An increasing number of potentially habitable terrestrial planets are found. The search for atmospheric signatures of planetary habitability and life might be possible in the future. We want to quantify the accuracy of retrieved atmospheric parameters obtained from infrared emission spectroscopy. We use synthetic observations of hypothetical habitable planets together with chi2 statistics and least-square fits. When adopting the design of currently planned or proposed exoplanet characterization missions, we find that emission spectroscopy could provide weak limits on surface conditions of terrestrial planets. However, these mission designs are unlikely to allow to characterize the composition of the atmosphere of a habitable planet. Upon increasing the signal-to-noise ratios by about a factor of 2-5 compared to current mission designs, the CO2 content could be characterized to within two orders of magnitude. The detection of the O3 biosignature remains marginal. The atmospheric temperature structure could not be constrained. A full atmospheric characterization seems to be beyond the capabilities of currently planned missions when using only emission spectroscopy.< Leer menos