Climate Disruption in the West: The Colorado River

The table above shows the averages from the multiple climate projections for mean river flows, compared to 1950-1999 levels, for each emissions scenario: a lower level, the so-called B1 scenario; a medium level, the A1B scenario; and a medium-high level, A2). (The Bureau study refers to the A2 scenario as assuming "higher" emissions, but at RMCO we follow the practice of the State of California and others, by referring more accurately to this scenario as having medium-high levels.) For each emissions scenario and each time period, the average of the projections is for a reduction in the river's flow compared to the historical level. (Some individual projections, though, are for future flow increases.) The Bureau's report correctly notes that the medium-emissions scenario actually assumes the highest level of emissions of any of these scenarios through about 2020. With this understanding, it is clear that in all time periods, the higher the level of emisssions, the lower the flows in the Colorado River are projected to be.

RMCO thinks this breakout of different futures should have been carried out throughout the report, including with respect to climate change effects on water demands. Certainly, water demands will be even higher the more that temperatures go up. Because the effects of different levels of emissions were not clearly and consistently spelled out in the Reclamation report, a major opportunity was lost for this important and expensive government effort to clearly show Westerners how higher emissions increase the threats to our water.

A second problem with the methodology used by the Bureau in its report is the way in which its single climate-change scenario was produced. The Bureau's method unintentionally skewed the average projection toward the projections from some climate models and away from the projections of others. Of the 112 model runs from 16 climate models in the database used by Reclamation, seven climate models contributed 84 projections and nine contributed the remaining 28 projections. Reclamation, in producing a single, averaged result of how climate change may alter the Colorado River, simply averaged together all 112 projections. This has the unintended consequence of skewing Reclamation’s number toward the models with the most individual projections.

This process issue may sound esoteric, but as a peer-review comment on a draft scientific journal article drawing on Reclamation’s approach says: "The methodology is fundamentally flawed in that it does not take proper account of the uneven number of ensemble members included across models. This is not a trivial omission considering that three models have ~5 ensemble members per scenario, while 8 other models have only 1 ensemble member per scenario. There is no logic nor justification for having scientific results that weight some models by a factor of 5 over others for reasons unrelated to model skill. The weighting used is essentially that of the donating institution’s computing budget, which is the main thing that determined the number of ensemble members available. . . . The methods used in this work need to be altered to weight results from all models equally."

The criticism by this reviewer reflects the scientific mainstream, embodied in a 2010 report of an Intergovernmental Panel on Climate Change expert meeting on best practices combining multi-model climate projections. The report squarely addressed the issue involved in Reclamation’s process and said: "Consideration needs to be given to cases where the number of ensemble members or simulations differs between contributing models. The single model’s ensemble size should not inappropriately determine the weight given to any individual model in the multi-model ensemble. In some cases ensemble members may need to be averaged first before combining different models, while in other cases only one member may be used for each model."

The same recommendation is in an article by Phil Mote of the Oregon Climate Change Research Institute, Levi Brekke of the Bureau of Reclamation, Phillip Duffy of Climate Central, and Ed Maurer of the University of Santa Clara, who wrote: “one should not simply lump all available simulations together, as this effectively gives more weight to the models contributing more simulations.”

RMCO does not have the raw data that would be needed to calculate the numerical difference that would have resulted had the Bureau first averaged together all model runs (or ensembles) from a single model, and then combined those averages into a single overall average. But we know that some models with multiple model runs project more precipitation than others, and that one of those models (by the National Center for Atmospheric Research), although having since been supplanted by a newer model, remains in the database used by the Bureau. It is entirely possible that using a better methodology would have increased the average reductions in Colorado River flows by more than one additional percent in all cases. For example, the Bureau's reported overall average climate-change effect (lumping together all three emissions scenaraios) of a niine percent reduction in water supply by 2041-2070 could instead have been a reduction by a double-digit percentage.

Although we at RMCO do not think the Bureau's report is perfect, we do welcome it as an invaluable contribution to the information available on how an altered climate may affect one of the most valuable resources to Westerners. From this report as well as other studies, it is obvious that we have a lot at stake as we continue changing the climate. To keep the West such a special place to live, we need both to reduce the extent to which we are changing the climate and to prepare for the changes that are coming, whether those changes are merely challenging or highly disruptive.

About 40 million Westerners -- more than one tenth of all Americans -- depend on the water of the Colorado River, most of which originates as snowfall in the Rocky Mountains. The river's water is used in seven states (California, Arizona, Colorado, Nevada, New Mexico, Utah, and Wyoming), including the largest city in each of those states (Los Angeles, Phoenix, Denver, Las Vegas, Albuquerque, Salt Lake City, and Cheyenne). According to an RMCO analysis, Colorado River water also is used by 22 of the 32 cities in those states with populations over 200,000. Many are among the fastest growing cities in the country, adding to local demands for water. And most areas served by the Colorado River are in arid or semi-arid regions, where the availability of adequate water is the primary limitation on life.

No wonder the U.S. Department of the Interior says the Colorado River is the most important natural resource in the southwestern United States.

This valuable resource is highly vulnerable to an altered climate, as has been shown by repeated studies over recent years. The Colorado River, in fact, is probably the most studied water source in the world in terms of the effects of climate disruption.

The latest and largest analysis of how climate change may affect the Colorado River is a study by the U.S. Bureau of Reclamation released in December 2012. The Colorado River Basin Water Supply and Demand Study is based on much new analysis and presents a wealth of new information that many people across the region, including those of us at RMCO, are still digesting. Its central importance is that it presents detailed information showing what other studies have shown: in the future, absent new water management actions, the water supplied by the river is increasingly likely to fall short of future demands for that water.

On the supply side, for 2041-2070 the average of many widely varying projections is that climate change will diminish typical river flows by about nine percent, compared to 1950-1999, mostly because of the effects of higher temperatures (as opposed to changes in precipitation). Demand by 2060 under different scenarios is projected to increase by about 10 to 28 percent, compared to 2015, because of both population growth and the effects of climate change, which is projected to increase agricultural and other needs for water and to increase water losses to evaporation. Clearly, new water management actions will be needed.

Although RMCO welcomes all the new information in the Bureau’s report, we also are disappointed with some of the ways in which the new data was analyzed and reported. Most importantly, the report primarily presents only a single future climate-change scenario, rather than separate analyses of different possible futures reflecting alternative levels of future emissions of heat-trapping pollution, as is standard with the best climate change reports, including others by the U.S. government. In the Bureau's report, however, for nearly all purposes a single climate-change future is presented, based on a simple averaging of 112 model runs from 16 climate models using three different, alternative levels of future emissions -- lower, medium, and medium-high levels.

With some digging, though, it is possible to discover that the Bureau’s data shows that higher future emissions of heat-trapping pollution would cause a greater imbalance of supply and demand than lower emissions would. The RMCO table in the next column is drawn from the one place in the report where this breakout appears (table B1 in Technical Appendix B, to be precise). In fact, it was only after RMCO and some of the other organizations active in our climate/water program sent joint written comments to the Bureau that the results from the different emissions scenarios were broken out in this one instance.