May 12, 2004

Drought Doubt

Filed under: Droughts, Precipitation

The New York Times makes, but doesn’t test, a hypothesis that the drought in the western United States is related to human-caused global warming. Our tests prove it false.

Is the current five-year-long drought in the western United States linked to anthropogenic global warming? The masthead editorial in the May 10 New York Times leads its readers to believe that’s the case. Think of their supposition as a scientific hypothesis. One that you can test.

In 10 minutes, using data that are only a few mouse clicks away on the Internet, you can run that test. But apparently, the Times didn’t. We’ll do what that esteemed newspaper neglected to: test the hypothesis that global warming and western drought are related.

The Times editorial concluded:

While there is some comfort in learning that periodic droughts are a part of natural climate change, the fact is this drought is occurring at a time when climate change is being driven by unnatural global warming. Drought may be normal, and yet there may be nothing historically normal about this drought.

In Figure 1 we present the Palmer Drought Severity Index, or PDSI (a standard measure of drought intensity) for the Southwestern United States (made up of the states of Arizona, New Mexico, Colorado, and Utah) and global average temperatures. Our comparison begins in 1895 and ends in 2003. This data shows about as much of a lack of a relationship than any two datasets can—none, nothing, zippo.

PDSI

Figure 1. The relationship between global temperatures and Southwestern drought from 1895 to 2003. Basically, there is none. Knowing the global temperature provides you with no useful information as to the drought conditions in the Southwestern United States.

In an attempt to be as fair as possible, Figure 2 shows same plot, only this time we use only the past 30 years—the so called “greenhouse era.” Once again, there is no statistically significant relationship between global temperatures and southwestern drought conditions. Sometimes it is dry in the southwest when global temperatures are high, and sometimes it is wet when global temperatures are high.

PDSI

Figure 2. The relationship between global temperatures and Southwestern drought from 1974 to 2003, the time of greatest human impact on the global atmosphere. Still no relationship. Again, knowing the global temperature provides you with no useful information as to the drought conditions in the Southwestern United States.

The fact of the matter is that the climate of the western United States features periods of very droughty conditions. Figure 3 shows the complete 108-year history of the PDSI for the Southwest. The current conditions were rivaled in the mid-1950s and exceeded in the first several years of the 20th century (and in case any of the editorial staff of the Times are reading—global temperatures were quite cooler then than they are now!).

PDSI

Figure 3. 108-year history of the Palmer Drought Severity Index in the Southwestern United States (Arizona, New Mexico, Colorado, and Utah), 1895 to 2003.

Figure 4 shows an 800-year reconstruction of PDSI for the Southwest derived from tree-ring data by Edward Cook, a noted paleoclimatologist who has worked extensively on paleoclimate records of drought in the United States. Cook’s reconstruction shows that the region has suffered from frequent droughts, several of them of a much greater extent and severity than any we have observed during the past century or so.

PDSI

Figure 4. 800-year history of the Palmer Drought Severity Index for the Southwestern United States as reconstructed using tree-ring data, 1200 to 1988.

And one last piece of information that our readers may find interesting is that there is evidence that the past 800 years have been relatively wet in the West. There are some indications that for at least the 1200 years prior to that (from 0 A.D. to about 1200 A.D.) average conditions were far drier than we have come to expect (for a more complete overview of the past climate of drought in the western United States see: http://www.ngdc.noaa.gov/paleo/drought/drght_home.html).

The evidence is obvious. Severe drought conditions occur in the western United States at the same rate, with or without global warming. And, if the past is any guide to the future, drought conditions will continue to occur there, some being much worse than any we have seen in the last century. Since we are currently living in a time when global warming is fingered in every weather/climate event, it should come as no surprise that the Times editorial said what it did. But, as we have conclusively demonstrated, that was a cheap shot with no real scientific backing.

References:

Global Temperatures
http://www.cru.uea.ac.uk/cru/data/temperature/

Palmer Drought Severity Index Values
http://www.ncdc.noaa.gov/oa/climate/onlineprod/drought/ftppage.html

Reconstructed Palmer Drought Severity Index Values
ftp://ftp.ngdc.noaa.gov/paleo/treering/reconstructions/swd_recon.txt

Western Drought Historical Overview:
http://www.ngdc.noaa.gov/paleo/drought/drght_home.html




No Comments

No comments yet.

RSS feed for comments on this post. TrackBack URI

Sorry, the comment form is closed at this time.

Powered by WordPress