century (Thambyahpillay, 1983; G. Wells, pers. commun.). Over the past 40 years, its characteristic response time te has varied from less than 1 year in 1985 to a maximum of 7.7 years in 1954-1969, when it covered an average of 21,520 km2 (Mason et al., 1994). Note, however, that a small component of seepage loss, approximately 5 percent of the water budget (Roche, 1977), has been ignored in making these estimates. The lake-level curve from A.D. 1900 onward lags the instrumental record of sub-Saharan rainfall (Nicholson, 1985) by a year or two at most.
The reconstructed Lake Chad water-level curve for the last millennium (Figure 6) shows large fluctuations of 20 to more than 100 years' duration. Particularly high levels were recorded during the twelfth to fourteenth and the seventeenth centuries.
On the time scales of the last 100 and the last 10,000 years, fluctuations in Sahelian rainfall have been attributed to large-scale changes in SST fields. Following earlier work by Lamb (1978) and Lough (1986), Folland et al. (1986) and Parker et al. (1988) showed that twentieth-century droughts in the Sahel were significantly correlated with cold SST anomalies in the northern oceans, particularly the North Atlantic, and warm anomalies in the southern oceans, particularly the South Atlantic and the Indian Ocean. This
hemispherically asymmetrical anomaly pattern, which also characterized important low stands of Lake Chad on the millennial time scale, was ascribed by Street-Perrott and Perrott (1990) to a reduction in the northward heat transport in the Atlantic by the global thermohaline circulation. There is as yet no evidence to determine whether or not drought episodes within the last millennium can be explained by the same mechanism.
The paleolimnological record provides examples of prolonged climatic fluctuations that have had profound significance for human communities in the tropics. For example, Lake Malawi (12°S), a large rift lake (28,750 km2) in equatorial Africa, has experienced variations in water level of at least 120 m vertical amplitude during the last few centuries (Owen et al., 1990). From A.D. 1860 onward, it fluctuated between 469 and 476 m above sea level. It was open throughout this period, with a response time te of about 3 to 8 years (R. Crossley, pers. commun.), apart from 1915 to 1935 when it ceased to overflow (Beadle, 1974). Between A.D. 1390 and 1860, however, there was a prolonged period of very low lake levels, represented by a widely traceable erosional unconformity and significant changes in diatom (algal) assemblages. During this low stand, the lake must have been closed, probably with a response time te much greater than 100 years. A rich oral tradition confirms that very low levels prevailed during the late eighteenth and early nineteenth centuries. Hydrological modeling by Owen et al. (1990) suggests that a reduction in rainfall of 40-50 percent, lasting for 100-150 years, could produce the required drop in lake level. The causes of such a prolonged anomaly remain uncertain, but probably involve large-scale oceanographic changes in the Indian and Atlantic Oceans.
An anomaly of similar duration is recorded by Lake Pátzcuaro (Mexico, 20°N) (O'Hara, 1992; Metcalfe et al., 1994). Lake Pátzcuaro is a small, closed lake with a modern te value of 22 years (from data in O'Hara, 1992). Sediment cores from the lake testify to a prolonged low stand, dated A.D. 800-1200. Low levels affected at least two other climatically sensitive lakes in the central Mexican highlands between A.D. 650 and 1200. This suggests that drought stress may have been a factor in the collapse of the great city of Teotihuacán (Basin of Mexico) and possibly of the Classic Maya ceremonial centers as well (Metcalfe et al., 1994; Hodell et al., 1995).
In contrast, isotopic investigations of Wallywash Great Pond, Jamaica (Street-Perrott et al., 1993; Holmes et al., 1995) indicate that a prolonged period of heavy rainfall, possibly of temperate origin, was centered on A.D. 750. Wet conditions also prevailed at this time in Lake Miragoane, Haiti (Curtis and Hodell, 1993). Paleolimnological studies of small lakes in the equatorial upper Amazon