Correspondence ASSESSMENT OF PRECISION IN TEMPERATURES FROM THE MICROWAVE SOUNDING UNITS Comments on James Hansen and Helene Wilson (Climatic Change 25: 2, 1993) 1. Importance of Upper Air Measurements Monitoring the earth system has traditionally focused on measurements at the earth's surface because this is where we live and perform our most obvious life sustaining functions. In just the past few decades, however, efforts have been initiated to assess the character of the atmosphere above the surface. Regular upper-air measurements by balloon ascents in scattered locations began in the 1940's and observations from satellites generally began in 1979. The upper air is important in the climate context because changes in these higher levels may offer more discernible relationships to such phenomena as global warming due to the enhanced greenhouse effect than seen at other levels. Barnett and Schlesinger (1987) point out the probability that climate-change trends in the mid-troposphere (5-8 km) will be more clearly evident against the background of natural variability than at, for example, the surface. Indeed, results from coupled ocean-atmosphere models indicate that in terms of a global mean quantity the troposphere will actually warm at a rate greater than that of the surface temperature (e.g. Manabe et al., 1991; Boer et al., 1992; Christy and McNider, 1994). At higher elevations, above 12 km, the IPCC 1990 indicates there is a high degree of certainty that a greenhouse-gas induced decrease in stratospheric temperatures will occur. The measure of change in these upper atmospheric layers, in combination with quantities observed at the surface, provide an ensemble of information to give the most robust opportunity for climate change detection. 2. Recent Reports Concerning Data Reliability Microwave Sounding Units (MSUs) on polar orbiting satellites have demonstrated high precision and global coverage for the temperature of deep layers in the tropo- sphere and stratosphere (Spencer and Christy, 1992b, 1993; Christy and Drouilhet, Climatic Change 30: 97-102, 1995.