Losing The Ability
To Absorb Man-Made Carbon
By Steve Connor
01 November, 2007
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
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
© 2007 Independent News
and Media Limited
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