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Recently, Poulsen et al. (2015) suggested that O2 has played a major role in climate forcing during the Phanerozoic. Specifically, they argued that decreased O2 levels during the Cenomanian stage of the middle Cretaceous (94–100 Ma) could help explain the extremely warm climate during that time. The postulated warming mechanism involves decreased Rayleigh scattering by a thinner atmosphere, which reduces the planetary albedo and allows greater surface warming. This warming effect is then amplified by cloud feedbacks within their 3-D climate model. This increase in shortwave surface forcing, in their calculations, exceeds any decrease in the greenhouse effect caused by decreased O2. Here we use a 1-D radiative-convective climate model (with no cloud feedback) to check their results. We also include a self-consistent calculation of the change in atmospheric ozone and its effect on climate. Our results are opposite to those of Poulsen et al.: we find that the climate warms by 1.4 K at 35% O2 concentrations as a result of increased pressure broadening of CO2 and H2O absorption lines and cools by 0.8 K at 10% O2 as a result of decreased pressure broadening. The surface temperature changes are only about 1 K either way, though, for reasonable variations in Phanerozoic O2 concentrations (10%–35% by volume). Hence, it seems unlikely that changes in atmospheric O2 account for the warm climate of the Cenomanian. Other factors, such as a higher-than-expected sensitivity of climate to increased CO2 concentrations, may be required to obtain agreement with the paleoclimate data.
Payne et al. (2016) studied this question.