Analysis of northern hemisphere mid‐latitude temperature data at 30 mbar over the last 20 years suggests a long‐term negative trend of about ‐0.24 K/decade when all observations between 10°N and 90°N are considered. This stratospheric cooling which could tentatively be attributed to the increase of the carbon dioxide amount in the stratosphere is in qualitative agreement at 30 mbar with a model calculation of the historical evolution of temperature and chemical composition in the middle atmosphere.