FIGURE 1

Time series of area-averaged seasonal and annual mean temperature and total precipitation for the contiguous United States. The smoothed curve is a nine-point binomial filter. The horizontal line reflects the mean over the period of record.

fying a mean climate state.) Nonetheless, there is evidence that climate fluctuations on longer time scales occur throughout the world (Jones and Briffa, 1995; Diaz and Bradley, 1995). As Karoly (1995) shows, there is a dearth of the data that would permit the identification of decadal-scale circulation variability in the Southern Hemisphere, but analysis of the longer-term surface data available suggests that significant climate fluctuations persist at least through decadal time scales.

The forcing agents of many climate fluctuations may have their origin in either anthropogenic or natural factors, or both. These fluctuations are important in terms both of their socioeconomic and biophysical effects and of our need to distinguish between natural climate variations and anthropogenic climate changes. In analyzing the climate record, the dangers of "data dredging" must be kept in mind. As access to climate data increases, the chance of finding trends and variations that appear to be significant also increases. For this reason decade-to-century-scale climate fluctuations have been referred to as the "gray area of climate change" (Karl, 1988). For instance, Keeling and Whorf (1995) find evidence for decadal fluctuations in the global temperature record that can be reproduced by assuming the existence of two oscillations with small differences in frequency that beat on time scales of about 100 years. The search for an explanation of this statistical result is a good example of the challenge presented by the existence of these decadal fluctuations.

IDENTIFYING CLIMATE FLUCTUATIONS

The instrumental record of atmospheric and related land and marine observations is fragmentary until at least the middle of the nineteenth century. Moreover, virtually all of



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