Jonathan Proctor, Angela Rigden, Duo Chan, Peter Huybers

It is well established that warming temperatures damage the yields of many crops across the globe. Yet the influence of water supply on global agricultural yield and its relation to water demand and direct temperature stress is unclear. A number of global studies found a minor influence for precipitation, whereas some regional analyses suggest a more prominent role for water availability. Here, we use satellite-based measurements to quantify how soil moisture and temperature jointly influence the global productivity of maize, soybeans, millet, and sorghum. Relative to empirical models using precipitation as a proxy for water availability, models using soil moisture better separate water supply stress from correlated heat stress, leading to a 30 to 120% increase in explained variance of inter-annual yield anomalies across crops. Historic yield anomalies are equally determined by temperature and soil moisture, whereas projected damages associated with climate change are substantially larger for temperature. Globally, yield damages of -9% to -32% are predicted across crops under SSP5-8.5 between 2015-2035 and 2080-2100. Projections using temperature and precipitation, instead of soil moisture, overestimate the magnitude of damages to agricultural productivity because they confound heat stress and dryness stress, and because dryness associated with historically hot days is proportionately more severe than that expected for global warming. These findings indicate that use of remotely sensed measurements of soil moisture improve the representation of water supply in empirical crop models and document the importance of accurately measuring and modelling the influence of water supply to predict historic and future changes in global agricultural productivity.