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Natural Climate Variability on Decade-to-Century Time Scales
temperature, precipitation, and pressure, but there are problems associated with even these records. Measurements have never been global in extent, nor as spatially uniform as required by statistical sampling theory (Madden et al., 1993; Trenberth et al., 1992). Furthermore, the expected signals of greenhouse-gas-induced changes (e.g., as predicted by modeling experiments) for precipitation and pressure are poorly understood for a variety of reasons (Barnett and Schlesinger, 1987; Wigley and Barnett, 1990).
In this paper we consider two aspects of temperature variability. First, we study hemispheric-mean temperature variability over the last 100 years. Second, in order to examine temperature variations over longer periods, we look at some long single-site and composite instrumental records, principally from Europe, and at the few millennium-long paleoclimatic reconstructions of summer temperature. Although imperfect, these long records currently represent one of our only methods of looking at century-time-scale variability with "real" data.
VARIATIONS IN HEMISPHERIC MEAN TEMPERATURE
Before about 140 years ago, instrumental temperature measurements were limited to Europe, parts of Asia and North America, and some coastal regions of Africa, South America, and Australasia. By the 1920s the only areas without instrumentation were some interior parts of Africa, South America, and Asia; Arctic coasts; and the whole of Antarctica.
Although incomplete, the coverage since 1850 allows the development of continental and hemispheric averages of temperature. Using compilations of homogeneous station records, Jones (1988) and Jones et al. (1986a,c) produced a gridded (5° latitude by 10° longitude) data set of surface air-temperature anomalies over land for each month since January 1851. The basic data were interpolated on a regular grid to mitigate the effects of uneven spatial density of the station network. Interpolating the station data in absolute degrees is not a viable option, since this would be affected by varying station numbers, different station elevations, and different formulae for calculating monthly averages. The use of anomaly values from a common reference period (1951-1970) overcomes these problems. A consequence of this procedure is that hemispheric-mean temperatures are expressed in relative rather than absolute terms.
Time series of hemispheric-mean seasonal and annual temperature anomalies are shown in Figure 1. The features exhibited by the two sets of curves have been discussed before (see, for example, Folland et al., 1990, 1992; Jones and Briffa, 1992; Jones et al., 1986a,c; Wigley et al., 1985, 1986). The differences in temperature between the second half of the record (1946-90) and the first half (1901-45) are listed in Table 1. The annual series for each hemisphere shows a warming of the order of 0.5°C since the late nineteenth century. The warming is considerably more erratic in the Northern Hemisphere, where a cooling of about 0.2°C clearly occurred between about 1940 and 1970 in all seasons except spring. In the Southern Hemisphere the warming is more monotonic, and there is no evidence of cooling after 1940.
The various seasonal curves in Figure 1 show considerable variation in periods of warming and cooling. The greater difference between seasonal trends over the last 140 years occurs over the Northern Hemisphere. In summer and autumn, the 1980s were barely warmer than the temperature levels of the 1930s and 1940s. In winter and spring the 1980s were clearly the warmest decade. All seasons except summer show the long-term warming evident in the annual data. In summer, the 1850s to 1870s were as warm as the most recent two decades. The cooling from the 1870s to the 1880s is the most pronounced feature of the summer
TABLE 1 Temperature Differences, 1946-1990 Average Minus 1901-1945 (land-only data)