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1—
Introduction

An overall increase in global-mean atmospheric temperatures is predicted to occur in response to human-induced increases in atmospheric concentrations of heat-trapping ''greenhouse gases" (IPCC, 1996). The most prominent of these gases, carbon dioxide, has increased in concentration by over 30% during the past 200 years, and is expected to continue to increase well into the future. Other changes in atmospheric composition complicate the picture. In particular, increases in the number of small particles (called aerosols) in the atmosphere regionally offset and mask the greenhouse effect, and stratospheric ozone depletion contributes to cooling of the upper troposphere and stratosphere.fr2],fr3]

Many in the scientific community believe that a distinctive greenhouse-warming signature is evident in surface temperature data for the past few decades. Some, however, are puzzled by the fact that satellite temperature measurements indicate little, if any, warming of the lower to mid-troposphere (the layer extending from the surface up to about 8 km) since such satellite observations first became operational incontinue

2 The troposphere is the atmospheric layer where the temperature generally decreases with height, extending from the surface up to approximately 10–15 km, and the stratosphere is the stable layer above that extending up to approximately 50 km.

3 Further complicating the response of the different atmospheric levels to increases in greenhouse gases are other processes such as those associated with changes in the concentration and distribution of atmospheric water vapor and clouds.



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Page 7 1— Introduction An overall increase in global-mean atmospheric temperatures is predicted to occur in response to human-induced increases in atmospheric concentrations of heat-trapping ''greenhouse gases" (IPCC, 1996). The most prominent of these gases, carbon dioxide, has increased in concentration by over 30% during the past 200 years, and is expected to continue to increase well into the future. Other changes in atmospheric composition complicate the picture. In particular, increases in the number of small particles (called aerosols) in the atmosphere regionally offset and mask the greenhouse effect, and stratospheric ozone depletion contributes to cooling of the upper troposphere and stratosphere.fr2],fr3] Many in the scientific community believe that a distinctive greenhouse-warming signature is evident in surface temperature data for the past few decades. Some, however, are puzzled by the fact that satellite temperature measurements indicate little, if any, warming of the lower to mid-troposphere (the layer extending from the surface up to about 8 km) since such satellite observations first became operational incontinue 2 The troposphere is the atmospheric layer where the temperature generally decreases with height, extending from the surface up to approximately 10–15 km, and the stratosphere is the stable layer above that extending up to approximately 50 km. 3 Further complicating the response of the different atmospheric levels to increases in greenhouse gases are other processes such as those associated with changes in the concentration and distribution of atmospheric water vapor and clouds.

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Page 8 1979. The satellite measurements appear to be substantiated by independent trend estimates for this period based on radiosonde data. Some have interpreted this apparent discrepancy between surface and upper air observations as casting doubt on the overall reliability of the surface temperature record,4 whereas others have concluded that the satellite data (or the algorithms that are being used to convert them into temperatures) must be erroneous. It is also conceivable that temperatures at the earth's surface and aloft have not tracked each other perfectly because they have responded differently to natural and/or human-induced climate forcing during this particular 20-year period. Whether these differing temperature trends can be reconciled has implications for assessing: • how much the earth has warmed during the past few decades, • whether observed changes are in accord with the predicted response to the buildup of greenhouse gases in the atmosphere based on model simulations, and • whether the existing atmospheric observing system is adequate for the purposes of monitoring global-mean temperature. This report reassesses the apparent differences between the temperature changes recorded by satellites and the surface thermometer network on the basis of the latest available information. It also offers an informed opinion as to how the different temperature records should be interpreted, and recommends actions designed to reduce the remaining uncertainties in these measurements.break 4 Unless specified otherwise, the "surface record" referred to in this report is a combination of the temperature of sea surface water and the temperature of surface air overland.