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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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