August 25, 2010

Southwest Drought?

Filed under: Droughts, Precipitation

As we have covered in previous essays, global warming alarmists insist that the southwestern United States is getting drier and will get substantially drier in the future due to the buildup of greenhouse gases. They bolster their claims by results from a relatively large number of articles in the professional scientific literature and countless comments in various UN IPCC reports. Throw in pictures of declining water levels at Lake Mead, some fountains in Las Vegas and golf courses in Phoenix, and just like magic, a scary scenario is produced.

As with virtually every other element of the climate change issue, the literature produces some surprises, and the drought in the Southwest claim runs up against some interesting realities. The latest article on this subject appears in a recent issue of the Journal of Geophysical Research and once again, the results are at odds with the popular perception of increased drought in the Southwest.

This recent work was produced by scientists with the U.S. Geological Survey and the University of Delaware and had to survive the peer-review process for this respected journal of the American Geophysical Union. The final two sentences reveal where this is going as McCabe et al. conclude “El Niño events have been more frequent, and this has resulted in increased precipitation in the southwestern United States, particularly during the cool season. The increased precipitation is associated with a decrease in the number of dry days and a decrease in dry event length.” What? More rain and fewer dry periods? We knew right away this would be featured in World Climate Report.

The authors focused on the Southwest “because (1) it has the highest consumptive use of water as a percentage of renewable supply in the United States and (2) dry event conditions in this region during the early 21st century have increased awareness of its vulnerability to water shortages.” There is no doubt that a lot of people have chosen to live in the Southwest and there is no doubt the desert climate of the region is prone to drought. In many respects, and depending on how one defines drought, the area is permanently in a state of drought (Phoenix has 7” of rain a year, Las Vegas averages about half of that amount).

McCabe et al. gathered data from 22 Weather Bureau-Army-Navy (WBAN) stations in the region “for water years (October through September) 1951 through 2006”. They explain that “During this period, 22 sites have nearly complete (99% complete) daily precipitation data. WBAN stations were selected because of the completeness of data record and the relative consistency of observational procedures.” They conducted their analysis for water years (October through September), cool seasons (October through March), and warm seasons (April through September).

They report that “trends in the fraction of dry days for water years, cool seasons, and warm seasons indicate that most trends are negative [i.e. towards more wet days, -eds.]. For water years, 18 sites exhibit negative trends in the fraction of dry days, and eight of these trends are statistically significant at a 95% confidence level. In contrast, only four sites indicate positive trends in the fraction of dry days for water years, and none of these trends is statistically significant at p = 0.05. For the cool season, 19 sites exhibit negative trends (12 are statistically significant at p = 0.05), and only 3 sites indicate positive trends (none are statistically significant).”

In this desert environment, cool season rain is far more important that rain in the summer. Rain falling in the hot summer season quickly evaporates and plays a relatively small role in water storage in the region. Nonetheless, the authors note that “For the warm season, 14 sites exhibit negative trends (seven are statistically significant), and 8 sites exhibit positive trends (six are statistically significant).”

The plot below (Figure 1) nicely reveals what has happened in the region. The number of dry days dropped over the entire study period but increased since 2000 (if you haven’t heard, the Southwest just experienced an unusually wet winter in 2009-2010). The authors varied the starting and ending dates in this time series of dry days and concluded “Examination of the counts of statistically significant trends in dry event length indicates small numbers of sites with significant positive trends for any period”.


Figure 1. Five year moving time series of the mean fraction of days with daily precipitation below 2.54 mm for water years (October through September), cool seasons (October through March), and warm seasons (April through September) (from McCabe et al., 2010).

McCabe et al. make a number of interesting comments that do not support the claims that the Southwest is drying; they state “Our results are consistent with analyses of trends in discharge for sites in the southwestern United States, an increased frequency in El Niño events, and positive trends in precipitation in the southwestern United States.” They elaborate noting “Since the mid-1970s, the frequency of El Niño events has been higher than the long-term average. Precipitation in the southwestern United States generally is greater during El Niño years than during normal and La Niña years. Increased precipitation in the southwestern United States associated with the higher frequency of El Niño events since the mid-1970s should result in decreased drought length”.

The McCabe et al. team concludes “Little evidence of long-term positive trends in dry event length in the southwestern United States is apparent in the analysis of daily WBAN precipitation data. During the mid-1990s to late 1990s, drought conditions began in the southwestern United States and persisted in the 21st century. This drought has resulted in positive trends in dry event length for some sites in the southwestern United States. However, most of the statistically significant trends in the number of dry days and dry event length are negative trends for water years and cool seasons.” Furthermore, they conclude “Since the mid-1970s, El Niño events have been more frequent, and this has resulted in increased precipitation in the southwestern United States, particularly during the cool season. The increased precipitation is associated with a decrease in the number of dry days and a decrease in dry event length.”

As with so many other articles we feature, had this group found general trends toward drier conditions, you would have heard about it already. They clearly did not, and their results are counter to the claims that the region should be trending to increased drought. That’s why you come to World Climate Report for a different perspective on what is really found in the scientific literature!

Reference:

McCabe, G. J., D. R. Legates, and H. F. Lins. 2010. Variability and trends in dry day frequency and dry event length in the southwestern United States, Journal of Geophysical Research, 115, D07108, doi:10.1029/2009JD012866.




No Comments

No comments yet.

RSS feed for comments on this post.

Sorry, the comment form is closed at this time.

Powered by WordPress