Long-term climate forcing by atmospheric oxygen concentrations

Abstract

Change was in the air The atmospheric fraction of molecular oxygen gas, O 2 , currently at 21%, is thought to have varied between around 35 and 15% over the past 500 million years. Because O 2 is not a greenhouse gas, often this variability has not been considered in studies of climate change. Poulson and Wright show that indirect effects of oxygen abundance, caused by contributions to atmospheric pressure and mean molecular weight, can affect precipitation and atmospheric humidity (see the Perspective by Peppe and Royer). These effects may thus have produced significant changes in the strength of greenhouse forcing by water vapor, surface air temperatures, and the hydrological cycle in the geological past. Science , this issue p. 1238 ; see also p. 1210