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5—
Introduction

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



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Page 29 5— Introduction 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

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Table 5.1 Summary of the characteristics of surface, MSU, and radiosonde observations.   Surface MSU Radiosonde Method of observations Thermometers in shelters (air) or sea water. Since 1982, satellite infrared (IR) oceanic observations blended with in situ observations. Atmospheric oxygen emits microwave radiation, the intensity of which is measured by the MSU and is proportional to temperature. Temperature sensors are carried upward through the atmosphere by the balloons and the data are radio-transmitted to ground receiving stations. Spatial coverage of measurements Good in most inhabited regions and shipping lanes. Spares elsewhere. Virtually complete global coverage. Very broad vertical layers (~ km). Poor in oceanic regions, in the developing world, and in sparsely populated land areas. Good elsewhere. Good vertical resolution from the surface to the lower stratosphere. Length of observation record Beginning in mid-nineteenth century, with expanding coverage in first half of twentieth century. Diminished land stations coverage in 1990s. Begins December 1978. Beginning in the mid- 1940s, with greatly expanded coverage in the early 1960s, but suffering some deterioration in the 1990s Directness of the temperature measurement Direct, in situ observation of temperature blended with satellite infrared for sea surfaces temperature. Remote measurement of radiative emissions. Direct, in situ observations of temperature Time-varying biases Raw data are influenced by changes in instruments, observing practices, and land use. Many biases related to, for example, spacecraft altitude, east-west orbital drift, solar heating, and instrument malfunctions. Many changes in instrumentation, observing methods, and the global network of stations. Multiplicity of instruments Sea surface temperature, marine air temperature, and land air temperature measured by tens of thousands of different thermometers of various types. Usually two spacecraft in orbit; 30,000 observations per day from each; 9 different satellites from 1979 to 1999. Each sounding made with a new instrument. Dozens of types used over time, varying from country to country, station to station. Number of independent efforts to produce the data sets Several groups, employing different methodologies. One main effort to date. A few groups, employing different methodologies.

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Page 31 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