FIGURE 1.2 Climate changes in central Greenland over the last 17,000 years. Reconstructions of temperature and snow accumulation rate (Cuffey and Clow, 1997; Alley, 2000) show a large and rapid shift out of the ice age about 15,000 years ago, an irregular cooling into the Younger Dryas event, and the abrupt shift toward modern values. The 100-year averages shown somewhat obscure the rapidity of the shifts. Most of the warming from the Younger Dryas required about 10 years, with 3 years for the accumulation-rate increase (Figure 2.2). A short-lived cooling of about 6°C occurred about 8,200 years ago (labeled 8ka event), and is shown with higher time resolution in Figure 2.3. Climate changes synchronous with those in Greenland affected much of the world, as shown in Figures 2.1 and 2.3.

ages and other features of deeper time and with studies of decadal-centennial climate modes and variability. In focusing on the tendency of climate to change in fits and starts rather than smoothly, and thus to surprise humans and ecosystems, the study of abrupt climate change is distinct from related branches of climatology.

SCIENTIFIC EVIDENCE, PROCESSES, AND CONSEQUENCES FOR SOCIETY AND ECOSYSTEMS

Abrupt climate change has affected societies. For example, evidence in geologic records suggests that abrupt but persistent droughts caused so-



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