Irrigation and Climate Change: Long-run Adaptation and its Externalities

As the largest user of global water, irrigated agriculture accounts for 20% of global cropland and 40% of food production. Irrigation is also a potential adaptive response to drought and extreme temperatures. This paper examines the extent to which the current global irrigation footprint reflects climate change over the last 50 years. Utilizing exogenous variation in geological structure, I find evidence that farmers adapt by increasing groundwater irrigation in places that became dryer and hotter. The results hold globally and in the US. Observed recent warming is responsible for 9% of global irrigation growth. GRACE satellite data shows that climate-driven irrigation contributes to aquifer stress, as well as increased soil salinity, thus representing large negative externalities of adaptation to climate change and a potential threat to future food security.