In the new study, researchers calculated past atmospheric oxygen levels by looking at air trapped inside ancient polar ice samples. Specifically, they looked at samples from Greenland and Antarctica.
The new estimates suggest that atmospheric oxygen levels have fallen by 0.7 percent over the past 800,000 years. The scientists concluded that oxygen sinks — processes that removed oxygen from the air — were about 1.7 percent larger than oxygen sources during this time.
Although a drop in atmospheric oxygen levels might sound alarming, the decrease the researchers found "is trivial in regard to ecosystems," Stolper told Live Science. "To put it in perspective, the pressure in the atmosphere declines with elevation. A 0.7 percent decline in the atmospheric pressure of oxygen occurs at about 100 meters (330 feet) above sea level — that is, about the 30th floor of a tall building."
One way out of this conundrum is a well-known but relatively untested concept that suggests "that on timescales longer than a few hundred thousand years, atmospheric carbon dioxide and Earth's temperature are regulated via a 'silicate weathering thermostat,'" Higgins said.
Basically, increasing atmospheric carbon dioxide levels will boost the rates at which volcanic rocks wear down and their components wash into the seas, which can then go on to trap atmospheric carbon dioxide in ocean minerals. This means that "one can have a change in atmospheric oxygen with no observable change in average carbon dioxide," Higgins said. "Importantly, this silicate weathering thermostat is one reason why Earth is thought to have remained habitable for billions of years despite changes in solar luminosity."
http://www.livescience.com/56219-earth-atmospheric-oxygen-levels-declining.html
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