Observed Temperature Trends a Trends over the Past Century

One of the obvious manifestations of anthropogenic emissions is expected to be an increase in the temperature of the air and sea surface (SST). As a result, there have been many analyses of such temperatures, for which there are substantial records based on instrumental measurements made in a number of locations back to approximately 1860 and in at least one location, Armagh Observatory, North Ireland, to 1795 (Wilson, 1998). A review of these data, as well as more limited temperature data at higher altitudes in the troposphere and stratosphere, is found in Bradley and Jones (1993) and IPCC (1996).

Figure 14.57 shows the globally averaged temperature anomalies for land and sea surface measurements from 1861 to 1994, relative to the 1961-1990 period (IPCC, 1996; Jones et al., 1994). Such data indicate there has been an increase in near-surface temperatures of ~0.3-0.6°C over this period, with an uncertainty of about 0.15°C. Measurements of underground temperatures from 358 boreholes in central Europe, southern Africa, Australia, and eastern North America show a similar temperature trend (Pollack et al., 1998). However, the increase has not been continuous, with substantial increases in temperature occurring between about 1920 and 1940, followed by a decrease and then an increase to the present time. The increase in the global average temperature over the past 40 years has been about 0.2-0.3°C. Similarly, the changes in temperature vary geographically and seasonally. For example, warming has occurred in the Northern Hemisphere over the continents, while cooling has occurred over the midlatitude North Pacific and over the northwestern Atlantic. The geographical and seasonal dependencies are summarized in IPCC (1996).

While the surface temperatures have clearly been increasing, some satellite measurements have suggested that the air temperatures in the troposphere have been cooling at altitudes where this was not expected. However, this is controversial (e.g., Pielke et al., 1998a, 1998b). For example, Wentz and Schabel (1998) have shown that the loss of satellite altitude with

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