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| A new NASA study suggests that tropical forests absorb more atmospheric carbon dioxide than is absorbed by forests in Alaska, Canada and Siberia. |
A new NASA-led study shows that tropical forests may be absorbing far
more carbon dioxide than many scientists thought, in response to rising
atmospheric levels of the greenhouse gas. The study estimates that
tropical forests absorb 1.4 billion metric tons of carbon dioxide out of
a total global absorption of 2.5 billion -- more than is absorbed by
forests in Canada, Siberia and other northern regions, called boreal
forests.
"This is good news, because uptake in boreal forests is already
slowing, while tropical forests may continue to take up carbon for many
years," said David Schimel of NASA's Jet Propulsion Laboratory,
Pasadena, California. Schimel is lead author of a paper on the new
research, appearing online in the Proceedings of National Academy of Sciences.
Forests and other land vegetation currently remove up to 30 percent
of human carbon dioxide emissions from the atmosphere during
photosynthesis. If the rate of absorption were to slow down, the rate of
global warming would speed up in return.
The new study is the first to devise a way to make apples-to-apples
comparisons of carbon dioxide estimates from many sources at different
scales: computer models of ecosystem processes, atmospheric models run
backward in time to deduce the sources of today's concentrations (called
inverse models), satellite images, data from experimental forest plots
and more. The researchers reconciled all types of analyses and assessed
the accuracy of the results based on how well they reproduced
independent, ground-based measurements. They obtained their new estimate
of the tropical carbon absorption from the models they determined to be
the most trusted and verified.
"Until our analysis, no one had successfully completed a global
reconciliation of information about carbon dioxide effects from the
atmospheric, forestry and modeling communities," said co-author Joshua
Fisher of JPL. "It is incredible that all these different types of
independent data sources start to converge on an answer."
The question of which type of forest is the bigger carbon absorber
"is not just an accounting curiosity," said co-author Britton Stephens
of the National Center for Atmospheric Research, Boulder, Colorado. "It
has big implications for our understanding of whether global terrestrial
ecosystems might continue to offset our carbon dioxide emissions or
might begin to exacerbate climate change."
As human-caused emissions add more carbon dioxide to the atmosphere,
forests worldwide are using it to grow faster, reducing the amount that
stays airborne. This effect is called carbon fertilization. "All else
being equal, the effect is stronger at higher temperatures, meaning it
will be higher in the tropics than in the boreal forests," Schimel said.
But climate change also decreases water availability in some regions
and makes Earth warmer, leading to more frequent and larger wildfires.
In the tropics, humans compound the problem by burning wood during
deforestation. Fires don't just stop carbon absorption by killing trees,
they also spew huge amounts of carbon into the atmosphere as the wood
burns.
For about 25 years, most computer climate models have been showing
that mid-latitude forests in the Northern Hemisphere absorb more carbon
than tropical forests. That result was initially based on the
then-current understanding of global air flows and limited data
suggesting that deforestation was causing tropical forests to release
more carbon dioxide than they were absorbing.
In the mid-2000s, Stephens used measurements of carbon dioxide made
from aircraft to show that many climate models were not correctly
representing flows of carbon above ground level. Models that matched the
aircraft measurements better showed more carbon absorption in the
tropical forests. However, there were still not enough global data sets
to validate the idea of a large tropical-forest absorption. Schimel said
that their new study took advantage of a great deal of work other
scientists have done since Stephens' paper to pull together national and
regional data of various kinds into robust, global data sets.
Schimel noted that their paper reconciles results at every scale from
the pores of a single leaf, where photosynthesis takes place, to the
whole Earth, as air moves carbon dioxide around the globe. "What we've
had up till this paper was a theory of carbon dioxide fertilization
based on phenomena at the microscopic scale and observations at the
global scale that appeared to contradict those phenomena. Here, at
least, is a hypothesis that provides a consistent explanation that
includes both how we know photosynthesis works and what's happening at
the planetary scale."
NASA monitors Earth's vital signs from land, air and space with a
fleet of satellites and ambitious airborne and ground-based observation
campaigns. NASA develops new ways to observe and study Earth's
interconnected natural systems with long-term data records and computer
analysis tools to better see how our planet is changing. The agency
shares this unique knowledge with the global community and works with
institutions in the United States and around the world that contribute
to understanding and protecting our home planet.
For more information about NASA's Earth science activities in the last year, visit: http://www nasa gov/ earthrightnow
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