Climate Change Increases Evaporative and Crop Irrigation Demand in North America

Abstract Across North America, warmer temperatures have increased reference evapotranspiration (ETo), taxing water resources. This problem is especially pronounced for semi‐arid regions with large amounts of irrigated agriculture, such as California's Central Valley. In this region, increased ETo has increased irrigation demand, but the role of anthropogenic climate change (ACC) in driving this increase has not yet been quantified. Here, we quantified the influence of ACC on ETo and how these changes have translated into increased irrigation demand. We calculated observational ETo from ERA5‐Land and counterfactual ETo that removes the forced changes simulated by 20 models from the Coupled Model Intercomparison Project Phase 6 from the observational records. At the scale of North America, we found that ACC drove a 64 mm increase in annual ETo from 1980 to 2022, compared to the observed 54 mm increase. The largest observed increases in ETo were found in the southwestern and central regions, where ACC has likely exacerbated trends linked to natural climate variability. The largest ACC contributor to increased ETo was increased vapor pressure deficit, while decreased solar radiation has tempered increased ETo. Finally, we found that ACC has increased annual crop irrigation demand in the Central Valley by 0.76 km 3 during 1980–2022, with cumulative increased irrigation demand of 9.2 km 3 , equivalent to ∼11% of the region's groundwater loss during this time. Our findings suggest that ACC is accelerating demand for water in this already water‐limited region, and this phenomenon is likely occurring in other semi‐arid agricultural regions of North America.