While several model estimates predict cooling of the upper atmosphere as a result of increasing concentrations of greenhouse gases, direct observational evidence of such a trend is scarce and partly susceptible, because the relevant data series do not cover sufficiently long time periods. We study the long-term data set from the ionosonde station at Sodankyl¿ (67.4 ¿N, 26.7 ¿E), which has been operated during almost 4 solar cycles. We find a close to linear decrease in the altitude of the F2 layer peak during the last 39 years, when the effect of solar cycle variations is removed from the data. This local trend is qualitatively consistent with the model predictions of a cooling of the lower thermosphere.¿ 1997 American Geophysical Union