Anthropogenic Global Warming and the Western USA's Diminishing Snowpack: A Link

The recently-concluded American Geophysical Union (AGU) Annual Meeting in San Francisco brought some interesting news about a strong link between human-induced global warming and the shrinking snowpack in the Western USA.

I've pasted a copy of the abstract below, taken from the AGU WWW site (Eos Transactions, AGU 88(52)).

Human Induced Changes in the Hydrological Cycle of the Western United States

T.P. Barnett et al. (Barnett is with the Scripps Institution of Oceanography (SIO); there are 10 co-authors)

Numerous studies have documented marked changes in components of the hydrological cycle of the western United States over the last 50 or so years. The snow pack has decreased and been observed to melt earlier in the calendar year. Main spring river runoff has also been coming earlier in the year over the entire region, while air temperatures, especially in Spring, have increased. Previous studies have speculated that such changes are due to global warming. However, strong interannual variability and marked decadal fluctuations in weather patterns greatly affect the region, and previous studies have not performed a formal detection and attribution analysis of these changes, nor attempted to quantify the comparative effect of natural and anthropogenic forcing. In this study we have conducted a rigorous, multi-variate detection and attribution analysis with a variety of climate, regional and hydrological models to determine the causes of the recent changes in the western snowpack, large river flow and surface air temperature. Natural variability of the western climate system has been ruled out as the major cause of the changes. Similarly, changes in solar variability and volcanic impacts are also ruled out by the analysis. We find that the observed changes in the hydrological components mentioned above can be explained well by anthropogenic forcing (green house gases and aerosols) alone. This is the first time, to our knowledge, that regional changes in the hydrological cycle have been attributed rigorously to human induced forcing. Further, since the climate models used here simulated the last 50 years of observations well, we might do well to place stock in model predictions of future changes in the West. The current simulations suggest that within the next few decades, the western United States will face a 'climate crisis' wherein its sustainability will be severely stressed.

Here is an earlier (February 2004) Science news article with the ominous title, "As the West Goes Dry":

Download as_the_west_goes_dry_science.pdf

The graphic, from the talk, is taken from the SIO press release. It was also printed in Science, 21 December 2007, as a news report by Richard A. Kerr. That report can be accessed here.

The graphic shows the change in snowpack in the next 43 years.

Some might question the predictions, based, of course, on models - climate models coupled to hydrological models. All models have uncertainties. Climate models are normally very coarse-scale because on a global scale they (GCMs - General Circulation Models) generally assume conditions are the same at a given time in a 400 km x 400 km (250 miles x 250 miles) region (plan view) of atmosphere. The thickness of that 160,000 square kilometer (62,500 square miles) region can vary, depending upon how many atmospheric "layers" are used in the model. So we are talking about the same conditions over a large area, or within a large volume of the atmosphere. Not very realistic, but we are getting better as computing power and data availability improve.

In the case at hand, however, Barnett et al. were modeling climate not on a global scale but a regional scale, which permitted them to use a much finer scale in the model. That usually means more accuracy.

On the other hand, hydrological models on a regional scale are generally not very accurate, so to model the hydrology of the region shown in the graphic would be quite a feat.   

But their work is quite impressive, and as they say, it's the first line of evidence linking anthropogenic global warming to changes in the hydrological cycle in the Western USA.

And that, my friends, is not good news for Western water supply.

"Sometimes we forget that the water cycle and the life cycle are the same." -- Jacques Cousteau