Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems

Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currentl y absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmosph eric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regi onal patterns of carbon exchange by terrestrial ecosystems. Atmospheric car bon dioxide and oxygen data confirm that the terrestrial biosphere was larg ely neutral with respect to net carbon exchange during the 1980s, but becam e a net carbon sink in the 1990s. This recent sink can be largely attribute d to northern extratropical areas, and is roughly split between North Ameri ca and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissi ons due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrow th on abandoned agricultural land and fire prevention, in addition to respo nses to environmental changes, such as longer growing seasons, and fertiliz ation by carbon dioxide and nitrogen. Nevertheless, there remain considerab le uncertainties as to the magnitude of the sink in different regions and t he contribution of different processes.