Recent studies have indicated the potential of using direct measurements of the resolved outgoing longwave radiation (OLR) by satellites to monitor the climatic state. Here we describe work undertaken at Imperial College which has sought to compare radiance observations made by two instruments flown on orbiting satellites well separated in time: the Infrared Interferometric Spectrometer (IRIS), operational from April 1970 to January 1971; and the Interferometric Monitor for Greenhouse Gases (IMG), recording from October 1996 to June 1997. The resolution of the measurements makes it possible to detect clear signatures of the effect of changes in greenhouse gas amounts upon the 'clear sky' OLR spectrum. These signatures are consistent with known increases in gas concentrations between the two observational periods. In addition, the 'all-sky' data may provide useful information concerning the response of the cloud field to the enhanced radiative forcing.