What’s Ahead for the Climate of the Southwest?

Recently, a colleague (Brad Udall, University of Colorado) and I wrote a short overview of climate change in the West—based on peer-reviewed literature—that appeared in the journal Science (Overpeck and Udall, 2010). The big story is really in the Southwest. What follows here is a summary of our findings.

First, like the globe, temperatures across the West are rising; the region is significantly warmer than the average during the 20th century (see Figure 1). In fact, warming in the Southwest is as dramatic as anywhere else in North America, save portions of the Arctic.

Second, this warming is having substantial impacts, including reduced late-season snowpack in the Colorado River headwaters, and it is contributing to the worst (and hottest) drought since the start of the 20th century. Combine this with an observed northward shift in winter storm tracks (a predictable response to a warming world), and we have significant reductions in the flow of the most important water source in the Southwest—the Colorado River. The two biggest reservoirs on the Colorado River, Lake Mead and Lake Powell, are now the emptiest they’ve been since Lake Powell was first filling back in the 1970s (Figure 2), and there hasn’t been a substantial refilling since they began their precipitous decline in volume around ten years ago. Lake Powell has refilled slightly, but Lake Mead continues to drop.

This unequivocal and unprecedented warming and drying in the West is also contributing to dramatic increases in vegetation death (see Figure 3) and large wildfires. In the Southwest, my UA colleague Dave Breshears and others have documented how more than a million hectares of pinyon pine tree death is linked to a record combination of warm temperatures and drought (Breshears et al. 2005). Others scientists have begun to document how plant death due to recent drought also extends down into the Southwestern deserts (McAuliffe and Hamerlynck, 2010). Moreover, Westerling et al. (2006) recently documented a major upsurge in large wildfire across the West due primarily to warming temperatures, not just the build-up of fuels following decades of human fire suppression.

What do all these dramatic changes mean for us? First and foremost, they give us a taste of what is at stake in the Southwest when it comes to climate change. All of the changes I’ve just described, plus others we discuss in our Science paper, have either been linked to, or are consistent with, human-caused climate change. In fact, our region is one of the few regions of the world where recent climate and hydrological (i.e., temperature, snowpack and river flow) change can be attributed to human-caused climate change with statistical confidence (Barnett et al., 2008).

The fact that there is such strong agreement between climate projections and what is now actually occurring gives us more confidence than other areas might have in anticipating what is to come. And, by all serious accounts, what we’ve seen so far is just a small taste of what lies ahead. Thus the current state of climate knowledge indicates that the choice for policy-makers in the Southwest is likely between slowing the rates of greenhouse gas emissions to the atmosphere, or planning for a hotter, more arid Southwest with all the associated impacts.

References Cited

• Barnett, T. P., D. W. Pierce, H. G. Hidalgo, C. Bonfils, B. D. Santer, T. Das, G. Bala, A. W. Wood, T. Nozawa, A. A. Mirin, D. R. Cayan, and M. D. Dettinger. 2008. Human-induced changes in the hydrology of the western United States. Science 319:1080-1083.
• Breshears, D. D., N. S. Cobb, P. M. Rich, K. P. Price, C. D. Allen, R. G. Balice, W. H. Romme, J. H. Kastens, M. L. Floyd, J. Belnap, J. J. Anderson, O. B. Myers, and C. W. Meyer. 2005. Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences of the United States of America 102:15144-15148.
• McAuliffe, J. R. and E. P. Hamerlynck. 2010. Perennial plant mortality in the Sonoran and Mojave deserts in response to severe, multi-year drought. Journal Of Arid Environments 74:885-896.
• Overpeck, J. and B. Udall. 2010. Dry Times Ahead. Science 328 1642-1643.
• Raffa, K. F., B. H. Aukema, B. J. Bentz, A. L. Carroll, J. A. Hicke, M. G. Turner, and W. H. Romme. 2008. Cross-scale Drivers of Natural Disturbances Prone to Anthropogenic Amplification: The Dynamics of Bark Beetle Eruptions. BioScience 58: 501-517.
• van Mantgem, P. J., N. L. Stephenson, J. C. Byrne, L. D. Daniels, J. F. Franklin, P. Z. Fule, M. E. Harmon, A. J. Larson, J. M. Smith, A. H. Taylor, and T. T. Veblen. 2009. Widespread Increase of Tree Mortality Rates in the Western United States. Science 323:521-524.
• Westerling, A., H. Hidalgo, D. Cayan, and T. Swetnam. 2006. Warming and earlier spring increase western US forest wildfire activity. Science 313:940-943.

Figure 2. Observed changes in the volumes of the two largest reservoirs on the Colorado River, highlighting the dramatic (and still ongoing, in the case of Lake Mead) drop in reservoir levels since the start of the 21st century western drought (data from U.S. Department of the Interior, Bureau of Reclamation).