Levi replies: Joseph Gallagher has misunderstood what I wrote in my news story. The first part of my story dealt with the western US as a whole, not Lake Mead in particular. I reported on a study led by Tim Barnett, of the Scripps Institution of Oceanography; the study made no assertions about particular local conditions. As I stated in my story, “Only by including greenhouse gases and aerosols in the model simulations could [the researchers] adequately reproduce the spatial and temporal pattern of the changes [in temperature-related hydrological variables] that have been observed over the past 50 years.” I mention the low water levels in Lake Mead among my examples, but I did not mean to imply that the study could impute the causes of such local conditions.

Later in the story, I discuss Lake Mead in more detail. Both Gallagher and Robert Ayers have read more into that part than is actually there. I made no assertions about the current drought but rather reported on a prediction about possible future drought conditions. In particular, I cite work by Barnett and David Pierce, who estimated that the current level of water withdrawals from Lake Mead is unsustainable if one folds in climate model predictions that river runoff into the region will fall 10%–30% by 2050. Barnett and Pierce are not claiming that the current dry conditions are due to global warming.