Estimating North American Carbon Sink with Process-Based Ecosystem Modeling and Satellite Observation

Atmospheric inverse modeling and direct inventories of the carbon on the ground indicated that Northern America is a substantial carbon sink, however, the sink size, spatio-temporal variation, and causes remain uncertain. In this study, we used two models - a biogeochemical model, CEVSA, driven with ground meteorological data and a production efficiency model, GLO-PEM, driving with remotely sensed NDVI and climate variables from NOAA/NASA AVHRR satellites- to quantify the changes in net primary productivity (NPP), heterotrophic respiration, carbon stocks in vegetation and soils, and net ecosystem production (NEP) in North America between 1981 and 1998. The results show NPP and NEP fluctuated interannually in response to the climate variability associated with the El Ni?o Southern Oscillation (ENSO) cycle, particularly in the middle and southwestern United States. Despite the high variability, both NPP and NEP increased consistently during the 1980s and 1990s, but the estimated annual carbon sink was much smaller than that estimated with atmospheric inverse modeling and the carbon accumulation occurred mainly in vegetation with litter changes in the carbon stocks in soils. The changes in NPP were closely correlated with climate variability, but the increasing trend seems to be driven mainly by the fertilization effect of increasing atmospheric CO2. The estimates with the two independent methods agree generally well, but the biogeochemical model predicted larger increases in NPP and weaker responses to the ENSO cycle than the production efficiency model did.