Because forests can absorb carbon dioxide (CO2) produced by the burning of fossil fuels, they have been labeled carbon sinks. But as the global atmospheric burden of CO2 continues to rise, scientists are realizing that the situation may not be quite so cut-and-dried. According to a report published online this week by the Proceedings of the National Academy of Sciences, increasing temperatures could cause rain forests to release unusually high levels of CO2, thereby amplifying the effects of future warming.

Deborah A. Clark of the University of Missouri-St. Louis and her colleagues measured the annual growth of six species of trees in an old-growth rain forest in La Selva, Costa Rica, between 1984 and 2000. The researchers also used data from global climate monitoring stations to calculate CO2 emissions from tropical lands over the same time period. Tree growth and the amount of carbon dioxide exchange both varied greatly over the 16-year period, and both were correlated with temperature. In addition, during the warmest years--particularly the record-breaking 1997-1998 El Nino episode--the rain-forest trees experienced the least growth and expelled the most carbon dioxide, the scientists report. They conclude that the carbon balance of the La Selva rain forest is remarkably sensitive to increasing temperatures. Tropical rain forests could thus potentially induce a large positive feedback for global CO2 atmospheric accumulation. Note the authors: "Such a feedback in future years would accelerate global warming."