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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
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OCR for page 7
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.
OCR for page 8
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.
Representative terms from entire chapter:
air overland
