In a study of long-term drought over North America, Stahle et al. (2000) found that the 1930s Dust Bowl drought in the United States - which was the nation's most severe, sustained, and widespread drought of the past 300 years - was eclipsed in all three of these categories by a 16th century "megadrought." So horrendous was this earlier climatic event, the authors unhesitatingly stated that it "far exceeded any drought of the 20th century," which the climate alarmists keep trying to tell us was the warmest century of the entire past millennium.
Think of what would happen if a drought of such magnitude were to commence today, or even a drought of mere Dust Bowl-intensity. Look at what happens when a regional drought of only a few months' duration occurs and you have your answer. Climate alarmists are quick to point to such events as "fingerprints" of CO2-induced global warming; while those who challenge their pronouncements are dismissed as out-of-touch skeptics. When we study the peer-reviewed scientific literature on the subject, however, the fingerprints we find do not appear to be those of either CO2 or global warming.
In a study of precipitation variability in the United States' Chesapeake Bay watershed, Cronin et al. (2000) found a high degree of decadal and multidecadal variability in wet and dry conditions over the past 1000 years. "Extremely rapid [shifts] occurring over about a decade" were noted, with precipitation totals fluctuating 25 to 30%. In addition, the authors determined that this region had experienced a number of "mega-droughts" of 60-70 years duration, some of which "were more severe than twentieth century droughts."
In a study of drought in the central United States, Woodhouse and Overpeck (1998) reviewed empirical evidence obtained from several proxy data sources covering the past 2000 years, wherein they discovered indications of numerous "multidecadal- to century-scale droughts." The most recent century, on the other hand, was characterized by the authors as having experienced droughts of "moderate severity and comparatively short duration, relative to the full range of past drought variability." Hence, they concluded that "the full range of past natural drought variability, deduced from a comprehensive review of the paleoclimatic literature, suggests that droughts more severe than those of the 1930s and 1950s are likely to occur in the future."
The United States is not the only region possessing evidence of more frequent and severe droughts in times past. Gan (1998) reported that drought is a natural feature of the Canadian Prairie region. At least twenty serious droughts were experienced there in the 19th century, but only ten in the 20th century, leading Gan to state that "there is no solid evidence to conclude that climatic warming, if it occurred, has caused the Prairie drought to become more severe."
Similar conclusions have been reached with respect to the non-anthropogenic origin of drought in the African Sahel, which has experienced, since the late 1960s, one of the most persistent droughts recorded in the global meteorological record. According to a palaeolimnological study by Holmes et al. (1997), a 5500-year lake sediment sequence extracted from northeast Nigeria showed "convincingly" that "the present drought is not unique and that drought has recurred on a centennial to interdecadal timescale during the last 1500 years." Hence, because the current drought is no different from what has been observed to naturally occur in the Sahel over and over again for the past 1500 years, the authors note that it "is not likely to be purely anthropogenic in origin."
What, then, might be the cause or causes of the recurring Sahel droughts, as well as others? According to Woodhouse and Overpeck (1998), there are a number of different possibilities that act, either directly or indirectly, to induce changes in atmospheric circulation and moisture transport that may lead to drought. One of the more prominently mentioned mechanisms in the literature is variable solar activity. Yu and Ito (1999), for example, reported that recurring intervals of drought in the Great Plains of North America, with periodicities of 100, 130, 200 and 400 years, match "in surprising detail" with several solar indices, such that "this spectral similarity forces us to consider solar variability as the major cause of century-scale drought frequency in the northern Great Plains." As for the future of drought in North America's interior, Yu and Ito's data indicate that this area is currently in the midst of a dry period that may persist for another century.
Similar solar-related drought conditions with periodicities of 200 and 400 years have also been noted for the Great Plains by Dean and Schwalb (2000); and Black et al. (1999) report finding a solar related influence on climate variability in the North Atlantic, which, they contend, "may play a role in triggering changes in the frequency and persistence of drought over North America." Lastly, Verschuren et al. (2000) reported on a solar-drought link for equatorial east Africa, noting that all three of the severest drought events of the past 700 years there were "broadly coeval with phases of high solar radiation, and the intervening periods of increased moisture were coeval with phases of low solar radiation."
From the studies highlighted above, it is abundantly clear that the frequency and magnitude of drought vary naturally, possibly forced by cyclical fluctuations in solar activity, on decadal, multidecadal and centennial time scales. It is also clear, according to Verschuren et al. (2000), that climatic data from the short period of the instrumental record are "inadequate to appreciate the full range of natural variation in drought intensity at timescales relevant to socio-economic activity."
What do these findings imply about the use of "fingerprints" to detect the cause of climate change? They suggest to us that one has to either have on file, or obtain after the fact, a set of clear fingerprints of a variety of suspects with which to compare those found at the crime scene, especially if one wants to bring one of them to trial and make the charges stick in a court of law. The studies here reviewed comprise a fingerprint of natural and potentially-solar-induced drought variability that provides a much better fit to what we observe in the world today than do the unverified (and likely unverifiable!) predictions (Shall we call them smudged fingerprints?) of global climate models of CO2-induced global warming. Indeed, there is not even enough evidence to prod a prosecutor to seek an indictment of CO2 in the case in question. Truly, if there ever was a time to say "case dismissed," this is it.
References
Black, D.E., Peterson, L.C., Overpeck, J.T., Kaplan, A., Evans, M.N. and Kashgarian, M. 1999. Eight centuries of North Atlantic Ocean atmosphere variability. Science 286: 1709-1713.
Verardo, S., McGeehin, J., Kerhin, R., Holmes, C., Colman, S. and Zimmerman, A. 2000. Climatic variability in the eastern United States over the past millennium from Chesapeake Bay sediments. Geology 28: 3-6.
Dean, W.E. and Schwalb, A. 2000. Holocene environmental and climatic change in the Northern Great Plains as recorded in the geochemistry of sediments in Pickerel Lake, South Dakota. Quaternary International 67: 5-20.
Gan, T.Y. 1998. Hydroclimatic trends and possible climatic warming in the Canadian Prairies. Water Resources Research 34: 3009-3015.
Holmes, J.A., Street-Perrott, F.A., Allen, M.J., Fothergill, P.A., Harkness, D.D., Kroon, D. and Perrott, R.A. 1997. Holocene palaeolimnology of Kajemarum Oasis, Northern Nigeria: An isotopic study of ostracodes, bulk carbonate and organic carbon. Journal of the Geological Society, London 154: 311-319.
Lins, H.F. and Slack, J.R. 1999. Streamflow trends in the United States. Geophysical Research Letters 26: 227-230.
Stahle, D.W., Cook, E.R., Cleaveland, M.K, Therrell, M.D., Meko, D.M., Grissino-Mayer, H.D., Watson, E. and Luckman, B.H. 2000. Tree-ring data document 16th century megadrought over North America. EOS, Transactions, American Geophysical Union 81: 121, 125.
Verschuren, D., Laird, K.R. and Cumming, B.F. 2000. Rainfall and drought in equatorial east Africa during the past 1,100 years. Nature 403: 410-414.
Woodhouse, C.A. and Overpeck, J.T. 1998. 2000 years of drought variability in the central United States. Bulletin of the American Meteorological Society 79: 2693-2714.
Yu, Z. and Ito, E. 1999. Possible solar forcing of century-scale drought frequency in the northern Great Plains. Geology 27: 263-266.