THERE is substantial evidence that many plants respond to increased concentrations of atmospheric carbon dioxide by increasing their productivity(1-4) This observation has led to the suggestion that, by taking up CO2, the terrestrial biosphere might mitigate the potential greenhouse warming associated with anthropogenic CO2 emissions(5). Whiting and Chanton(6) have found, however, that for wetlands of varying productivity around the world, higher net primary production is associated with higher emissions of methane-another important greenhouse gas. Here we present measurements of methane emissions from a marsh that has been exposed to twice the present ambient concentration of atmospheric CO2. We find that over a one-week period, the CO2-enriched sites had significantly higher emissions of methane than the control sites. Our results suggest that future increases in atmospheric CO2 concentration may lead to significant increases in methane emissions from wetlands.