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Suggested Citation:"5 Introduction." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
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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

Suggested Citation:"5 Introduction." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
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Page 30

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.

Suggested Citation:"5 Introduction." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
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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

Suggested Citation:"5 Introduction." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
×
Page 29
Suggested Citation:"5 Introduction." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
×
Page 30
Suggested Citation:"5 Introduction." National Research Council. 2000. Reconciling Observations of Global Temperature Change. Washington, DC: The National Academies Press. doi: 10.17226/9755.
×
Page 31
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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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