Dennis Baldocchi and Riccardo Valentini

Through the long-term monitoring of CO2 concentration at Mauna Loa (Keeling and Whorf 1996) and an expanded network of CO2 monitoring stations across the globe (Tans et al. 1996), we now possess an ability to observe the "breathing" of the biosphere. In general, the biosphere inhales CO2 during its hemisphere's summer and exhales CO2 during its winter. Superimposed upon this general pattern, however, is much complexity and variability due to temporal and spatial gradients in climate, available resources, plant structure and function, land use, and soil development.

Technically, the biosphere's breathing can be quantified by assessing net ecosystem productivity, the balance between natural sources and sinks of carbon dioxide. Net ecosystem productivity (NEP; grams of carbon per square meter per year [g C m-2 y-1]) is defined as the difference between gross primary productivity (GPP) and ecosystem respiration (Reco):

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