changes of temperature were never collected for that purpose; they were made primarily to aid in navigation, agriculture, commerce, and, in recent decades, weather forecasting. Thus, many uncertainties remain about important details of the temperature increase. The IPCC (1996a) has summarized known changes in the temperature record; this summary is presented in graphic form in the upper panel of Figure 6-1.
Recent global-scale measurements of layer-averaged atmospheric temperatures and sea surface temperatures from instruments aboard satellites have greatly aided our ability to monitor global temperature change (Spencer and Christy, 1992a,b; Reynolds, 1988), but the situation is far from satisfactory (Hurrell and Trenberth, 1996). Changes in satellite temporal sampling (e.g., orbital drift), changes in atmospheric composition (e.g., volcanic emissions), and techni-