Putting Together Maps and Measurements

   
 

Despite the aid of satellite data and years of observations, Richey and his colleagues couldn’t hope to study the whole Amazon. Instead, they focused their efforts on a large area in the central Amazon basin. They categorized the waters of the 1.77-million-square-kilometer study area into four geographic regions based on the hydrological characteristics: the main Amazon channel, the main channel floodplain, tributaries greater than 100 meters wide, and tributaries less than 100 meters wide. The region was further subdivided into up-, mid- and downriver regions. Based on the carbon dioxide detected in the river samples from each of these categories, they came up with an estimate for the entire study area.
 

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Graph of Water Area over Time

Richey said they had suspected for years that the amount of carbon dioxide evasion could be large, but until they could combine their ground-based measurements with the satellite maps of the total flooded area, they had no hard evidence, no “smoking gun.” When the amount of carbon dioxide emitted from the sampled water surfaces was extrapolated to the entire flooded area within the study site, it totaled 120 million grams (264,550 pounds) of carbon per square kilometer per year. A rough estimate for the amount of carbon given off by the entire Amazon River basin was half a gigatonnne of carbon every year—a mass of carbon equivalent to more than 90 million adult elephants!
 

  The area covered by the Amazon River and its tributaries more than triples over the course of a year. In an average dry season 110,000 square km of land are water-covered, while in the wet season, the flooded area of the Amazon Basin rises to 350,000 square km. (Graph adapted from Richey, Melack, Aufdenkampe, Ballester, and Hess)

Graph of Carbon Dioxide Emissions over Time

Says Richey, “When we put our measurements together with the satellite-based flood maps, we got an estimate of carbon dioxide emissions that was greater than 10 times the amount of carbon that washes out to sea in the river outflow. Hydrologists had long thought that the most important role of river systems in the global carbon cycle was in the carbon that flowed out to sea as dissolved organic and inorganic compounds. And now we had an estimate that the carbon dioxide flowing into the atmosphere directly from the river surface was almost 13 times larger than that amount.” For the first time, there was solid evidence of a large carbon source within the forest sink.

Identifying the Source of the Source
The carbon in the rivers comes from a number of places. Richey and his colleagues’ believe that most of the carbon originates in the non-flooded, upland forests. Accounting for 35 percent of the total, they believe, is forest litter that washes down from highland forests. The litter decomposes, giving off carbon dioxide. Another 25 percent of the carbon comes into the system directly as carbon dioxide when plant and tree roots give off carbon dioxide during respiration. The carbon dioxide becomes dissolved in groundwater that flows into streams and rivers. Another 15 percent comes from carbon-containing compounds that leach out of soil, leaf litter, and other biological matter. Those dissolved organic carbon compounds get metabolized by river life, ultimately returning to the atmosphere as carbon dioxide.
 

  The rivers in the Amazon Basin carry a large amount of dissolved carbon dioxide gas, created by rotting leaves and other sources in the forest upstream. As the river system floods each year, some of this carbon dioxide is released into the atmosphere, peaking at a level of about 35 teragrams of carbon per month. (Graph adapted from Richey, Melack, Aufdenkampe, Ballester, and Hess)
 

Photograpg of a Boy Paddling A Boat Through the Jungle

Richey estimates that only about 25 percent of the carbon given off by the Amazon River and its tributaries actually originates within the river itself, mostly in the form of aquatic vegetation that first fixes carbon dioxide during photosynthesis and then respires some of it back into the water. He admits those numbers are only estimates at this time. Despite the surprising discovery of this large source of carbon emissions, he says, so far the scientific community doesn’t seem bothered by the magnitude of his estimate. “There is definitely a sense of ‘here is a missing piece’ of the tropical carbon budget puzzle.”

Answers Produce More Questions
Where that carbon is coming from is more hotly debated. If most of the carbon dioxide released from the Amazon waters comes from carbon originally absorbed by the upland forests and washed down into rivers and streams, as Richey believes, then it would represent a real carbon loss from the ecosystem. But if it turns out the carbon dioxide is produced by vegetation in the river and in the adjacent flooded forests and lakes, rather than the upland forests, then the large emissions only counterbalance a large carbon intake by the aquatic vegetation. The source of the carbon dioxide seeping out of the Amazon waters is the subject of several ongoing studies.

Richey’s enthusiasm for the project and his excitement about the results don’t seem to have dimmed since the paper was published in the journal Nature in April 2002. “This study was a terrific assemblage of water chemistry data, GIS, theory, remote sensing, and tower dynamics. That’s why this was so fun—the integration—all these disciplines coming together to work on a problem.” The implication is that the coupling between the land and the atmosphere, and also between the terrestrial Amazon and the aquatic Amazon, is tighter than scientists previously thought.

Those who say that for every question science answers, it generates a dozen more can find evidence in Richey’s work. Richey himself is already thinking ahead. He wonders about the effect on this source of carbon from global warming and land-use change. He’s also beginning to think globally, and has also begun planning a similar study of the rivers and rainforests near the Mekong River in southeast Asia. And he’s not done with the Amazon yet either. Says Richey, “Not all the data we used in this study was gathered specifically to answer this question. Now we have to go back and get better, more detailed measurements, specifically targeted to answering our questions.”

References:

1. Richey, J. E., Melack, J.M., Aufdenkampe, A.K., Ballester, V.M., and Hess, L.L. (2002) Outgassing from Amazonian rivers and wetlands as a large source of tropical CO2, Nature, 416: 617-620
2. Houghton, R.A, Skole, D. L., Nobre, C.A., Hackler, J.L., Lawrence, K.T., and Chomentowski, W.H. (2002) Annual fluxes of carbon from deforestation and regrowth in the Brazilian Amazon, Nature, 403: 301-304

Resources:

1. Amazon Facts from the Smithsonian National Zoo.
2. LBA-ECO Website

back Seeing Through Clouds

 

By identifying the carbon dioxide being transferred from the rivers of the Amazon Basin to the atmosphere, scientists are enhancing their understanding of the role the Amazon plays in the global carbon cycle. This understanding will help clarify how natural and human-caused changes in the Amazon could change the world. (Photograph courtesy Jeffrey Richey)

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