Abstract. While greenhouse gases trap heat emanating from the Earth and thereby heat the surface atmosphere, they act as emitters in the high atmosphere and cool the air there. In 1989 Roble and Dickinson (1989) estimated the cooling that would occur in the thermosphere (250–500 km altitude) due to a doubling of greenhouse gas densities. Ever since, long-term data bases have been scoured for evidence of this thermospheric "global cooling." Here we show evidence that the thermosphere did indeed cool over the period 1966–1987, but the data suggest that the cooling accelerated at a "breakpoint year" around 1979 to a rate far larger than may be attributed to greenhouse cooling. This 1979 breakpoint year appears to coincide with a breakpoint year in ozone (O₃) column density. Further, the cooling was confined largely to the daytime thermosphere while the nighttime showed only a small trend. These results suggest, first, that the greenhouse cooling of the thermosphere may well not be detectable with current data sets and, second, that the long-term cooling that is clearly seen may be due largely to O₃ depletion.