“When Will Lake Mead Go Dry?” – A New Paper That Uses Multi-Decadal Global Models for Regional Predictions

Professor Chris Castro alerted us to the following paper;

Barnett, T. P, and D. W. Pierce, 2008: When will Lake Mead go dry? Water Resour. Res., 44, W03201, doi:10.1029/2007WR006704.

The abstract reads

“A water budget analysis shows that under current conditions there is a 10% chance that live storage in Lakes Mead and Powell will be gone by about 2013 and a 50% chance that it will be gone by 2021 if no changes in water allocation from the Colorado River system are made. This startling result is driven by climate change associated with global warming, the effects of natural climate variability, and the current operating status of the reservoir system. Minimum power pool levels in both Lake Mead and Lake Powell will be reached under current conditions by 2017 with 50% probability. While these dates are subject to some uncertainty, they all point to a major and immediate water supply problem on the Colorado system. The solutions to this water shortage problem must be time-dependent to match the time-varying, human-induced decreases in future river flow.”

The text includes the statements

We consider human-induced reductions in runoff of 10 to 30%, in accordance with estimates from global climate models and statistical analysis, and take these reductions to be linear in time over the next 50 years (i.e., runoff slowly decreases until it reaches a total reduction of, say, 10% below current levels in 2057)”;

“….we begin with deterministic estimates of when the live storage will be depleted by global warming-driven runoff reductions alone, without the outside impacts of evaporation and natural variability in the river flow”;

The climate models which have produced estimates of decreasing runoff have a host of problems of their own in handling the water budget from coarse resolution (little in the way of Rocky Mountains) to the variety of ways they handle soil processes and vegetation representations. However, a recent study of changes in hydrology of the western U.S. over that last 50 years shows several of the models, when run with observed anthropogenic forcings, reproduce extremely well the observed changes in river flow timing, snow pack decline and increasing air temperatures in the western United States [Barnett et al., 2008]. So these models, while not perfect, have a message to tell; a message supported by their ability to reproduce well the last 50 years of multivariate hydrological observations“;

and 

“…..the Colorado River will continue to lose water in the future, if the global climate models are correct.”

This paper correctly identifies that there is risk associated with the limited water available from the Colorado River. Indeed their statement that

Tree ring data suggest the long term flow of the Colorado experiences more variability than has been observed over the last century [NAS, 2007]. These data also suggest prolonged droughts far worse and more extensive than seen in the last 100 years of flow record on the River are possible

shows that the water resource is at risk regardless of how humans have altered the system. This is a conclusion we also reached in our paper

 Pielke Sr., R.A., N. Doesken, O. Bliss, T. Green, C. Chaffin, J.D. Salas, C. Woodhouse, J.L. Lukas, and K. Wolter, 2005: Drought 2002 in Colorado – An unprecedented drought or a routine drought?Pure Appl. Geophys., Special Issue in honor of Prof. Singh, 162, 1455-1479, doi:10.1007/200024-005-2679-6.

However, the paper suffers from their reliance on the multi-decadal global models as quantitative predictions of what will happen in terms of climate in the coming years. They even recognize this in their text “…..the Colorado River will continue to lose water in the future, if the global climate models are correct.” 

Thus while Climate Science agrees that there is a significant concern on water available from the Colorado River, and planning should be a major priority with respect to long-term drought, the multi-decadal global model predictions are just hypotheses and their use as part of the computation as definitive, skillful predictions to present quantitative probabilities of Lake Mead drying out is misleading to the policymakers. This is yet another example of overselling the skill that exists in using these models as predictions. The large amounts of precipitation this past winter (2007-2008) in large areas of the West should be a wake-up call on the serious limitations of the IPCC models.

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