scale climate change," he told New Scientist. "Imagine molecules in a bottle. As the bottle heats up, the molecules move faster."
In November 2006, a team led by James Randerson of the University of California at Irvine in the US showed that forest fires in high latitudes tend to cool the local climate in the long-term.
But Baltzer says this does not necessarily mean Siberia will undergo a negative feedback loop, where global warming will cause forest fires which will in turn result in cooling, and return the ecosystem to normal. He cautions that such reasoning does not take into account the other effects of global warming on forests. For instance, modelling studies have suggested that forests will move towards the poles as a result of climate change.
Journal reference: Journal of Climate (DOI:10.1175/JCLI4226).