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Reconciling Observations of Global Temperature Change (2000)

Chapter: Executive Summary

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Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
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EXECUTIVE SUMMARY

The global-mean temperature at the earth's surface is estimated to have risen by 0.25 to 0.4 °C during the past 20 years. On the other hand, satellite measurements of radiances indicate that the temperature of the lower to mid-troposphere (the atmospheric layer extending from the earth's surface up to about 8 km) has exhibited a smaller rise of approximately 0.0 to 0.2 °C during this period. Estimates of the temperature trends of the same atmospheric layer based on balloon-borne observations (i.e., radiosondes) tend to agree with those inferred from the satellite observations. The panel was asked to assess whether these apparently conflicting surface and upper air temperature trends lie within the range of uncertainty inherent in the measurements and, if they are judged to lie outside that range, to identify the most probable reason(s) for the differences.

To address these questions the panel had to consider:

• the factors that contribute to uncertainties in the trends inferred from three categories of instrumental measurements—Microwave Sounding Units (MSU) carried aboard National Oceanic and Atmospheric Administration (NOAA) satellites, radiosondes, and surface observations;

• the technical issues involved in making comparisons between global-mean temperature trends derived from measurements with differentcontinue

Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
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physical characteristics, different spatial and temporal sampling characteristics, and different error characteristics;

• the impact of the recent corrections to the algorithms for processing measurements derived from the MSU to account for satellite drifting and changes in instrument response;

• the contribution of natural climate variability to decade-to-decade climate changes, including changes in the atmosphere's vertical structure associated with natural variability;

• the changes in the atmosphere's vertical structure associated with human-induced climate changes; and

• the results of recent climate model simulations of temperature trends that take into account both natural variability and human-induced forcing.1

In the opinion of the panel, the warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century. The disparity between surface and upper air trends in no way invalidates the conclusion that surface temperature has been rising. The recent corrections in the MSU processing algorithms (referred to above) bring the global temperature trend derived from the satellite data into slightly closer alignment with surface temperature trends, but a substantial disparity remains. The various kinds of evidence examined by the panel suggest that the troposphere actually may have warmed much less rapidly than the surface from 1979 into the late 1990s, due both to natural causes (e.g., the sequence of volcanic eruptions that occurred within this particular 20-year period) and human activities (e.g., the cooling of the upper part of the troposphere resulting from ozone depletion in the stratosphere). Regardless of whether the disparity is real, the panel cautions that temperature trends based on data for such short periods of record, with arbitrary start and end points, are not necessarily indicative of the long-term behavior of the climate system.

Reducing uncertainties in the evaluation of the trends will require: (1) implementing an improved climate monitoring system designed to ensure the continuity and quality of critically needed measurements ofcontinue

1 A climate forcing is a perturbation to the energy balance of the earth-atmosphere system and may bring about climate change.

Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
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temperature, other climatic variables, and concentrations of aerosols and trace gases; and (2) making raw and processed atmospheric measurements accessible in a form that enables a number of different groups to replicate and experiment with the processing of the more widely disseminated data sets such as the MSU tropospheric temperature record. A number of possible research strategies for improving the understanding of uncertainties inherent in the various measurement systems and the relationship between surface and upper air temperature trends are proposed in the report.break

Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
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Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
×
Page 1
Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
×
Page 2
Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
×
Page 3
Suggested Citation:"Executive Summary." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
×
Page 4
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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." 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.

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 in 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, 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.

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