Study: Oxygen Levels Have Varied Little Over Ages

A new study finds oxygen has probably not varied all that dramatically since dinosaurs first roamed the Earth. Scientists in Ireland say the atmosphere seems to have been at least 15 percent oxygen for hundreds of millions of years.

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The Earth's atmosphere is about 21 percent oxygen. But it hasn't always been. There have been ups and downs in oxygen levels and they may have played a role in mass extinctions and the evolution of mammals. A new experiment suggests the history of the Earth's oxygen may need a serious rewrite. NPR's Richard Harris has our story.

RICHARD HARRIS: Much as they'd love to, scientists can't travel back into time to measure the oxygen content of ancient air. So they need to find hints in the rocks. And one hint is the presence of charcoal dating back hundreds of millions of years. Charcoal means there were fires, and those fires needed oxygen. So, Claire Belcher and a colleague at University College in Dublin set out to answer a seemingly easy question, how much oxygen do you need to make a fire?

HARRIS: Being able to create a low-oxygen environment that's realistic is quite hard.

HARRIS: They spent well over a million dollars to build chambers that would allow them to control oxygen levels and other gases as well.

HARRIS: I had to wear what kind of looks like a spacesuit...

(SOUNDBITE OF LAUGHTER)

HARRIS: ...to actually work in the chambers because it was down to nine percent oxygen. Humans would have trouble below 18 percent oxygen.

HARRIS: Belcher and her colleague tried to light pine twigs and peat moss on fire, and they very gradually increased the amount of oxygen in the chambers.

HARRIS: I couldn't just go in there and strike one match, say, oh, look, it didn't light. So I was in there for a good few months in this spacesuit igniting fires.

(SOUNDBITE OF LAUGHTER)

HARRIS: And as they now report in Science magazine, it actually takes quite a bit of oxygen in the atmosphere to sustain a fire.

HARRIS: You needed atmospheric oxygen concentrations at 15 percent to ignite a wildfire, essentially.

HARRIS: That 15 percent is a lot more than some previous estimates. In fact, some scientists, using other methods to estimate oxygen levels, say there are times when they are a lot lower, around 10 percent. And one of those periods of low oxygen coincides with an enormous mass extinction 240 million years ago. So, scientists suspect low oxygen could even have been a major reason for that die-off. Maybe not if this new estimate of oxygen is correct.

Now, oxygen levels aren't just about understanding extinction, they could be involved in the evolution of animals. Paul Falkowski at Rutgers University has proposed that since the days of the dinosaurs, oxygen levels on Earth have gradually risen, from 10 percent to present levels of more than 20 percent.

HARRIS: All data show there was a rise of oxygen from about 150 million years ago to present time. And we believe that about 55 million years ago, there was a big tick of oxygen.

HARRIS: And oxygen boost gradually made the environment more favorable for our own lineage, the mammals.

HARRIS: Mammals started to get very large, and they rose to massive prominence over the next 20 million years. And so, the rise of placental mammals really is a story we believe that's still based upon high concentrations of atmospheric oxygen.

HARRIS: But the story gets more complicated when you look back farther in time. If, as Falkowski believes, the atmosphere was only 10 percent oxygen before the dinosaur boom times, that suggests a world with forest of ferns and conifers, but without fire.

So, there could've been a period of time on Earth where, essentially, there was not enough oxygen to create wildfires.

HARRIS: Exactly.

HARRIS: That's a wild thought.

HARRIS: It is a wild thought.

HARRIS: That's also possibly contrary to some of the fossil evidence. So right now, the pieces aren't all fitting together neatly. It's a work in progress. But the payoff would be understanding how our atmosphere came to be full of life-giving oxygen today.

Richard Harris, NPR News.

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