Variations in the percentage of atmospheric oxygen may have influenced climate in the past 500 million years, according to new calculations by Christopher Poulsen and colleagues.

Since oxygen isn't a greenhouse gas like carbon dioxide, it typically hasn't been included in studies of past climate change. But the proportion of atmospheric oxygen has varied between 10 percent and 35 percent over the past 500 million years, and Poulsen and colleagues have developed a model to show how those fluctuations might impact climate.

In general, the percentage of oxygen contributes to the atmosphere's mass and density, which in turn influences how the atmosphere absorbs and scatters incoming solar radiation. When oxygen concentrations are low and the atmosphere is less dense, the changes to solar radiation scattering can lead to higher temperatures and precipitation, the researchers note.

They applied their model to data from the Cenomanian time period, about 100 to 94 million years ago - one of the warmest periods in the past 100 million years and a time of very low atmospheric oxygen concentration. By accounting for the climatic effects of oxygen, the researchers' model was able to predict the Cenomanian's temperatures more consistently than models that use carbon dioxide data alone. Daniel Peppe and Dana Royer discuss the results in a related Perspective.