Abstract
More accurate projections of future carbon dioxide concentrations in the atmosphere and associated
climate change depend on improved scientific understanding of the terrestrial carbon cycle. Despite the
consensus that U.S. terrestrial ecosystems provide a carbon sink, the size, distribution, and interannual
variability of this sink remain uncertain. Here we report a terrestrial carbon sink in the conterminous U.S.
at 0.63 pg C yr−1 with the majority of the sink in regions dominated by evergreen and deciduous forests
and savannas. This estimate is based on our continuous estimates of net ecosystem carbon exchange
(NEE) with high spatial (1 km) and temporal (8-day) resolutions derived from NEE measurements from
eddy covariance flux towers and wall-to-wall satellite observations from Moderate Resolution Imaging
Spectroradiometer (MODIS). We find that the U.S. terrestrial ecosystems could offset a maximum of 40%
of the fossil-fuel carbon emissions. Our results show that the U.S. terrestrial carbon sink varied between
0.51 and 0.70 pg C yr−1 over the period 2001–2006. The dominant sources of interannual variation of
the carbon sink included extreme climate events and disturbances. Droughts in 2002 and 2006 reduced
the U.S. carbon sink by ∼20% relative to a normal year. Disturbances including wildfires and hurricanes
reduced carbon uptake or resulted in carbon release at regional scales. Our results provide an alternative,
independent, and novel constraint to the U.S. terrestrial carbon sink.
