Carbon Sources and Sinks of North America as Affected by Major Drought Events During the Past 30 Years

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 remained 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 from long-term (1980 - 2010) means in modeled leaf area indices (LAI), fully prognostic in the model, during major drought events in 1988 and 2002 agreed well with those in AVHRR NDVI (geographically weighted regression, R2 = 0.84 in 1988, 0.71 in 2002). NA modeled net ecosystem productivity (NEP) declined by 92% (0.50 Pg C yr-1) and 90% (0.49 Pg C yr-1) from the long-term mean (+0.54 Pg C yr-1), in 1988 and 2002 respectively. These significant drops in NEP offset 28% of the long-term carbon gains from the long-term mean 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 that may be associated to frequent occurrences of El Niño-Southern Oscillation, controlled much of the continental scale interannual variability and mainly occurred in parts of the Great Plains, southwest US and northern Mexico. Although stronger carbon sinks were modeled in northern ecosystems as a result of greater gross primary productivity with longer growing season, projected increases in frequency and intensity of drought could enhance carbon release hence may reduce net carbon sink of the continent.