May 14, 2012

Future Southwest Drought in Doubt?

Filed under: Droughts, Precipitation

One of the most “robust” signals from global climate models run under scenarios of increasing human greenhouse gas emissions is an even drier climate in the Southwestern U.S. than exists there currently.

The 2009 report “Global Climate Change Impacts in the United States” from the U.S. Global Change Research Program (a report which the EPA relied upon in making its “Endangerment Finding” from carbon dioxide and other greenhouse gases) has this to say about the prospects of future drought in the U.S. (p. 33):

“In the future, droughts are likely to become more frequent and severe in some regions. The Southwest, in particular, is expected to experience increasing drought as changes in atmospheric circulation patterns cause the dry zone just outside the tropics to expand farther northward in the United States.”

The 2007 Fourth Assessment Report of the IPCC (another report which the EPA relied heavily upon in making its “Endangerment Finding”) had this to say (p. 890):

“Annual mean precipitation is very likely to increase in Canada and the northeast USA, and likely to decrease in the southwest USA.”

Not surprisingly, the EPA included this statement about projected changes in precipitation in the Executive Summary of its Technical Support Document for its “Endangerment and Cause or Contribute Findings for Greenhouse Gases under Section 202(a) of the Clean Air Act”:

“Increases in the amount of precipitation are very likely in higher latitudes, while decreases are likely in most subtropical latitudes and the southwestern U.S., continuing observed patterns.”

But new research published in Geophysical Research Letters suggests that the USGCRP, the IPCC, and consequently, the EPA may be overdoing things a bit.

A research team led by Yanhong Gao of the University of Washington investigated that the differences between how large-scale global climate models and finer-scale regional climate models handled the characteristics of moisture flow in the atmosphere over the southwestern U.S.

The regional climate models (RCMs) include much finer scale processes than are included in the global climate models (GCMs). In the Southwest, this includes a finer representation of the complex, mountainous terrain which plays a key role in the regional precipitation processes. Here is how Gao et al. describe the situation:

While GCMs are internally consistent with respect to their representation of the water cycle in the land-atmosphere-ocean system, they suffer from coarse spatial resolution. This is especially problematic in topographically complex areas like the western U.S. For instance, within the Colorado River Basin, high elevation headwater areas are disproportionately important to the hydrology of the basin. GCMs, which typically use spatial resolutions of several degrees latitude by longitude, smooth the topography and do not capture the high elevations that are associated with these headwater areas. Therefore, GCMs only crudely represent the Continental Divide, a major topographic barrier that forms the eastern boundary of the basin.

Gao et al. compared how the RCMs handled the processes that lead to precipitation across the Southwest compared to how the processes were simulated in GCMs. They generally found that the better representation of the terrain by the RCMs allowed them to generate more future rainfall (based on better resolved atmospheric physics and terrain interaction) than that simulated by the coarser GCMs. According to Gao et al.:

The ability of RCMs to better resolve transient eddies and their interactions with mountains allows RCMs to capture the response of transient flux convergence to changes in stability. This leads to reduced susceptibility to hydrological change in the RCMs compared to predictions by GCMs.

In summary, this study suggests that limitations in how GCMs represent terrain and its effects on moisture convergence have important implications for their ability to project future drying in the SW where mountains play an important role in the regional water cycle.

This result from Gao et al. showing that RCMs generated more future precipitation than GCMs in the Southwestern U.S. couples with an earlier result from many of the same authors published last year (Gao et al., 2011) which showed that the enhanced resolution of RCMs allowed them to better simulate the snow accumulation and ablation at high elevations and consequently “runoff in the Colorado River Basin is less susceptible to a warming climate in RCMs than in GCMs.”

Together these two studies suggest that the future Southwest is considerably less threatened by water shortages from climate change than the USGCRP, IPCC, and EPA have assessed it to be.

Now, as our long-timer readers surely recognize, we are among the first to express doubt about the
ability of regional climate models to accurately portray future climate states (see here, for example), much less the ability for global climate models (from which the regional climate models get their driving data) to get things right. So, we are not touting the results presented by Gao et al. as being the final, or even more accurate, word on the subject of future southwest U.S. precipitation characteristics. But what we are pointing out is that the more in depth scientists study the mechanisms involved, the less confidence they have in the scope and magnitude of future changes in the system—or at least how they manifest themselves in the total amount of precipitation that the atmosphere delivers to the Southwest.

And, since the EPA lists a whole lot of follow-on (negative) impacts from decreasing precipitation in the Southwest (e.g., increased wildfires, more insect outbreaks, increased tree mortality, stress on municipal water systems, stresses on agriculture) if the magnitude of the drying is less than currently assessed, so too will be the magnitude of the related impacts.

At the very least, the Gao et al., study, and numerous others like it, illustrate that the detailed science of climate change continues to evolve, and that assessments of the current state of the understanding of climate change science are outdated as soon as (or even prior to when) they are published. Oftentimes, as in this case, the new science erodes the basis for which the EPA has justified its finding that human greenhouse gas emissions endanger public health and welfare and the string of regulations that result.

References

Gao, Y., J. Vano, C. Zhu, and D. P. Lettenmaier, 2011. Evaluating climate change over the Colorado River basin using regional climate models. Journal of Geophysical Research, 116, D13104, doi:10.1029/2010JD015278.

Gao, Y., et al., 2012. Moisture flux convergence in regional and global climate models: Implications for drought in the southwestern United States under climate change. Geophysical Research Letters, 39, L09711, doi:10.1029/2012GL051560




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