North American Droughts

By | August 6, 2007

Droughts are becoming longer and more intense … at least according to Al Gore, who made this unequivocal declaration in his 21 March 2007 testimony before the United States Senate’s Environment & Public Works Committee. So what do you think? Is he right? So right, in fact, that he need only declare it, and without the citing of a single shred of evidence? Perhaps a few quotes from a recent comprehensive review of the subject by Cook et al. (2007) will help to set the record straight, at least for Gore’s "home continent" of North America.

"Recent advances in the reconstruction of past drought over North America," in the words of Cook et al., "have revealed the occurrence of a number of unprecedented megadroughts over the past millennium that clearly exceed any found in the instrumental records." Indeed, they state that "these past megadroughts dwarf [our italics] the famous droughts of the 20th century, such as the Dust Bowl drought of the 1930s, the southern Great Plains drought of the 1950s, and the current one in the West that began in 1999," all of which dramatic droughts fade into almost total insignificance when compared to the granddaddy of them all, which they describe as "an epoch of significantly elevated aridity that persisted for almost 400 years over the AD 900-1300 period."

Although these observations clearly refute Gore’s flippant assertion that "droughts are becoming longer and more intense," they do much more than merely clarify this issue, as we will demonstrate shortly. First, however, we must report what Cook et al. have to say about the likely cause of the great megadroughts of the past.

Of central importance to North American drought formation, in the words of the four researchers, "is the development of cool ‘La Niña-like’ SSTs in the eastern tropical Pacific." Paradoxically, as they describe the situation, "warmer conditions over the tropical Pacific region lead to the development of cool La Niña-like SSTs there, which is drought inducing over North America."

In further explaining the mechanics of this phenomenon, on which both "model and data agree," Cook et al. state that "if there is a heating over the entire tropics [our italics] then the Pacific will warm more in the west than in the east because the strong upwelling and surface divergence in the east moves some of the heat poleward," with the result that "the east-west temperature gradient will strengthen, so the winds will also strengthen, so the temperature gradient will increase further … leading to a more La Niña-like state." What is more, they add that "La Niña-like conditions were apparently the norm [our italics] during much of the Medieval period when the West was in a protracted period of elevated aridity and solar irradiance was unusually high."

In light of these several observations, it would appear that throughout the AD 900-1300 period of what Cook et al. call "significantly elevated aridity" in North America, the tropical Pacific Ocean likely experienced significantly elevated temperature, which may well have been far greater than anything experienced over the course of the 20th century, because there was no period of time over the past ten to twelve centuries or more when North America experienced anything like the seemingly endless aridity of that 400-year megadrought that coincided with the great central portion of the Medieval Warm Period. And in light of this observation, we conclude that much of the Medieval Warm Period had to have been much warmer than even the warmest portion of the 20th century, or any time since. In fact, there is reason to believe that the world as a whole may well have been warmer during the bulk of the Medieval Warm Period than it is currently, for Cook et al. write that "the persistent droughts over North America all arose as part of the response of the global climate [our italics] to persistent La Niña-like conditions in the tropical Pacific Ocean." And this conclusion contradicts the climate-alarmists’ primary but unfounded claim that the world is currently warmer than it has been at any other time over the past two millennia or more.

Sherwood, Keith and Craig Idso

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.