Multi-model projections and uncertainties of irrigation water demand under climate change

Crop irrigation is responsible for 70% of humanity’swater demand. Since the late 1990s, the expansion ofirrigated areas has been tapering off, and this trend isexpected to continue in the future. Future irrigation waterdemand (IWD) is, however, subject to large uncertaintiesdue to anticipated climate change. Here, we use a set ofseven global hydrological models (GHMs) to quantify theimpact of projected global climate change on IWD oncurrently irrigated areas by the end of this century, and toassess the resulting uncertainties arising from both theGHMs and climate projections. The resulting ensembleprojections generally show an increasing trend in futureIWD, but the increase varies substantially depending onthe degree of global warming and associated regionalprecipitation changes. Under the highest greenhouse gasemission scenario (RCP8.5), IWD will considerably increaseduring the summer in the Northern Hemisphere (>20% by2100), and the present peak IWD is projected to shift onemonth or more over regions where ≥80% of the globalirrigated areas exist and 4 billion people currently live.Uncertaint ies arising from GHMs and global climatemodels (GCMs) are large, with GHM uncertainty dominatingthroughout the century and with GCM uncertaintysubstantially increasing from the midcentury, indicating thechoice of GHM outweighing by far the uncertainty arisingfrom the choi ce of GCM and associated emission scenario.