We show that the cumulative CO emission from galaxies throughout cosmic history distorts the spectrum of the cosmic microwave background (CMB) at a level that is well above the detection limit of future instruments, such as the Primordial Inflation Explorer (PIXIE). The modeled CO signal has a prominent bump in the frequency interval 100-200 GHz, with a characteristic peak intensity of ~ 2×10−23 W m−2 Hz−1 sr−1. Most of the CO foreground originates from modest redshifts, z ~ 2-5, and needs to be efficiently removed for more subtle distortions from the earlier universe to be detected.
