Understanding how climate varies in time is a central issue of climate research. Of particular interest are climate variations which occur within the human lifespan, say over 5- to 100-y time scales. Climate changes might occur as a gradual drift to a new state, a series of long-term swings, or a sequence of abrupt steps. The climate record over the last 100 years or so exhibits ample evidence for all these types of variations (Jones et al., 1986), but we have little understanding of what causes and controls these regime changes (Karl, 1988; Wunsch, 1992). Though many of these variations in climate are certainly natural, some components could be associated with increased concentrations of greenhouse gases or other anthropogenic effects. To advance our understanding of mankind’s potential influence on climate, the study of various natural climate variations is of paramount importance.

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