The association between net primary productivity and rainfall in CMIP5 20th and 21st century simulations

Tuesday, 16 December 2014
Robinson I Negron Juarez1, William J Riley1, Charles D Koven2, Ryan G Knox2, Philip Taylor3 and Jeffrey Q Chambers4, (1)Lawrence Berkeley Natl Lab, Berkeley, CA, United States, (2)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (3)University of Colorado at Boulder, Boulder, CO, United States, (4)University of California Berkeley, Berkeley, CA, United States
Tropical forests fix large amounts of atmospheric CO2 into biomass via net primary productivity (NPP). In this study we use the NPP–MAR (mean annual rainfall) relationship observed in tropical forests to evaluate the performance (20th century) and predictions (21st century) of tropical NPP from ten Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth System Models (ESMs). Most of the CMIP5 models showed an increase in NPP concurrent with an increase in rainfall and a decrease in surface solar radiation. Models that better represented the NPP–MAR pattern did not better represent the climate, and vice versa. By the end of the 21st century the models that best reproduced the observed NPP–MAR relationship projected an increases in NPP between ~2% (RCP 4.5) and ~19% (RCP 8.5) relative to current observations (11.88±5 MgC ha-1 yr-1, 327 field sites) and increases of ~9% and ~25% relative to historical simulations (2005). By separating the effects of climate forcing and CO2 fertilization models showed that maximum productivity is likely occurring during the current climate, but this signal is masked by increases in NPP due to CO2 fertilization. Further studies addressing the individual and simultaneous effect of other climate variables on NPP are needed.