Are Today’s Droughts Exceptional?

We published a paper this year in Pure and Applied Geophysics entitled “Drought 2002 in Colorado – An unprecedented drought or a routine drought?“. We concluded that the recent drought was not exceptional and that the state is actually more vulnerable to drought than it was in the past as a result of population growth and increased competition for water among different sectors of society.

New research on droughts in other locations in the country from the National Climate Data Center (NCDC) provides additional evidence that the recent droughts over the last few years are not exceptional. One study in Montana based on proxy data from tree rings for the period 1540-2000 (see Climate of 2005 – August Montana Drought) has the following conclusions,

“Comparison of the MSD (mean summer deficit) reconstruction with the instrumental records of summer drought for northwest Montana leads to several observations. First, the most severe single-year summer droughts of the 20th century were probably matched or exceeded on only a few occasions in the prior four centuries. Second, the cumulative deficit during the extended period of dry summers in the 1920s and 1930s appears to be unprecedented in the 461-year reconstruction. This period was one of rapid retreat of glaciers in Glacier National Park (Pederson et al. in press). The current drought, though it includes two very dry summers, pales in comparison with that event and many other extended drought events seen in the tree ring record.”

Pederson, G. T., S. T. Gray, D. B. Fagre, and L. J. Graumlich, in press. “Long-Duration Drought Variability and Impacts on Ecosystem Services: A Case Study from Glacier National Park, Montana USA.” Ecological Applications. in press.

For Illinois, the following conclusions have been presented (see Climate of 2005 – August
Illinois Drought

“The current drought was very unusual in the speed of its onset, with PDSI (Palmer Drought Severity Index) dropping from slightly wet to severe drought in just five months. In the Illinois Division 1 instrumental record, this has occurred only once before, in 1936. But the drought’s current severity has been matched or exceeded on numerous occasions in the past, according to both the instrumental and tree-ring records. ”

All existing evidence has shown that earlier historic droughts and paleo-droughts in the United States clearly were of greater severity than what has occurred recently. This should be sobering to any climate scientist and policymaker. Even without human climate change, we are vulnerable to severe social, economic, and environmental disruption. This is why we have proposed the vulnerability paradigm as a more effective framework to address our future risks as has been discussed in our July 19, August 16, August 31, and September 6 weblogs.

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