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Figure 2.1.
Time series of seasonally averaged global surface temperature (December 1879–August 1999)
based on the Quayle et al. (1999) data set, computed as differences from the 1880–1998 mean.
The time series uses-an area-weighted average of the surface air temperature over land and
the temperature of water at the ocean's surface.

Temperature changes at and just above the earth's surface are of singular importance from the standpoint of societal and human impacts, and they are also widely regarded as an important indicator of human-induced climate change. However, if global warming is caused by the build-up of greenhouse gases in the atmosphere, it should be evident not only at the earth's surface, but also in the lower to mid-troposphere. Temperatures aloft can be measured in a number of ways, two of which are useful for climate monitoring: by radiosondes (balloon-borne instrument packages, including thermometers, released daily or twice daily at a network of observing stations throughout the world), and by satellite measurements of microwave radiation emitted by oxygen gas in the lower to mid-troposphere, taken with an instrument known as the Microwave Sounding Unit (MSU).5 The balloon measurements are taken at the same Greenwich mean times each day, whereas the times of day of the satellite measurements for a given location drift slowly with changes in the satellite orbits. The radiosonde network has been operative since the late 1940s and substantialcontinue

5 The Microwave Sounding Unit senses radiation in a number of different channels, each of which is representative of a different layer of the atmosphere. The measurements discussed in this report are derived from channel 2—a channel that senses radiation in the layer extending from the surface up to about 15 km. To eliminate the influence of the stratospheric radiation, rather elaborate processing is required. The processed data are referred to as MSU 2LT (lower to mid-troposphere). Successive, improved versions of the MSU 2LT data have been produced over the past several years. The current version (D) was released in early 1999. For further discussion see chapter 7.