Escaping carbon


At first glance, the simplest explanation might appear to be deforestation. When forests are cut down or burned, the carbon stored in the forest biomass is released into the atmosphere. Combined with the other processes that carry carbon out of the rainforest ecosystem—decomposition, respiration, soil and sediment run off into the Atlantic—deforestation might be the big source of carbon scientists are seeking. But calculations suggest otherwise. In Brazil alone, deforestation is proceeding at a rate of about 20,000 square kilometers per year as the Amazon is cleared for farming and ranching (Houghton, et al., 2002), but these losses still do not appear to be large enough to offset the large carbon intake measured by the flux towers.

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If deforestation wasn’t the culprit, then how could scientists account for the apparent discrepancy between how much carbon the flux towers indicated was coming into the forest and the lesser amount of carbon actually contained in the biological material? Researchers had no lack of alternative explanations. Maybe the global models were wrong. Maybe estimates of the rates of deforestation were too low. Maybe there was something wrong with how scientists were collecting the flux tower data. A few scientists, though, did not discount the possibility that the Amazon could be hiding a large, yet-to-be-discovered source of carbon emissions. Richey thought he knew where.

“We had been working in the Amazon for almost 20 years, collecting all kinds of river samples, including measurements of the carbon dioxide dissolved in the water. So as far back as 20 years ago, we were publishing papers saying that the amount of carbon in the waters of the Amazon was greater than that in the air. For years I had been listening to the carbon modelers complaining about the discrepancies in the tropics, and I said to myself, ‘I know that carbon dioxide is moving out of the water into the atmosphere.’ But at that time the scientists doing the carbon modeling didn’t talk to the people doing the flux tower measurements, and they didn’t talk to those of us who were down on the water.”

  The state of Acre in Brazil, shown in the satellite image above, is undergoing rapid deforestation. Move your mouse cursor over the image to compare the landscape in 2000 with how it appeared in 2002. The tan areas are recently cleared patches. The 2002 image is also shrouded in haze from nearby fires set to clear land. (Images by Clare Averill, NASA JPL MISR Science Team)

Photographs of Scientists Sampling Water from a Boat and Air from a Tower


LBA brings the right scientists together
But then in 1998, the Brazilian science community, joined by an international team of scientists, launched the Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA). Their aim was to study how Amazonia functions as a regional entity within the larger Earth system and how changes in land use and climate will affect the biological, physical, and chemical functioning of the region’s ecosystem. With the Amazon as their laboratory, scientists have been studying climate, atmospheric chemistry, the carbon cycle, nutrient cycling, land surface hydrology and water chemistry, land use and land cover, and the interaction of humans with the landscape.

Richey credits the LBA project for bringing a diverse group of scientists together and encouraging them to speak a common language. It was on a return flight from an LBA conference that Richey began a dialogue with a carbon cycle modeler. He says,“On the plane we started comparing notes. I realized that we had always talked in terms of pressures of carbon dioxide, and they spoke in terms of mass, so many tons of carbon in and out of the ecosystem each year. I realized we would need to put our results into that common language.”

  Although direct measurements of the air in the Amazon [from flux towers (left)] showed the forest removing large amounts of carbon dioxide from the atmosphere, mathematical models of the global atmosphere showed the Amazon as a source of CO2. Jeffrey Richey and a team of scientists had been studying the region’s rivers and streams for 20 years, and knew that high concentrations of CO2 were dissolved in the water (right). Perhaps the excess CO2 was coming from the Amazon River and its tributaries. [Photographs courtesy Michael Keller, USDA Forest Service Institute of Tropical Forestry (right), and Jeffrey Richey, University of Washington (left).]

Richey knew that what they needed was a grand total: how much total carbon was emitted from water surfaces (a process called evasion) across the Amazon every year. To get a grand total, they required two pieces of information; as many measurements as they could get of the amount of carbon dioxide released by numerous areas within the basin and an estimate of the total surface area covered by water in the Amazon. To come up with these numbers, Richey and his colleagues made use of data sources that ranged from low tech— more than a decade’s worth of air and water samples collected from the bows of small fishing boats—to a sophisticated, satellite-based radar.

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Photograph of Scientists Measuring Carbon Dioxide in the Water
Small teams of scientists working in remote reaches of the Amazon discovered that large amounts of carbon dioxide were escaping from the surface of Amazon flood waters. Carbon dioxide was trapped by bowls turned upside down over the water. (Photograph courtesy Jeffrey Richey. )