What causes droughts? There is clearly no single simple answer to this question. Much recent research, however, is pointing to solar activity as the primary behind-the-scenes phenomenon that determines the broad outlines of drought severity and frequency.
Yu and Ito (1999), for example, studied a sediment core from a closed-basin lake in the northern Great Plains of North America, producing a 2100-year record that revealed four dominant periodicities of drought that matched "in surprising detail" similar periodicities of various solar indices. The correspondence was so close, in fact, that they say "this spectral similarity forces us to consider solar variability as the major cause of century-scale drought frequency in the northern Great Plains."
Dean and Schwalb (2000) derived a similar-length record of drought from sediment cores extracted from Pickerel Lake, South Dakota, which also exhibited recurring incidences of major drought on the northern Great Plains. They too reported that the cyclical behavior appeared to be in synchrony with similar variations in solar irradiance. After making a case for "a direct connection between solar irradiance and weather and climate," they thus concluded that "it seems reasonable that the cycles in aridity and eolian activity over the past several thousand years recorded in the sediments of lakes in the northern Great Plains might also have a solar connection."
Hodell et al. (2001) analyzed sediment cores obtained from Lake Chichancanab on the Yucatan Peninsula of Mexico, reconstructing the climatic history of this region over the past 2600 years. Long episodes of drought were noted throughout the entire record; and spectral analysis revealed a significant periodicity that matched well with a cosmic ray-produced 14C record preserved in tree rings that is believed to reflect variations in solar activity. Hence, they too concluded that "a significant component of century-scale variability in Yucatan droughts is explained by solar forcing."
Expanding the geographical scope of such studies still more, Black et al. (1999) found evidence of substantial decadal and centennial climate variability in a study of ocean sediments in the southern Caribbean that were deposited over the past 825 years. Their data suggested that climate regime shifts are a natural aspect of Atlantic variability; and in relating these features to records of terrestrial climate, they concluded that "these shifts may play a role in triggering changes in the frequency and persistence of drought over North America." In addition, because there was a strong correspondence between these phenomena and similar changes in 14C production rate, they further concluded that "small changes in solar output may influence Atlantic variability on centennial time scales."
Verschuren et al. (2000) conducted a similar study in a small lake in Kenya, documenting the existence of three periods of prolonged dryness during the Little Ice Age that were, in their words, "more severe than any recorded drought of the twentieth century." In addition, they discovered that all three of these severe drought events "were broadly coeval with phases of high solar radiation" - as inferred from 14C production data - "and the intervening periods of increased moisture were coeval with phases of low solar radiation." They thus concluded that variations in solar activity "may have contributed to decade-scale rainfall variability in equatorial east Africa."
In considering the similar findings of these several palaeoclimatic studies, there seems to be little question but what variations in solar activity have been responsible for much of the drought variability of the Holocene in many parts of the world.
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.
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.
Hodell, D.A., Brenner, M., Curtis, J.H. and Guilderson, T. 2001. Solar forcing of drought frequency in the Maya lowlands. Science 292: 1367-1370.
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.
Yu, Z. and Ito, E. 1999. Possible solar forcing of century-scale drought frequency in the northern Great Plains. Geology 27: 263-266.