  We compute the circularly polarized signal from atmospheric molecular oxygen.
Polarization of O2 rotational lines is caused by Zeeman effect in the Earth
magnetic field. We evaluate the circularly polarized emission for various sites
suitable for CMB measurements: South Pole and Dome C (Antarctica), Atacama
(Chile) and Testa Grigia (Italy). An analysis of the polarized signal is
presented and discussed in the framework of future CMB polarization
experiments. We find a typical circularly polarized signal (V Stokes parameter)
of ~ 50 - 300 {\mu}K at 90 GHz looking at the zenith. Among the other sites
Atacama shows the lower polarized signal at the zenith. We present maps of this
signal for the various sites and show typical elevation and azimuth scans. We
find that Dome C presents the lowest gradient in polarized temperature: ~ 0.3
{\mu}K/\circ at 90 GHz. We also study the frequency bands of observation:
around {\nu} \simeq 100 GHz and {\nu} \simeq 160 GHz we find the best
conditions because the polarized signal vanishes. Finally we evaluate the
accuracy of the templates and the signal variability in relation with the
knowledge and the variability of the Earth magnetic field and the atmospheric
parameters.
