One of the most “robust” signals from global climate models run under scenarios of increasing human greenhouse gas emissions is an even drier climate in the Southwestern U.S. than exists there currently.
The 2009 report “Global Climate Change Impacts in the United States” from the U.S. Global Change Research Program (a report which the EPA relied upon in making its “Endangerment Finding” from carbon dioxide and other greenhouse gases) has this to say about the prospects of future drought in the U.S. (p. 33):
“In the future, droughts are likely to become more frequent and severe in some regions. The Southwest, in particular, is expected to experience increasing drought as changes in atmospheric circulation patterns cause the dry zone just outside the tropics to expand farther northward in the United States.”
The 2007 Fourth Assessment Report of the IPCC (another report which the EPA relied heavily upon in making its “Endangerment Finding”) had this to say (p. 890):
“Annual mean precipitation is very likely to increase in Canada and the northeast USA, and likely to decrease in the southwest USA.”
Not surprisingly, the EPA included this statement about projected changes in precipitation in the Executive Summary of its Technical Support Document for its “Endangerment and Cause or Contribute Findings for Greenhouse Gases under Section 202(a) of the Clean Air Act”:
“Increases in the amount of precipitation are very likely in higher latitudes, while decreases are likely in most subtropical latitudes and the southwestern U.S., continuing observed patterns.”
But new research published in Geophysical Research Letters suggests that the USGCRP, the IPCC, and consequently, the EPA may be overdoing things a bit.