Recent Amplified Global Gross Primary Productivity Due to Temperature Increase Is Offset by Reduced Productivity Due to Water Constraints

Satellite remote sensing observations show an increased greenness trend over land in recent decades. While greenness observations can indicate increased productivity, estimation of total annual productivity is highly dependent on vegetation response to climate and environmental conditions. Models have been struggling to determine how much carbon is taken up by plants as a result of increased atmospheric CO2 fertilization. Current remote sensing light use efficiency (LUE) models contain considerable uncertainty due to the lack of spatial and temporal variability in maximum LUE parameter and climate sensitivity defined for global plant functional types (PFTs). We used the optimum LUE (LUEopt) previously derived from the global FLUXNET network to improve estimation of global gross primary productivity (GPP) for the period 1982-2016. Our results indicate increasing GPP in northern latitudes owing to reduced cold temperature constraints on plant growth, thereby suggesting increasing negative carbon-climate feedback in high latitudes. In the tropics, by contrast, our results indicate an emerging positive climate feedback, mainly due to increasing atmospheric vapor pressure deficit (VPD). Further pervasive VPD increase is likely to continue to reduce global GPP and amplify carbon emissions. Plain Language Summary In light use efficiency (LUE) models, plant production is linearly related to canopy absorbed photosynthetically active radiation (APAR), based on the assumption that plants absorb and convert solar radiant energy into vegetation biomass with a given efficiency rate. Here, we used an enhanced LUE model driven with remote sensing observations to estimate plant productivity for 1982-2016. We found that over the study period, plant photosynthetic activity has increased over northern latitudes, which may partially offset the CO2 emissions from fossil fuel consumption. However, our results show that productivity in the tropical zones is declining rapidly due to increased water stress. With increased warming, water limitations are expected to increasingly limit global plant productivity. Key Points We used a new dynamic LUE model to estimate global GPP for 1982-2016 Increasing GPP in northern high latitudes is offset by GPP decline in the tropics Global GPP is shifting from being temperature limited to VPD limited