Stambaugh, M.C., Guyette, R.P., McMurry, E.R., Cook, E.R., Meko, D.M. and Lupo, A.R. 2011. Drought duration and frequency in the U.S. Corn Belt during the last millennium (AD 992-2004). Agricultural and Forest Meteorology 151: 154-162.
In his 21 March 2007 testimony before the United States Senate's Environment & Public Works Committee, Al Gore declared that "droughts are becoming longer and more intense," implying that global warming was the cause of it all.
What was done
In a study directly related to the validity of this declaration/implication, Stambaugh et al. "used a new long tree-ring chronology developed from the central U.S. to reconstruct annual drought and characterize past drought duration, frequency, and cycles in the agriculturally-important U.S. Corn Belt region during the last millennium," which chronology they calibrated and verified against monthly values of the instrumental Palmer Hydrologic Drought Index during the summer season of June, July and August.
What was learned
The six scientists report that "20th century droughts, including the Dust Bowl, were relatively unremarkable when compared to drought durations prior to the instrumental record." They note, for example, that the 19th century was the driest of the past millennium, with major drought periods occurring from about 1816 to 1844 and 1849 to 1880, during what they describe as the transition out of the Little Ice Age. Prior to that, there had been 45 years of drought in the latter part of the 17th century that were coincident with the Maunder Minimum of solar activity, which is associated with the coldest period of the current interglacial. And going back further in time, there was an approximately 35-year drought in the mid- to late-15th century during "a period of decreased radiative forcing and northern hemisphere temperatures."
Eclipsing them all, however, Stambaugh et al. write that "the approximately 61-year drought in the late 12th century (ca. AD 1148-1208) appears to be the most significant drought of the entire reconstruction," noting that it "corresponds to the single greatest megadrought in North America during the last 2000 years (Cook et al., 2007), as well as "unmatched persistent low flows in western U.S. river basins (Meko et al., 2007)." And this drought, as they describe it, occurred during the middle of the Medieval Warm Period -- "an interval of warmer temperatures between approximately AD 800-1300 characterized by greater drought duration and frequency in the Northern Great plains compared to more modern times."
What it means
It is abundantly clear from Stambaugh et al.'s findings that there is nothing unusual, unnatural or unprecedented about any 20th or 21st century droughts that may have occurred throughout the agricultural heartland of the United States. It is also clear that the much greater droughts of the past millennium occurred during periods of both relative cold and relative warmth, as well as the transitions between them. Thus, to testify that "droughts are becoming longer and more intense," and to imply that they are doing so because of global warming, is to be doubly disingenuous.
Cook, E.R., Seager, R., Cane, M.A. and Stahle, D.W. 2007. North American drought: reconstructions, causes, and consequences. Earth Science Reviews 81: 93-134.
Meko, D.M., Woodhouse, C.A., Baisan, C.A., Knight, T., Lukas, J.J., Hughes, M.K. and Salzer, M.W. 2007. Medieval drought in the upper Colorado River Basin. Geophysical Research Letters 34: 10.1029/2007GL029988.Reviewed 6 July 2011