Lensing of the cosmic microwave background by modes that are larger than the size of the survey dilates intrinsic scales in the temperature and polarization fields and coherently shifts their observed power spectra with respect to the ensemble or all-sky mean. The effect can be simply encapsulated as a contribution to the power spectrum covariance matrix in accordance with the lensing trispectrum or as an additional parameter, the mean convergence in the field, for parameter estimation. It should be included for upcoming surveys that precisely measure acoustic polarization features deep into the damping tail at multipoles of $\ell \gtrsim 1500$ with less than 10% of the sky. Its omission may lead to seemingly conflicting values for the angular scale of the sound horizon, which may then provide erroneous cosmological parameters when compared to baryon acoustic oscillation measurements.

• Received 7 February 2014

DOI:https://doi.org/10.1103/PhysRevD.90.023003