Earth & Environmental Sciences

The North American (NA) terrestrial biosphere has been a long-term carbon sink but impacts of climate extremes such as drought on ecosystem carbon exchange remain largely uncertain. Here, changes in biospheric carbon fluxes with recent climate change and impacts of the major droughts of the past 30 years on continental carbon cycle across NA were studied using a comprehensive mathematical process model, ecosys. In test of these model responses at continental scale, the spatial anomalies in modeled leaf area indices, fully prognostic in the model, from long-term (1980–2010) means during major drought events in 1988 and 2002 agreed well with those in AVHRR NDVI (R2=0.84 in 1988, 0.71 in 2002). Net ecosystem productivity (NEP) modeled across NA declined by 92% (0.50PgC yr−1) and 90% (0.49PgC yr−1) from the long-term mean (+0.54PgC yr−1), in 1988 and 2002 respectively. These significant drops in NEP offset 28% of the carbon gains modeled over the last three decades. Although the long-term average modeled terrestrial carbon sink was estimated to offset ∼30% of the fossil fuel emissions of NA, only 0.03% and 3.2% were offset in 1988 and 2002 leaving almost all fossil fuel emissions to the atmosphere. These major drought events controlled much of the continental-scale interannual variability and mainly occurred in parts of the Great Plains, southwest US and northern Mexico. Although warming in northern ecosystems caused increasing carbon sinks to be modeled as a result of greater gross primary productivity with longer growing seasons, elsewhere in the continent frequent drought events of the past 30 years reduced carbon uptake and hence net carbon sinks of the NA.