Cornell Chronicle-Cornell University Nov. 18, 2008
Global warming predictions are overestimated,
suggests study on black carbon
By Krishna Ramanujan
A detailed analysis of black carbon–the residue of burned organic
matter–in computer climate models suggests that those models may
be overestimating global warming predictions.
A new Cornell study, published online in Nature Geosciences,
quantified the amount of black carbon in Australian soils and found
that there was far more than expected, said Johannes Lehmann, the
paper’s lead author and a Cornell professor of biogeochemistry. The
survey was the largest of black carbon ever published.
As a result of global warming, soils are expected to release more
carbon dioxide, the major greenhouse gas, into the atmosphere, which,
in turn, creates more warming. Climate models try to incorporate
these increases of carbon dioxide from soils as the planet warms, but
results vary greatly when realistic estimates of black carbon in
soils are included in the predictions, the study found.
Soils include many forms of carbon, including organic carbon from
leaf litter and vegetation and black carbon from the burning of
organic matter. It takes a few years for organic carbon to decompose,
as microbes eat it and convert it to carbon dioxide. But black carbon
can take 1,000-2,000 years, on average, to convert to carbon dioxide.
By entering realistic estimates of stocks of black carbon in soil
from two Australian savannas into a computer model that calculates
carbon dioxide release from soil, the researchers found that carbon
dioxide emissions from soils were reduced by about 20 percent over
100 years, as compared with simulations that did not take black
carbon’s long shelf life into account.
The findings are significant because soils are by far the world’s
largest source of carbon dioxide, producing 10 times more carbon
dioxide each year than all the carbon dioxide emissions from human
activities combined. Small changes in how carbon emissions from soils
are estimated, therefore, can have a large impact.
“We know from measurements that climate change today is worse than
people have predicted,” said Lehmann. “But this particular aspect,
black carbon’s stability in soil, if incorporated in climate models,
would actually decrease climate predictions.”
The study quantified the amount of black carbon in 452 Australian
soils across two savannas. Black carbon content varied widely,
between zero and more than 80 percent, in soils across Australia.
“It’s a mistake to look at soil as one blob of carbon,” said Lehmann.
“Rather, it has different chemical components with different
characteristics. In this way, soil will interact differently to
warming based on what’s in it.”
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