Impacts of irrigation expansion on moist-heat stress: first results from IRRMIP

Irrigation rapidly expanded during the 20th century, thereby affecting climate via changes in water, energy, and biogeochemical cycling. Previous assessments of these historical climate effects of irrigation expansion predominantly relied on a single Earth System Model, and therefore suffered from structural model uncertainties. Here we quantify the impacts of historical irrigation expansion on climate by analysing simulation results from six Earth system models participating in the Irrigation Model Intercomparison Project (IRRMIP). Despite the large range of simulated irrigation water withdrawal from those models (~900 to ~4000 km3 after the year 2000), our results show that irrigation expansion causes a rapid increase in irrigation water withdrawal, which leads to less frequent 2-meter air temperature heat extremes across heavily irrigated areas (>4 times less likely). However, due to the irrigation-induced increase in air humidity, the cooling effect of irrigation expansion on moist-heat stress is less pronounced or even reversed, depending on the heat stress metric. In summary, this study suggests the priorities in irrigation dataset collection and parameterisation development, and shows that irrigation deployment is not an efficient adaptation measure to escalating human heat stress under climate change, calling for carefully dealing with the increased exposure of local people to moist-heat stress.