The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, "˜dust-bowl-like' megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming.
Nature's Quirin Schiermeier has an article on the study, and this eye-popping quote from Richard Seager, a Columbia University climate researcher:
The drying we expect for the twenty-first century is entirely the result of increased greenhouse forcing.
But we're not there yet, Seager tells Nature:
A signal of anthropogenic drying is emerging, but it is still small. I'd expect that by mid-century the human signal will exceed the amplitude of natural climate variability. Then we can safely say that the Southwest has entered a new climate stage.
UPDATE: Prehistoric drought in the SW is a big interest of mine, so I'm going to provide all the relevant press coverage links, as they come in. John Fleck, a science writer for The Albuquerque Journal, has a story and a post at his blog.