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Cover: Instruments on a treetop tower have been measuring fluxes of carbon dioxide since 1989, in an experiment led by Steven Wofsy of Harvard University. This is one of many studies worldwide of how and why terrestrial ecosystems absorb or release carbon. Complementary efforts are focused on learning about the oceanic uptake of carbon. In their article on page 30, Jorge Sarmiento and Nicolas Gruber discuss both the land and ocean sinks for the carbon that humankind is pumping into the atmosphere. (Photograph courtesy of Allen Goldstein.)
