Climatically sensitive tropical ecosystems provide important information that may help us to fill the gaps in our knowledge concerning the evolution of rainforests during periods of full glaciation. Small changes in precipitation in the Amazon Basin have immediate consequences for the survival of the Andean cloud forest, because its dominant source of moisture today is the Atlantic Ocean. A 40 k.y. lacustrine record from the Eastern Cordillera in Bolivia in an endemic species–rich and ecologically threatened region shows a dry Last Glacial Maximum, indicating a drastic decrease of the Amazonian moisture source. To explain this aridity, we infer steep temperature gradients between the pole and equator in both hemispheres that would have reduced considerably the size and displacement of the Intertropical Convergence Zone and the austral summer precipitation. This major change in water supply induced a dramatic reduction in species diversity and suggests that the Andean cloud forest did not provide refugia for tropical lowland taxa during full glacial times.