In Africa, Verschuren et al. (2000) developed a decadal-scale history of rainfall and drought for the past thousand years based on lake-level and salinity fluctuations evident in a small crater-lake basin in Kenya, as reconstructed from sediment stratigraphy and the species compositions of fossil diatom and midge assemblages, after which they compared this history with an equally long record of atmospheric ¹⁴C production, which is a proxy for solar radiation variations. This work revealed that equatorial east Africa was much drier than today during the Medieval Warm Period from AD 1000 to 1270, but that it was relatively wet during the Little Ice Age from AD 1270 to 1850, although this latter period was interrupted by three shorter periods of significant dryness: 1390-1420, 1560-1625 and 1760-1840. These "episodes of persistent aridity," as they describe them, were "more severe than any recorded drought of the twentieth century." In addition, they discovered that "all three severe drought events of the past 700 years were broadly coeval with phases of high solar radiation, and the intervening periods of increased moisture were coeval with phases of low solar radiation."

There are a couple of important lessons to be learned from the results of this study. First, the researchers note that their results "corroborate findings from north-temperate dryland regions that instrumental climate records are inadequate to appreciate the full range of natural variation in drought intensity at timescales relevant to socio-economic activity." This point is important, for with almost every new storm of significant size and intensity, with every new flood, and with every new hint of drought somewhere in the world, there are claims that the weather is becoming more extreme than ever before as a consequence of global warming. In fact, such a claim was actually part of U.S. President Bill Clinton's final State of the Union address. We learn from this study, however, that there were far more intense droughts in the centuries preceding the recent rise in the air's CO2 content than have occurred during the current era of increasing greenhouse gas concentrations.

Another important point has to do with the strong correlation Verschuren et al. observed between the drought history of their study region and the concomitant history of solar variability. On the basis of this relationship, they concluded that variations in solar radiative output "may have contributed to decade-scale rainfall variability in equatorial east Africa." In a commentary on their work, this conclusion was hailed as robust by Oldfield (2000), who states that the thinking of Verschuren et al. on this point is "not inconsistent with current views." In fact, he too suggests that their results "provide strong evidence for a link between solar and climate variability."

In Europe, Starkel (2002) reviewed what was known at the time about the relationship of extreme weather events to climate during the Holocene. This exercise indicated that more extreme fluvial activity, of both the erosional and depositional type, was associated with cooler climates, and that "continuous rains and high-intensity downpours" were major problems, the "most distinct" of which was "from the Little Ice Age." These flood phases "were periods of very unstable weather and frequent extremes of various kinds," and Starkel's review revealed that "most of the phases of high frequency of extreme events during the Holocene coincide with the periods of declined solar activity." An example of a major recovery from one such period is the Younger Dryas-Preboreal transition, when temperatures in Germany and Switzerland rose by 3-5°C over several decades, which "fast shift," says Starkel, "caused a rapid expansion of forest communities, rise in the upper treeline and higher density of vegetation cover."

Also working in Europe, Mauquoy et al. (2002) extracted peat monoliths from ombrotrophic mires at Lille Vildmose (LVM), Denmark and Walton Moss (WLM), United Kingdom, which sites, in their words, "offer the possibility of detecting supraregional changes in climate," as they are separated from each other by about 800 km. From these monoliths, vegetative macrofossils were extracted at 1-cm intervals and examined using light microscopy; and where increases in abundances of S. tenellum and S. cuspidatum were found, a closely spaced series of ¹⁴C AMS-dated samples immediately preceding and following each of these wet-shifts was used to "wiggle-match" date them (van Geel and Mook, 1989), thereby enabling comparison of the wet-shifts with the history of ¹⁴C production during the Holocene.

This protocol revealed the existence of wet-shift climatic deteriorations that began in the mid-1400s and early 1600s in the VLM and WLM data, while the WLM data additionally revealed a wet-shift that began about 1215. These three climatic deteriorations mark the beginnings of periods of wet conditions that correspond closely in time with the Wolf, Sporer and Maunder minima of solar activity, as inferred from ¹⁴C data. Mauquoy et al. further report that these same time intervals "correspond to periods of peak cooling in 1000-year Northern Hemisphere climate records." Hence, they concluded that their work adds to the "increasing body of evidence" that "variations in solar activity may well have been an important factor driving Holocene climate change,"

In North America, Dean and Schwalb (2000) extracted sediment cores from Pickerel Lake, South Dakota, USA, in the 1960s and again in 1995, analyzing them for magnetic susceptibility, percent organic matter and percent calcium carbonate. This work revealed that over the past 2000 years there have been recurring incidences of major drought on the Northern Great Plains of the United States at approximately 400-year intervals, which cyclic behavior appears to have been in synchrony with similar variations in solar irradiance. The most recent of these droughty periods occurred between 200 and 400 years ago, contemporaneous with the Maunder Minimum of sunspot activity and coincident with the main cold phase of the Little Ice Age, which findings, in the words of the two researchers, implies "a direct connection between solar irradiance and weather and climate."

In South America, Haug et al. (2001) examined the titanium and iron concentrations of an ocean sediment core taken from the Cariaco Basin on the Northern Shelf of Venezuela, hoping to infer variations in the hydrologic cycle throughout the surrounding region over the past 14,000 years. They found that the titanium and iron concentrations were lower during the Younger Dryas cold period between 12.6 and 11.5 thousand years ago, corresponding to a weakened hydrologic cycle with less precipitation and runoff. During the Holocene Optimum (10.5 to 5.4 thousand years ago), however, concentrations of these metals were at or near their highest values, suggesting wet conditions and an enhanced hydrologic cycle for over five thousand years. Closer to the present, the largest century-scale variations in precipitation occurred between approximately 3.8 and 2.8 thousand years ago, as the amounts of titanium and iron in the sediment record varied widely over short time intervals. Higher precipitation was also noted during the Medieval Warm Period from 1.05 to 0.7 thousand years ago, followed by drier conditions associated with the Little Ice Age (between 550 and 200 years ago). As for the mechanics behind the phenomenon, Haug et al. say "these regional changes in precipitation are best explained by shifts in the mean latitude of the Atlantic Intertropical Convergence Zone," which, in turn, "can be explained by the Holocene history of insolation, both directly and through its effect on tropical Pacific sea surface conditions."

In conclusion, although lower solar activity generally results in lower temperatures worldwide (see Little Ice Age (Solar Influence - Temperatures), it can result in either lower or higher precipitation, depending on geographic location, as demonstrated by the findings reviewed above, where the results for the North and South American sites are different from the African and European sites.

References
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.

Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C. and Rohl, U. 2001. Southward migration of the intertropical convergence zone through the Holocene. Science 293: 1304-1308.

Mauquoy, D., van Geel, B., Blaauw, M. and van der Plicht, J. 2002. Evidence from northwest European bogs shows 'Little Ice Age' climatic changes driven by variations in solar activity. The Holocene 12: 1-6.

Oldfield, F. 2000. Out of Africa. Nature 403: 370-371.

Starkel, L. 2002. Change in the frequency of extreme events as the indicator of climatic change in the Holocene (in fluvial systems). Quaternary International 91: 25-32.

Van Geel, B. and Mook, W.G. 1989. High resolution ¹⁴C dating of organic deposits using natural atmospheric ¹⁴C variations. Radiocarbon 31: 151-155.

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.