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Date: October 16, 1998
Scientists Find Evidence of Large Carbon Sink over North America
Princeton, N.J. -- Researchers from Princeton University, with collaborators from the National Oceanic Atmospheric Administration and Columbia University, have found evidence of higher-than-expected absorption of the greenhouse gas carbon dioxide by ecosystems in North America.
The findings of the research team, the Carbon Modeling Consortium, are published today in the journal Science. The carbon-absorbing zone, known to geoscientists as a carbon "sink," soaked up high amounts of carbon dioxide during the period studied, from 1988 to 1992, confirming earlier studies. Evidence has been accumulating in recent years that a large component of this sink must be on land. Today's findings suggest that the North American continent plays a much larger role than what would be proportional to its size.
The findings are the result of a collaboration among geoscientists and biologists from several institutions: Song-Miao Fan, a research scientist with Princeton's Atmospheric and Oceanic Sciences Program; Emanuel Gloor, research scientist with Ecology and Evolutionary Biology; Jerry Mahlman, director of the Geophysical Fluid Dynamics Laboratory (GFDL), a NOAA facility on the Princeton campus; Princeton Professor of Ecology and Evolutionary Biology Stephen Pacala, associate coordinator of the Carbon Modeling Consortium; Princeton Professor of Geosciences Jorge Sarmiento, coordinator of the consortium; Taro Takahashi, of the Lamont-Doherty Earth Observatory at Columbia; and Pieter Tans, of the Climate Modeling and Diagnostics Laboratory, a NOAA facility in Boulder, Colo.
The researchers used atmospheric data provided by Tans, air-sea fluxes developed by Takahashi, and one ocean and two separate atmospheric models developed by GFDL. The team created a three-dimensional grid of the earth to model the flow of carbon dioxide. Researchers anticipated that as they moved from point to point on the grid across North America, atmospheric carbon levels would rise, based on the fact that North America is a major producer of carbon dioxide by burning of fossil fuels. Instead, carbon levels actually dropped between the North Pacific and the North Atlantic.
This suggests the presence of a carbon sink, which occurs when carbon dioxide absorbed by plants as they grow exceeds carbon dioxide released by dead material when it decays. Although the method does not identify the causes, there are a number of possible mechanisms that could be responsible for the sink. Forest regrowth in areas where generations of pioneers leveled trees to create farmland almost certainly plays an important role. Millions of acres east of the Mississippi have returned to forest.
Forest regrowth, and carbon absorption, in North America may be enhanced by some side effects of industrialization. Nitrogen deposition (a dilute form of acid rain) caused by combustion processes in automobiles and power plants can act as a fertilizer, as can the higher concentrations of atmospheric carbon dioxide in the air. Global warming can contribute to longer growing seasons, which have been observed in studies of satellite measurements cited by the team.
The researchers stress that all of these mechanisms are temporary. It is thus inevitable that this sink will eventually go to zero.
Team members emphasized that while the North American sink may prove important in worldwide management of atmospheric carbon absorption, its value will come at a global level, not a regional level. Sources of carbon dioxide from all parts of the earth mix together rapidly, certainly much more quickly than a local sink can act. Thus, today's results should not be interpreted as justification for claiming that pre-existing carbon sinks in a given region act to offset that region's combustion-produced carbon dioxide. Put simply, the fact that a pollution source, such as a factory, happens to be located near a large forest does not in any way diminish that facility's contribution to rising carbon levels.
The researchers stress that their result remains highly uncertain. The relevance of the result to mitigating future greenhouse warming cannot be assessed until this uncertainty is reduced.
The researchers also caution that the size and location of the sink is variable. Other studies of carbon dioxide in the atmosphere show that global sinks vary by almost a factor of five from year to year and may also vary in location. It is currently not known if the results in this paper are representative of periods outside 1988 to 1992.
Nonetheless, the identification of the location and timing of a major ecological sink is an important step toward an understanding of the global carbon cycle. Such an understanding is essential to manage greenhouse gases in the atmosphere.