Forest Regrowth A Likely Cause, Say Columbia, Princeton, NOAA Researchers
Ecosystems in North America are absorbing carbon dioxide at a rate that is greater than expected, according to findings by a team of scientists from Columbia University's Lamont-Doherty Earth Observatory, Princeton University and the National Oceanic and Atmospheric Administration.
The study, which its authors said was subject to confirmation, may mean that land-based carbon-absorbing zones could play a greater role than expected in managing greenhouse warming of the atmosphere. Carbon dioxide is one of several gases implicated in greenhouse warming.
The findings of the research team, the Carbon Modeling Consortium
"We know that we who reside in the United States emit about 6.2 billion tons of carbon dioxide into the atmosphere each year," said Taro Takahashi, Doherty Senior Research Scientist, associate director of Lamont-Doherty, Columbia's earth sciences campus in Palisades, N.Y., and an author of the report. "As an air mass travels from west to east, it should receive carbon dioxide and the East Coast concentration of CO2 should be higher than on the West Coast.
"But observations tell us otherwise. The mean atmospheric CO2 concentration on the East Coast has been observed to be lower than that over the Pacific coast. This means that more CO2 is taken up by land ecosystems over the United States than is released by industrial activities."
Team members emphasized that while the North American sink may prove important in worldwide management of atmospheric carbon absorption, their 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.
The findings are the result of a collaboration among geoscientists and biologists from several institutions: Song-Miao Fan and Emanuel Gloor, Princeton research scientists; Jerry Mahlman, director of the Geophysical Fluid Dynamics Laboratory (GFDL), a NOAA facility on the Princeton campus; Stephen Pacala, professor of ecology and evolutionary biology at Princeton; Jorge Sarmiento, professor of geosciences at Princeton; Pieter Tans, of the Climate Modeling and Diagnostics Laboratory, a NOAA facility in Boulder, Colo., and Dr. Takahashi.
The researchers used atmospheric data provided by Dr. Tans, observations of ocean-based sinks contributed by Dr. Takahashi, and one ocean and two separate atmospheric models developed at Princeton's Geophysical Fluid Dynamics Laboratory. The team created a three-dimensional grid of the earth to model the worldwide 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 from the burning of fossil fuels. Instead, carbon levels actually dropped between the North Pacific and the North Atlantic.
The results suggest the presence of a carbon sink, which occurs when carbon dioxide absorbed by plants as they grow exceeds carbon dioxide released by dead material as 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 in soil, a result of 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 itself 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 stop absorbing carbon dioxide at these levels. They also stressed that the findings are subject to confirmation. "Our sampling density and frequency were inadequate for estimating the ecosystem uptake of carbon dioxide over other areas of the world," Dr. Takahashi said. "So it is really too early to say that the North American continent is so unique."
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. The results in this paper may not be representative of periods outside 1988 to 1992, they added.
Nonetheless, the identification of the location and timing of a major ecosystem that absorbs carbon dioxide is an important step toward an understanding of the global carbon cycle, they said. Such an understanding is essential to managing greenhouse gases in the atmosphere.