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.
What was done
The authors developed a decadal-scale history of rainfall and drought in equatorial east Africa for the past thousand years based on lake-level and salinity fluctuations of a small crater-lake basin in Kenya, as reconstructed from sediment stratigraphy and the species compositions of fossil diatom and midge assemblages. They also compared this history with an equally long record of atmospheric 14CO2 production, which is a proxy for solar radiation variations.
What was learned
Equatorial east Africa was significantly drier than today during the Medieval Warm Period from AD 1000 to 1270, while it was relatively wet during the Little Ice Age from AD 1270 to 1850. However, this latter period was interrupted by three periods of prolonged dryness: 1390-1420, 1560-1625 and 1760-1840. These "episodes of persistent aridity," in the words of the authors, 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."
What it means
There are a couple of important lessons to be learned from the results of this study. First, the authors 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, with every new flood, or with every new hint of drought almost anywhere 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 President William Jefferson 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 between the rainfall and drought history of this study and the contemporaneous history of solar radiation variability. Based on this relationship, the authors state that variations in solar radiative output "may have contributed to decade-scale rainfall variability in equatorial east Africa." This conclusion is hailed as robust by Oldfield (2000), who states that the thinking of the authors on this point is "not inconsistent with current views." Indeed, he too suggests that their results "provide strong evidence for a link between solar and climate variability." Hence, it is very possible that solar effects play a much bigger role in determining the climatic state of the earth than is typically acknowledged by those who claim that the ongoing rise in the planet's surface air temperature is primarily a function of the contemporaneous increase in the air's CO2 content.
Oldfield, F. 2000. Out of Africa. Nature 403: 370-371.
Reviewed 15 February 2000