Tropical forests absorb more CO2 than previously believed

Chuck Bednar for – Your Universe Online
Good news for the environment: Tropical forests may be absorbing far more carbon dioxide than previously believed.
In fact, new NASA-led research indicates that tropical forests are absorbing an estimated 1.4 billion metric tons of the global 2.5 billion metric tons of CO2 being absorbed worldwide. That means that these forests are absorbing more than the boreal forests found in the north.
“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 in Pasadena, California and lead author of a new paper published online Monday in the journal Proceedings of National Academy of Sciences (PNAS).
Up to 30 percent of manmade carbon dioxide emissions are removed from the atmosphere by forests and other land vegetation during photosynthesis, the researchers explained. If that rate of absorption slowed down, it would cause the rate of global warming to speed up.
The new research “is the first to devise a way to make apples-to-apples comparisons of carbon dioxide estimates from many sources at different scales,” including computer simulations of various ecosystem processes, atmospheric models that run backwards to locate the sources of current concentrations, satellite images and experimental forest plot data, NASA said.
Schimel and his colleagues combined various types of analyses and determined the accuracy of the results based on their ability to reproduce independent, ground-based measurements. The models they determined to be the most accurate, the most trusted  and the most verifiable were used to obtain the new estimate of tropical carbon absorption.
“Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry and modeling communities,” explained study co-author Joshua Fisher, also from JPL. “It is incredible that all these different types of independent data sources start to converge on an answer.”
Co-author Britton Stephens, from the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, said that determining which type of forest absorbed the greatest amount of carbon was “not just an accounting curiosity. 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 more CO2 is added to the atmosphere as the result of human-caused emissions, forests all over the world are using it to grow more rapidly, thus reducing the amount that remains airborne, the researchers said. The effect, which is known as carbon fertilization, tends to be stronger at higher temperatures, meaning that it will typically be higher in tropical forests than boreal ones.
However, since climate change also decreases the amount of water available in some parts of the Earth, it can lead to more frequent and more damaging wildfires. This issue is compounded in the tropics, where people intentionally burn wood during deforestation. These fires not only halt carbon absorption by killing trees, but also release massive amounts of CO2 into the atmosphere.
For the past quarter of a century, most computer models of the climate have shown that the mid-latitude forests in the Northern Hemisphere absorb more carbon than tropical forests. Those findings were based on the understanding of global air flows and limited data suggesting that deforestation was causing tropical forests to release more CO2 than they were absorbing.
In the mid-2000s, however, Stephens used measurements of carbon dioxide made from aircraft to show that many of those simulations were not accurately representing flows of carbon above ground level. The new study used findings reported by other scientists since Stephens’ initial paper to compile regional and national data into a new global-scale database.
“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,” Schimel said. “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.”
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