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(1) Surface temperature is rising. Because global warming is a long-term process, which can be masked by year-to-year climate variability, it is most clearly revealed in the longest available record of global temperature—i.e., that of surface temperature (Figure 2.1), which is based on stations determined not to have been substantially impacted by urbanization. In the opinion of the panel, the disparity between satellite and surface temperature trends during 1979–98 in no way invalidates the conclusion of the Intergovernmental Panel on Climate Change report (IPCC, 1996) that global surface temperature has warmed substantially since the beginning of the twentieth century. Accelerated warming during the late 1990s has raised the estimated warming to 0.40–0.8 °C in the past 100 years. The warming of surface temperature that has taken place during the past 20 years is undoubtedly real, and it is at a rate substantially larger than the average warming during the twentieth century.
(2) Based on current estimates, the lower to mid-troposphere has warmed less than the earth's surface during the past 20 years. For the time period from 1979 to 1998, it is estimated that on average, over the globe, surface temperature has increased by 0.25 to 0.4 °C and lower to mid-tropospheric temperature has increased by 0.0 to 0.2 °C.9break
9 The range of these trend estimates is determined by applying different trend algorithms to the different versions of the surface and tropospheric data sets. Further discussion of the uncertainties inherent in these estimates is provided in chapters 6–9.
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3—
Findings
(1) Surface temperature is rising. Because global warming
is a long-term process, which can be masked by year-to-year climate
variability, it is most clearly revealed in the longest available
record of global temperature—i.e., that of surface temperature
(Figure 2.1), which is based on stations determined not to have
been substantially impacted by urbanization. In the opinion of the
panel, the disparity between satellite and surface temperature
trends during 1979–98 in no way invalidates the conclusion of
the Intergovernmental Panel on Climate Change report (IPCC, 1996)
that global surface temperature has warmed substantially since the
beginning of the twentieth century. Accelerated warming during the
late 1990s has raised the estimated warming to 0.40–0.8
°C in the past 100 years. The warming of surface temperature
that has taken place during the past 20 years is undoubtedly real,
and it is at a rate substantially larger than the average warming
during the twentieth century.
(2) Based on current estimates, the lower to mid-troposphere
has warmed less than the earth's surface during the past 20
years. For the time period from 1979 to 1998, it is estimated
that on average, over the globe, surface temperature has increased
by 0.25 to 0.4 °C and lower to mid-tropospheric temperature has
increased by 0.0 to 0.2 °C.9break
9 The range of
these trend estimates is determined by applying different trend
algorithms to the different versions of the surface and
tropospheric data sets. Further discussion of the uncertainties
inherent in these estimates is provided in chapters 6–9.
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(3) Current estimates of surface and lower to
mid-tropospheric temperature trends are subject to a level of
uncertainty that is almost as large as the apparent disparity
between them. The factors contributing to this uncertainty
are:
• the uncertainty inherent in temperature trends determined
over relatively short time periods;
• the complexity of the algorithms for processing the
satellite data, and the limited validation that has been performed
on them thus far;
• the possibility of biases remaining in the data sets that
have not yet been recognized and corrected;
• the uneven and, in some places, sparse spatial coverage
of radiosonde observations and, to a lesser extent, surface
observations; and
• the inherent difficulties in correcting for changes in
instrumentation and in the siting of radiosonde and surface
stations.
(4) The observed trends have been partially, but not fully,
reconciled with climate model simulations of human-induced climate
change. The simulated three-dimensional spatial pattern of the
temperature changes induced by increasing concentrations of a
well-mixed greenhouse gas, such as carbon dioxide, is complicated
and varies from model to model, but one common aspect is the
tendency for the lower to mid-troposphere to warm more rapidly than
the surface, except over high latitudes. More realistic model
simulations that take into account radiative forcing with combined
changes in human-induced and natural factors, including the
three-dimensional structure of the changing distribution of ozone,
are in better agreement with the observed changes, but they still
predict that the lower to mid-troposphere should warm at least as
rapidly as the earth's surface. The models used to perform these
simulations are subject to uncertainties and subject to change as
more realistic treatments of physical processes are incorporated
into them.
(5) The record of satellite observations of lower to
mid-tropospheric temperature is still short and subject to large
sampling fluctuations. Recent experiments with a number of
different climate models indicate that the inclusion of natural
climate forcings such as volcanic eruptions, stratospheric ozone
depletion, and solar variability can lead to a broad spectrum of
simulated 20-year surface and lower to mid-tropospheric temperature
trends. In light of this new information, thecontinue
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panel cautions that trends in such short periods of record with
arbitrary start and end points are not necessarily representative
of how the atmosphere is changing in response to long-term
human-induced changes in atmospheric composition. Given reliable
measurements, as outlined in Recommendation #1, the level of
confidence that can be attached to the trends will increase as the
period of record of upper air measurements lengthens.
(6) It is not currently possible to determine whether or not
there exists a fundamental discrepancy between modeled and observed
atmospheric temperature changes since the advent of satellite data
in 1979. Measurement uncertainties (Finding #3), modeling
uncertainties (Finding #4), and sampling uncertainties (Finding #5)
were all considered by the panel as possible causes of the
disagreement between models and observations. None of them can be
singled out as the dominant factor, nor can any one of them be
shown to be unimportant. Surface temperature and lower to
mid-tropospheric temperature are different entities, which should
not be expected to vary in precisely the same manner in response to
human-induced and natural climate forcings during a particular
20-year period of record. Hence, the panel concludes that at least
part of the observed disparity between the 20-year changes in
surface and mid-tropospheric temperature is probably real, but the
measurement, modeling, and sampling uncertainties alluded to above
make it difficult to precisely attribute the disparity to any
particular sources. A more definitive reconciliation of modeled and
observed temperature changes awaits the extension and improvement
of the observations and the algorithms used in processing them,
better specification of the natural and human-induced climate
forcings during this period, and improvement of the models used to
simulate the atmospheric response to these forcings.break
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