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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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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
OCR for page 30
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.
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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
Representative terms from entire chapter:
air temperature
