We present simulations of the impact of climate change on global water scarcity for five greenhouse gas emissions mitigation policy scenarios and compare them with a business-as-usual emissions scenario. A global water scarcity model is driven by climate change projections from 21 global climate models (GCMs). An aggressive policy scenario that gives a 50% chance of avoiding a 2 ¡C global-mean temperature rise from pre-industrial times could avoid almost 40% of the business as usual global impacts by 2100. However, mitigation policy does not completely eliminate the impacts of climate change. For any given GCM, the avoided impacts are affected more by the year at which emissions peak than to the rate at which emissions are subsequently reduced and the uncertainty across the 21 forcing GCMs is large.