Indeed, scientists see signs of the relationship between warming and drought in western North America by analyzing trends over the last 100 years; evidence suggests that the more frequent drought and low precipitation events observed for the West during the 20th century are associated with increasing temperatures across the Northern Hemisphere.

These climate-model projections suggest that what we consider today to be an episode of severe drought might even be classified as a period of abnormal wetness by the end of the century and that a coming megadrought — a prolonged, multidecade period of significantly below-average precipitation — is possible and likely in the American West.

The current drought plaguing the country is worryingly consistent with these expectations. Although we do not attribute any single event to global warming, the severity of both the turn-of-the-century drought and the current one is consistent with simulations accounting for warming from increased greenhouse gases. The Northern Hemisphere has just recorded its 327th consecutive month in which the temperature exceeded the 20th-century average. This year had the fourth-warmest winter on record, with record-shattering high temperatures in March. And 2012 has already seen huge wildfires in Colorado and other Western states. More than 3,200 heat records were broken in June alone.

And yet that may be only the beginning, a fact that should force us to confront the likelihood of new and painful challenges. A megadrought would present a major risk to water resources in the American West, which are distributed through a complex series of local, state and regional water-sharing agreements and laws.