The Late Ordovician glaciation differs from other major Phanerozoic glaciations because of its short duration (∼1 m.y.). A satisfactory explanation for this glaciation must be able to account for its sudden initiation, short duration, and rapid termination; mechanisms operating on tectonic time scales appear to be precluded. Given recent studies of a major perturbation in the carbon cycle during the glaciation, a climate model investigation of the sensitivity of the Late Ordovician climate to changes in atmospheric pCO2 was undertaken. Under the condition of a 4.5% reduction in solar luminosity, permanent snow cover (taken as a key indicator of potential for glaciation) is dramatically different between five experiments. The range of 18X present atmospheric level CO2 (ice free) to 8X (“runaway” icehouse) lies within the uncertainty of previous geochemical estimates of Late Ordovician atmospheric pCO2. The strong sensitivity to the modest direct forcing from pCO2 changes is due to the ice-albedo feedback. A plausible increase in organic carbon burial could have drawn down enough atmospheric CO2 to have briefly lowered the climate system below a critical glacial inception threshold at the end of the Ordovician. Conversely, a high pCO2 is required for the rest of the early Paleozoic, which was essentially ice free.