Drought Risk of Global Terrestrial Gross Primary Productivity Over the Last 40 Years Detected by a Remote Sensing-Driven Process Model

Gross primary productivity (GPP) is the largest flux in the global terrestrial carbon cycle. Drought has significantly impacted global terrestrial GPP in recent decades, and has been projected to occur with increasing frequency and intensity. However, the drought risk of global terrestrial GPP has not been well investigated. In this study, global terrestrial GPP during 1981-2016 was simulated with the process-based Boreal Ecosystem Productivity Simulator model. Then, the drought risk of GPP was quantified as the product of drought probability and reduction of GPP caused by drought, which was determined using the standardized precipitation evapotranspiration index. During the study period, the drought risk of GPP was high in the southeastern United States, most of South America, southern Europe, central and eastern Africa, eastern and southeastern Asia, and eastern Australia. It was low at some high latitudes of the Northern Hemisphere and in part of tropical South America, where terrestrial GPP increased slightly in drought years. The drought risk of terrestrial GPP was greater during 2000-2016 than during 1981-1999 in 21 out of 24 climatic zones. The global mean drought risk of GPP increased from 13.6 g C m(-2) yr(-1) during 1981-1999 to 19.3 g C m(-2) yr(-1) during 2000-2016. The increase in drought risk of GPP was mainly caused by the increase in drought vulnerability. Simulation experiments indicated that the drought vulnerability of GPP was mainly induced by climatic variability. This study advances our understanding on the impact of drought on GPP over the globe. Plain Language Summary Drought generally affects gross primary productivity (GPP) of terrestrial ecosystems. However, the drought risk of global terrestrial GPP has not been well investigated. We assessed the drought risk of GPP based on the simulation by a process-based model for the years 1981-2016. The drought risk of GPP was quantified in terms of the chances of drought occurring and the anticipated reduction of GPP that would be caused by the drought. The results were variable across the world, being serious in the southeastern United States, most of South America, southern Europe, central and eastern Africa, eastern and southeastern Asia, and eastern Australia. The drought risk of GPP at some high latitudes of the Northern Hemisphere and in part of tropical South America was not found to be problematic. In those areas, GPP increased slightly in drought years in comparison with that in normal years. The drought risk of terrestrial GPP was greater during 2000-2016 than during 1981-1999 in 21 out of 24 climatic zones. The global mean drought risk of GPP increased from 13.6 g C m(-2) yr(-1) during 1981-1999 to 19.3 g C m(-2) yr(-1) during 2000-2016. The increase in drought risk of GPP was mainly caused by climatic variability.