Meteorological reanalyses provide best estimates of the true state of the whole atmosphere, particularly over the past 40 years. As such, they are of eminent importance for detecting changes in the climate system both in the troposphere and in the stratosphere. However, reanalyses cannot yet produce reliable distributions of the atmospheric trace gases that determine the radiative properties of the atmosphere, such as water vapor, ozone, methane or carbon dioxide. Deficiencies in the representations of these trace gases are particularly pronounced in the upper troposphere and stratosphere (UTS).