Experiments with atmospheric general circulation models indicate that increased concentrations of carbon dioxide in the atmosphere may lead to reduced temperatures in the lower stratosphere as well as increased temperatures in the troposphere. An index designed to identify this signal in atmospheric observations has been computed from annual radiosonde temperature data for 19 stations in the Southern Hemisphere for the period 1964--85. This index has a trend of the same sign, consistent with the impact of increased concentrations of greenhouse gases, at all stations. This trend is significant at most of the stations. ¿ American Geophysical Union 1987