The April 2008 issue of Physics Today features a short item about the Colorado River basin’s current drought (page 16). The writer of that story, Barbara Goss Levi, tries to tie the drought to anthropogenic global warming.

The US Geological Survey publication Climatic Fluctuations, Drought, and Flow in the Colorado River Basin (USGS Fact Sheet 2004-3062, helpfully supplies a graph of the Colorado flow since measuring began in 1890; it identifies for the reader four droughts since then, roughly centered on 1900, 1933, 1960, and 2003—one every 30–40 years.

According to the USGS report, tree-ring data show that droughts more severe than any of the 20th- and 21st-century ones occur fairly often; there have been 13 since 1226.

Colorado River droughts are not new. Climate varies.

A fancy computer-simulation giving human activity as an explanation of the current Colorado River basin drought is not needed. It will suffice to say that the current drought has the same cause that produced the drought of 1896–1906 and two or three per century for the last millennium. Climate happens.