A Model based Investigation of the Relative Importance of CO2-fertilization, Climate Warming, Nitrogen deposition and Land Use Change on the Global Terrestrial Carbon Cycle in the Historical Period

Abstract:

In this work, using the fully coupled NCAR Community Earth System Model (CESM1.0.4), we investigate the relative importance of CO₂-fertilization, climate warming, anthropogenic nitrogen deposition, and land use and land cover change (LULCC) for terrestrial carbon uptake during the historical period (1850-2005). In our simulations, between the beginning and end of this period, we find an increase in global net primary productivity (NPP) on land of about 4 PgCyr^-1 (8.1%) with a contribution of 2.3 PgCyr^-1 from CO₂-fertilization and 2.0 PgCyr^-1 from nitrogen deposition. Climate warming also causes NPP to increase by 0.35 PgCyr^-1 but LULCC causes a decline of 0.7 PgCyr^-1. These results indicate that the recent increase in vegetation productivity is most likely driven by CO₂ fertilization and nitrogen deposition. Further, we find that this configuration of CESM projects that the global terrestrial ecosystem has been a net source of carbon during 1850-2005 (release of 45.1±2.4 PgC), largely driven by historical LULCC related CO₂ fluxes to the atmosphere. During the recent three decades (early 1970s to early 2000s), however, our model simulations project that the terrestrial ecosystem acts as a sink, taking up about 10 PgC mainly due to CO₂ fertilization and nitrogen deposition. Our results are in good qualitative agreement with recent studies that indicate an increase in vegetation production and water use efficiency in the satellite era and that the terrestrial ecosystem has been a net sink for carbon in recent decades.