CESM-simulated 21st Century Changes in Large Scale Crop Water Requirements and Yields

Abstract:

We assess potential changes in crop water requirements and corresponding yields relative to the late 20^th century in major crop producing regions of the world by using the Community Land Model (CLM) driven with 21^st century meteorology from RCP8.5 and RCP4.5 Community Earth System Model (CESM) simulations. The RCP4.5 simulation allows us to explore the potential for averted societal impacts when compared to the RCP8.5 simulation. We consider the possibility for increased yields and improved water use efficiency under conditions of elevated atmospheric CO₂ due to the CO₂ fertilization effect (also known as concentration-carbon feedback). We address uncertainty in the current understanding of plant CO₂ fertilization by repeating the simulations with and without the CO₂ fertilization effect. Simulations without CO₂ fertilization represent the radiative effect of elevated CO₂ (i.e., warming) without representing the physiological effect of elevated CO₂ (enhanced carbon uptake and increased water use efficiency by plants during photosynthesis). Preliminary results suggest that some plants may suffer from increasing heat and drought in much of the world without the CO₂ fertilization effect. On the other hand plants (especially C3) tend to grow more with less water when models include the CO₂ fertilization effect. Performing 21^st century simulations with and without the CO₂ fertilization effect brackets the potential range of outcomes. In this work we use the CLM crop model, which includes specific crop types that differ from the model’s default plant functional types in that the crops get planted, harvested, and potentially fertilized and irrigated according to algorithms that attempt to capture human management decisions. We use an updated version of the CLM4.5 that includes cotton, rice, and sugarcane, spring wheat, spring barley, and spring rye, as well as temperate and tropical maize and soybean.