FIGURE 5.1 Core stratigraphy from Lake Chichancanab showing measurements of calcium carbonate, sulphur, and δ18O of ostracods and the gastropod P. coronatus. General lithology is shown at the right of the diagram. Two periods of particularly dry climate (high evaporation/precipitation) are marked by coincident peaks of sulphur content and δ18O of both ostracods and gastropods. The first occurs between 8,000 and 7,200 years ago, and the second is centered at 1200 during the late Holocene. The latter is linked to the Mayan collapse (from Hodell et al. 1995).

features associated with the El Niño-Southern Oscillation (ENSO) in modern times (e.g., Kitzberger et al., 2001). While the discussion of future climate changes has been dominated by the paradigm of gradual climate warming, we highlight the paleoclimate evidence of Holocene drought because such abrupt changes are likely to be more disruptive of human societies, especially in those parts of the globe that currently have water shortages and marginal rain-fed agriculture.

The recognition of abrupt changes in past climates reinforces concerns about the potential for significant impacts of anthropogenic climate change. Current trends along with forecasts for the next century indicate that the

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