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Western US droughts: Climate happens

In her report on the drop in Lake Mead’s water level (PHYSICS TODAY, April 2008, page 16), Barbara Goss Levi concludes that when the impact of greenhouse gases and aerosols is included in global climate models, the drop in water level seems to be related to greenhouse gases and not to natural variability.

A NASA article about Lake Mead (see http://www.nasa.gov/vision/earth/lookingatearth/Lake_Mead2004.html) suggests that the current drop in water level is associated with a drought that began in that area of the Southwest in 1999. Furthermore, the NASA article points out that a more severe drought occurred in the same area over a five-year period in the 1950s.

I agree with Levi that local climate variations can contribute to repeated droughts in an area; that connection is substantiated by tree-ring data in the Southwest from back before human impact was even a factor. Since we cannot correlate those early droughts with increases in anthropogenic greenhouse gases, we must be careful with assertions that everything can be deduced from global climate-model simulations, especially when the simulations are downscaled to finer grids. While you can certainly make the block averages of properties equal in fine and course grids, you cannot reproduce the initial spatial correlations that would exist when starting with a fine-grid model based on actual data. Furthermore, from my experience in flow simulation models, I know that any forecasts derived from those models depend on grid cell size and orientation.

We also need to further understand the causes of repeated events, like droughts, in the climate of an area before we make the jump to a global climate model; the correlation lengths of local and global climate phenomena are not equivalent. We must be careful with our assertions of global climate change and be aware of the limitations in our models, especially in the absence of actual data.

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, http://pubs.usgs.gov/fs/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.

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.