A new study examining carbon emissions in the Amazon basin by a Boulder researcher and two other scientists indicates its tropical ecosystems may now emit more carbon dioxide into the atmosphere than they absorb.
The long-term implications of the study, which provide the basis for the first carbon budget estimate of the Amazon basin, could spell accelerated global warming.
The international team of scientists discovered that the annual rainfall was the key factor determining how much carbon dioxide was taken up and released from the Amazon region in the two years that were studied, 2010 and 2011.
The Amazon forests, during a wet year, 2011, were found to be roughly carbon-neutral. Forests took in more carbon dioxide than they emitted, the study found — but the burning of biomass, which releases carbon dioxide, made up for the difference, the researchers found.
During a very dry year such as 2010, however, forest growth stalled, photosynthesis slowed and the burning of biomass increased. The result was that the region "exhaled " significant amounts of carbon dioxide into the Earth's atmosphere.
One of the study's three co-authors is John Miller, a scientist with the National Oceanic and Atmospheric Administration's Cooperative Institute for Research in Environmental Sciences at the University of Colorado. It was published today in the journal Nature.
He was joined in the project by Luciana Gatti from the Instituto de Pesquisas Energeticas e Nucleares in Sao Paulo, Brazil and Emanuel Gloor from the University of Leeds in England.
Miller said the significance of the Amazon's threatened status as an efficient "sink," or absorber of carbon emissions, is considerable.
"If we can't count on the Amazon (region) to do that, going into the future, that means more of the global 10 billion tons and rising (of carbon from fossil fuels) is going to stay in the atmosphere, and that will accelerate climate change not just in the Amazon, but globally," Miller said.
"It's not just a local or regional issue in the Amazon, but it is globally important because the Amazon is such an important part of the global carbon system."
Gloor, in an email, wrote, "The Amazon forests are in a sense a human treasure with the largest biodiversity on the planet. They are experiencing warmer conditions and also more variation in climate. What will their fate be?... If the forests suffer they will also release carbon to the atmosphere and thus add to greenhouse warming, another concern."
The scientists used aircraft to gather air samples between the surface and an altitude of 14,500 feet across Amazonia, in about 200 flights over the course of two years. Samples were then analyzed at Gatti's lab in Brazil with high-precision carbon dioxide and carbon monoxide sensors, enabling researchers to track changes in the concentrations of those gases as air masses moved across the region.
Miller said a summary of the team's research stated that the Amazon region "may" emit more greenhouse gases than they absorb because there is no certainty about future climate trends there.
"The reason there's a 'may' in there is that this is what we saw was in two years," Miller said. "We're very confident that in 2010, the dry year, more carbon was released. The 'may' relates to the fact that we cannot predict climate enough to know what will happen.
"We think we have a good idea from these two years what the sensitivity is. But what we don't know is how much and how often droughts will occur in the Amazon in coming years."
Miller doesn't want to settle for just a two-year record of the carbon exchange in the Amazon region.
"For the relatively modest expense of money you can monitor what is going on in the Amazon and start to draw stronger conclusions," Miller said. "Two years does not make a trend. One of my big hopes is that by making this research result more prominent, we can attract more funding, and with that, the ability to continue the monitoring of this critically important region of the world."
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