A multiyear decrease in the total global outgoing longwave radiation (OLR) is revealed by the Nimbus 7 data set. The size of the change is of the order of 1 W m-2. The data have been analyzed using spectral harmonic decomposition on the monthly averages. The two lowest order fitting coefficients (i.e., C00 and C10) directly yield the temporal behavior of the global OLR and the hemispheric difference in OLR between the northern and southern hemispheres, respectively. Using a hypothetical detrending technique, it is found that the decreasing signal in the global OLR cannot be ascribed to a drift in the instrumental sensitivity. The secular change in terms of decreasing with time is also found in C10. This represents that the OLR from the northern hemisphere is decreasing faster than that from the Southern hemisphere. Thus a single detrending fit to the instrumental sensitivity cannot remove the drift in the OLR. As a previous study showed that the annually averaged C00 variability is strongly correlated with observed variability of global mean surface air temperature (Ts), we show that C10 variability is also consistent with the relative variabilities of the hemispheric mean Ts. Finally, the analysis on the solar insolation during the same period shows that the decreasing insolation may be a contributing factor to the decrease of OLR. ¿American Geophysical Union 1991