Effects of Anthropogenic Activity on Global Terrestrial Gross Primary Production

Gross primary production (GPP) has been identified as the largest terrestrial carbon flux and the major driver of the growing biosphere uptake of carbon. Factorial simulations using several biosphere models have been used to estimate the effects of long-term (>50 years) climate change on global terrestrial GPP. However, no study has integrated large-ensemble climate simulation data into a biosphere model to realistically estimate global terrestrial GPP with associated uncertainty. Here we present a novel approach to estimate the global terrestrial long-term GPP with associated climate data-induced uncertainty that combines a diagnostic-type biosphere model with a large-ensemble climate simulation data set. We distinguish the effects of recent anthropogenic activity on global GPP (the anthropogenic GPP effect) from the effects of interannual climate variability (the natural GPP effect) by comparing GPP model estimates forced by historical and "nonwarming" climate data. We provide evidence for an increasing anthropogenic effect on global terrestrial GPP. The anthropogenic GPP effect is driven by CO₂ fertilization, which is projected to weaken or saturate by 2050-2150, depending on the representative concentration pathway scenario used. Model results suggest that shortwave radiation couples with El Niño-Southern Oscillation conditions and volcanic eruptions to drive the natural GPP effect. Because shortwave radiation at the surface is related to cloud cover, we encourage future studies to focus on cloud-radiation feedbacks on the carbon cycle.