Net Greenhouse Gas Impacts of US Irrigation: Integrating Local Emissions and Global Land Sparing

Irrigation increases cropland productivity, improves resilience to intensifying climatic stressors, and is accordingly recognized as an effective strategy for climate change adaptation. Irrigation also produces greenhouse gas emissions through energy use for pumping, increased N₂O emissions, and degassing of CO₂ from supersaturated groundwater, and therefore involves a potential tradeoff between climate change adaptation and mitigation goals. However, irrigation may also decrease global demand for agricultural land by increasing yields, preventing land use change emissions. Here, we quantify the net greenhouse gas impact of US irrigation via both direct emissions and avoided land use change. First, we find that irrigation produces 18.9 Mt CO₂e yr^-1, 72% of which is due to energy use and thus can be mitigated through adoption of electric pumps coupled with decarbonization of the electric grid. Next, we use empirical models of irrigated to rainfed yield ratios to estimate the production benefits of irrigation in the US for 16 crop groups. Based on these production estimates, we use a global economic model for evaluating land use (GTAP-AEZ) to project hypothetical land use change in response to the loss of irrigated crop production in the US. Land use change projections are downscaled to 300 m resolution using the Spatial Economic Allocation Landscape Simulator (SEALS) model, calibrated on historical land use change. Finally, we leverage existing estimates of biomass and soil carbon stocks to quantify the carbon impacts of the projected land use change. Preliminarily, we find the carbon benefits attributable to avoided land use to be ~4.6 Gt CO₂e in total, equivalent to roughly 240 years of annual direct emissions from irrigation. These findings improve clarity regarding the environmental and economic tradeoffs of irrigation, particularly with respect to irrigation expansion for the sake of climate change adaptation.