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could be viewed as an arid glacial phase. Similarly, the early Holocene transgression
represents a humid interglacial phase, and the late Holocene interval of low lake levels
represents a dry interglacial phase. This simplified fourfold subdivision ignores local
hydrological and geomorphic controls over rainfall, run-off, evaporation, seepage
losses and groundwater inflow, but it is probably closer to reality than the simple
dichotomy between arid glacial and humid interglacial climates.
12.10 Conclusion
From what we have discussed so far, it is evident that the glacial pluvial climates of
the North American deserts do not have equivalents in the tropical deserts and semi-
deserts of North and East Africa, Asia and Australia. This lack of synchrony prompted
Broecker et al. ( 1998 ) to argue for what they termed 'antiphasing' between rainfall
in the Great Basin of North America and the Rift Valley in East Africa. Milly ( 1999 )
accepted the primary conclusion of this work but questioned the fourfold decrease
in rainfall posited by these workers which reduced Lake Victoria to one-tenth of its
present area during the LGM, calculating instead that only a halving was needed. Be
that as it may, there now seems to be little dispute that when late Pleistocene lake
levels were high in the deserts of the south-west United States, they were low in the
lakes of the African Rift Valley. Conversely, when the East African lakes were high
during the early Holocene, the lakes were low in the Great Basin of North America.
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