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Although persistent drought in West Africa is well documented from the instrumental record and has been primarily attributed to changing Atlantic sea surface temperatures, little is known about the length, severity, and origin of drought before the 20th century. We combined geomorphic, isotopic, and geochemical evidence from the sediments of Lake Bosumtwi, Ghana, to reconstruct natural variability in the African monsoon over the past three millennia. We find that intervals of severe drought lasting for periods ranging from decades to centuries are characteristic of the monsoon and are linked to natural variations in Atlantic temperatures. Thus the severe drought of recent decades is not anomalous in the context of the past three millennia, indicating that the monsoon is capable of longer and more severe future droughts.

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This resulted in extraordinarily low lake levels, even in Africa's deepest lakes. On the basis of well dated paleoecological records from Lake Malawi, which reflect both local and regional conditions, we show that this aridity had severe consequences for terrestrial and aquatic ecosystems. During the most arid phase, there was extremely low pollen production and limited charred-particle deposition, indicating insufficient vegetation to maintain substantial fires, and the Lake Malawi watershed experienced cool, semidesert conditions (<400 mm/yr precipitation). Fossil and sedimentological data show that Lake Malawi itself, currently 706 m deep, was reduced to an Ϸ125 m deep saline, alkaline, well mixed lake. This episode of aridity was far more extreme than any experienced in the Afrotropics during the Last Glacial Maximum (Ϸ35-15 ka). Aridity diminished after 95 ka, lake levels rose erratically, and salinity/alkalinity declined, reaching near-modern conditions after 60 ka. This record of lake levels and changing limnological conditions provides a framework for interpreting the evolution of the Lake Malawi fish and invertebrate species flocks. Moreover, this record, coupled with other regional records of early Late Pleistocene aridity, places new constraints on models of Afrotropical biogeographic refugia and early modern human population expansion into and out of tropical Africa. cichlid evolution ͉ Lake Malawi ͉ Out-of-Africa Hypothesis ͉ paleoclimate ͉ paleolimnology

Stable isotope profiles of fossil freshwater bivalve shells and mammal teeth provide a record of the seasonal ␦ 18 O variation in surface waters of the Himalayan foreland over the past 11 m.y. Between 3.1 and 10.7 Ma the ␦ 18 O of surface waters approached or exceeded 0‰ standard mean ocean water (SMOW) in the dry season. Since 9.5 Ma the magnitude of seasonal variability in ␦ 18 O has remained essentially unchanged. Both observations imply that the Tibetan Plateau had attained sufficient elevation and area prior to 10.7 Ma to support a strong Asian monsoon. These data also imply that the ␦ 18 O of wet-season rainfall was significantly more negative (؊9.5‰ SMOW) prior to 7.5 Ma than after (؊6.5‰ SMOW). If this change is attributable to a lessening of the amount effect in rainfall, this agrees with floral and soil geochemical data that indicate increasing aridity beginning at 7.5 Ma.

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