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6.7.3. Other Climatic Variations

A number of other climatic indices reveal changes during the twentieth century and are discussed in the following sub-sections. Tropospheric and Stratospheric Temperatures Changes

Tropospheric and stratospheric temperatures are central to the problem of greenhouse warming because GCMs predict that temperature changes with enhanced concentrations of greenhouse gases will have a characteristic profile in these layers, with warming in the mid-troposphere and cooling in the much of the stratosphere. The cooler stratospheric temperatures would be an expected consequence of the increased trapping of terrestrial radiation in the troposphere.

Layer mean temperatures from a set of 63 radiosonde stations covering most of the globe have been derived by Angell (1988). Layer mean temperatures from this network have been globally integrated. Figure 6.11 shows that, over the globe as a whole, mid-tropospheric (850-300mb) temperatures have increased between the 1970s and 1990s, in parallel with surface temperature. In the upper troposphere (300-100mb) there has been a steady decline in temperature of about 0.4C since the 1960s. Such a finding is in general disagreement with model simulations that show warming at these levels in response to greenhouse forcing. Temperatures in the lower stratosphere (100-50mb) show the greatest change, especially since 1980. It is mostly attributed to changes over and around Antarctica, probably related, in part, to the decrease in springtime stratospheric ozone.