Princeton - PWB 102698 - Carbon sink over North America

Princeton Weekly Bulletin October 26, 1998

Carbon sink over North America

By Mary Caffrey

Princeton researchers and collaborators from the National Oceanic and Atmospheric Administration (NOAA) 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, were published in Science on October 16. Earlier studies have suggested that there is a carbon-absorbing zone, known to geoscientists as a carbon "sink," in the northern part of the western hemisphere. The results in the Science paper confirm that this sink soaked up high amounts of carbon dioxide between 1988 and 1992 and indicate 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: geosciences professor Jorge Sarmiento is coordinator of the consortium, and Stephen Pacala, professor of ecology and evolutionary biology, is associate coordinator; other collaborators are research scientist Song-Miao Fan of the Atmospheric and Oceanic Sciences Program, research scientist Emanuel Gloor of EEB, and Jerry Mahlman, director of the Geophysical Fluid Dynamics Lab (GFDL), at Princeton; Taro Takahashi of the Lamont-Doherty Earth Observatory at Columbia; and Pieter Tans of the Climate Modeling and Diagnostics Lab in Boulder, Colo. (Both GFDL and the Boulder lab are NOAA facilities.)

Three-dimensional grid of earth

Using atmospheric data provided by Tans, air-sea fluxes developed by Takahashi, and one oceanic 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. They 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 the 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.

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.

According to the Carbon Modeling team, all of these mechanisms are temporary, and it is inevitable that this sink will eventually go to zero.

Team members emphasize that the findings published in Science should not be interpreted as justification for claiming that preexisting carbon sinks in a given region act to offset that region's combustion-produced carbon dioxide. 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 point out 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, much faster than a local sink can act.

They also stress that their result remains highly uncertain, and the relevance of the result to mitigating future greenhouse warming cannot be assessed until this uncertainty is reduced.

The researchers 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 a factor of almost five from year to year and may also vary in location. It is not currently known if the results in the Science paper are representative of periods outside the studied span of 1988 to 1992.

Nonetheless, they say, the identification of the location and timing of a major ecological sink is an important step toward understanding the global carbon cycle. Such an understanding is essential for managing greenhouse gases in the atmosphere.