Global carbon cycle models suggest that CO2 degassing from the solid Earth has been a primary control of paleoatmospheric CO2 contents and through the greenhouse effect, of global paleotemperatures. Because such models utilize simplified and indirect assumptions about CO2 degassing, improved quantification is warranted. Present-day CO2 degassing provides a baseline for modeling the global carbon cycle and provides insight into the geologic regimes of paleodegassing. Mid-ocean ridges (MORs) discharge 1–3 × 1012 mol/yr of CO2 and consume ∼3.5 × 1012 mol/yr of CO2 by carbonate formation in MOR hydrothermal systems. Excluding MORs as a net source of CO2 to the atmosphere, the total CO2 discharge from subaerial volcanism is estimated at ∼2.0–2.5 × 1012 mol/yr. Because this flux is lower than estimates of the global consumption of atmospheric CO2 by subaerial silicate weathering, other CO2 sources are required to balance the global carbon cycle. Nonvolcanic CO2 degassing (i.e., emission not from the craters or flanks of volcanos), which is prevalent in high heat flow regimes that are primarily located at plate boundaries, could contribute the additional CO2 that is apparently necessary to balance the global carbon cycle. Oxidation of methane emitted from serpentinization of ultramafics and from thermocatalysis of organic matter provides an additional, albeit unquantified, source of CO2 to the atmosphere. Magmatic CO2 degassing was probably a major contributor to global warming during the Cretaceous. Metamorphic CO2 degassing from regimes of shallow, pluton-related low-pressure regional metamorphism may have significantly contributed to global warming during the Cretaceous and Paleocene/Eocene. CO2 degassing associated with continental rifting of Pangaea may have contributed to the global warming that was initiated in the Jurassic. During the Cretaceous, global warming initiated by CO2 degassing of flood basalts, and consequent rapid release of large quantities of CH4 by decomposition of gas hydrates (clathrates), could have caused widespread extinctions of organisms.