Variations in global-mean temperature are inferred from three different sets of measurements: surface observations, satellite observations, and radiosonde observations. Each of these kinds of measurements has its own particular strengths and weaknesses, as summarized in Table 5.1.
The satellite measurements of tropospheric temperature are the only ones that provide comprehensive global coverage, but rather intricate processing is required in order to infer global-mean temperature trends from the raw radiance data, and these trends have proven difficult to validate independently. Temperature measurements retrieved from the hundreds of balloon-borne radiosonde instruments that are released each day by the various national weather services provide much more detailed information on the vertical structure of atmospheric temperature changes than is available from satellites. The processing of these observations is straightforward, but large gaps in spatial coverage compromise the reliability of global averages, and changes in instrumentation can give rise to spurious trends. Surface temperature measurements derived from thermometers at land stations (housed in instrument shelters) and aboard ships (mostly engine intake temperatures) are more densely spaced than the radiosonde measurements. However, spatial sampling is still an issue in the higher latitudes of the Southern Hemisphere, and ensuring the homogeneity of these data in the face of urbanization and changes incontinue
instrumentation and observing protocols has proven to be a major challenge.
To appreciate the issues involved in comparing estimates of surface and lower tropospheric temperature trends, it is necessary to have at least a rudimentary understanding of these three kinds of measurements and the uncertainties inherent in each of them. Chapters 6, 7, and 8 present this basic background information, and the final chapter (9) discusses the issues involved in making comparisons between the different kinds of measurements. Collectively, these last four chapters of the report are the basis for the findings and recommendations presented in chapters 3 and 4.break