Changing oxygen levels in Earth's atmosphere could have significantly altered global climate throughout the planet's history, a US study said on Thursday.
Although the proportion of atmospheric oxygen has varied between 10 percent and 35 percent over the past 541 million years, oxygen typically hasn't been included in studies of past climate change because it's not a greenhouse gas like carbon dioxide, said the study published in the US journal Science.
In the new study, paleoclimatologist Christopher Poulsen of the University of Michigan and two colleagues quantified the effect of changing oxygen levels on climate using an atmospheric global climate model to account for changes in atmospheric density, mass and molecular weights, Xinhua reported.
They developed a series of simulations in which oxygen levels varied from a low of 5 percent to a high of 35 percent and found that the percentage of oxygen contributes to the atmosphere's mass and density, which in turn influences how the atmosphere absorbs and scatters incoming sunlight.
In periods when oxygen levels declined, the resulting drop in atmospheric density increased the likelihood that incoming sunlight will make it to the surface without getting scattered away.
More sunlight means more evaporation from the surface, which leads to higher humidity levels and increased precipitation, they said. As humidity levels rise, temperatures also increase because water vapor is a potent heat-trapping "greenhouse" gas.
Rising oxygen levels have the opposite effect: a thicker atmosphere, more scattering of incoming sunlight, reduced surface evaporation, and less heat trapped by water vapor.
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"The connection between oxygen levels and climate has never been considered," said Poulsen. "It turns out that it's an important factor over geological timescales."
Changing oxygen concentrations could help explain features of the paleoclimate record not accounted for by variations in carbon dioxide levels, such as warm polar temperatures and unexpectedly high precipitation rates in some periods, he said.
Though previously unappreciated for its influence on climate, changing atmospheric oxygen levels have long been recognised for shaping the course of life on Earth.
Billions of years ago, for example, photosynthesising cyanobacteria in the oceans released massive amounts of oxygen that eventually made it possible for animals to colonise the land.