Anthropogenic Enhancement of Sahel Precipitation by Remote Agricultural Irrigation

Abstract:

Irrigated croplands expanded to almost 2% of the global land area by the year 2015. In climate models, the resultant impacts of irrigation on precipitation, particularly those far from irrigated regions, are poorly understood, and the governing mechanisms generally have not been rigorously analyzed. In this study, we describe historical global irrigation experiments with the Geophysical Fluid Dynamics Laboratory earth-system model ESM2Mb. Our results show that irrigation in the Middle East and South Asia enhances Sahel precipitation, fully counteracting the reduction of precipitation induced by other anthropogenic factors in ESM2Mb. By energy conservation, the irrigation-induced increase of the latent heat content of the air upwind of the Sahel leads to an increase of rising motion in the Sahel itself, thereby driving the enhancement of precipitation; this process involves a reversal of the mechanisms of Sahel drying associated with global warming. Our experiments imply that the 1970s-1980s drought and famine could have been far more harmful without the inadvertent benefit of remote irrigation. Future water availability in the Sahel could be affected by such factors as groundwater depletion in South Asia and the pace of rehabilitation of war-damaged irrigation infrastructure in the Tigris-Euphrates valley.