Date:Fri, 16 Oct 1998 15:48:45 -0400  

Understanding global climate change requires knowledge of the comings and goings of carbon in the atmosphere. Three papers in the current issue of Science investigate this phenomenon.

In "A Large Terrestrial Carbon Sink in North America Implied by Atmospheric and Oceanic CO2 Data and Models," S. Fan, J. Sarmiento, M. Gloor, S. Pacala, and J. Mahlman at Princeton U. in Princeton, NJ; T. Takahashi at Columbia U. in Palisades, NY; P. Tans at Climate Modeling and Diagnostics Laboratory, NOAA in Boulder, CO. suggest a possible location for a carbon sink in the Northern Hemisphere.

In the last decade, a number of studies have compared emissions of CO2 into the atmosphere over the Northern Hemisphere with measurements of actual atmospheric CO2 levels -- and have come up short: A portion of the carbon that has entered the atmosphere seems to be taken up somewhere else.

Where and how this is happening is a subject of active debate; researchers have proposed a variety of possibilities, including the oceans, forests, and soil.

In this study, the authors compiled information from several sources such as measured CO2 emissions, patterns of atmospheric CO2 levels from monitoring sites, and climate models.

Their results suggest that North America may be a site for a major carbon sink, which may be at least partly due to regrowth of vegetation on abandoned farmland and previously logged forests. They agree that more work is needed to evaluate this result.

The possibility that the tropical rain forest is an important carbon sink is also a hot topic under debate in environmental science circles. In "Changes in the Carbon Balance of Tropical Forests: Evidence from Long-Term Plots," O. L. Phillips at U. of Leeds in Leeds, UK; Y. Malhi and J. Grace at U. of Edinburgh in Edinburgh, UK; N. Higuchi, W. F. Laurance, S. G. Laurance, and L V. Ferreira at Instituto Nacional de Pesquisas da Amaznia in Manaus, Brazil; P. V. Nez at Biodiversidad Amaznica in Cusco, Peru; R. M. Vsquez at Missouri Botanical Garden - Proyecto Flora del Per in Iquitos, Peru; M. Stern at New York Botanical Garden in Bronx, NY; S. Brown at U. of Illnois in Urbana, IL. present new long-term, broad-scale evidence that mature tropical forests sequester significant amounts of carbon.

They suggest that Neotropical forests (in tropical Central and South America) may help slow the rate of increase in atmospheric CO2 and reduce the impacts of global climate change.

By measuring permanent sample plots that had been established by foresters and ecologists to monitor tree growth and mortality, the authors found that biomass gain by tree growth exceeded losses in old-growth humid tropical forests. The neotropical forest plots accumulated 0.71 tons of carbon per hectare per year in recent decades.

The enhanced forest activity may be explained by several mechanisms: response to cyclical climate change; recovery from widespread natural or man-made disturbance; and increased nutrient availability. The authors caution that deforestation, logging, and regional drying and warming may limit or reverse the carbon sink provided by mature forests.

As scientists try to determine what impact human activity is having on the carbon dioxide load in the atmosphere, it's necessary to track natural carbon dioxide sources as well. In "Carbon Dioxide Emission From European Estuaries," M. Frankignoulle, G. Abril, A. Borges, I. Bourge, C. Canon, B. Delille, E. Libert and J.-M. Thate at U. de Lige in Lige, Belgium, show that European estuaries are significant natural sources of atmospheric carbon dioxide.

Estuaries are rich marine ecosystems that are created when freshwater mixes with saltwater. The authors measured CO2 emissions from nine European estuaries and found that they emit between 30 and 60 million tons of carbon per year to the atmosphere.

That represents 5 to 10 percent of the total amount of carbon dioxide emissions from human activity in Western Europe. The largest amounts of CO2 occurred in the upper estuary, where there is low salinity and a decrease in saturated oxygen. Although there is limited data available, the authors expect high carbon amounts to be reflected in other estuaries around the world.