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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
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