Fossils Suggest an Ancient CO2-Climate Link

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Some of the best evidence linking rising carbon dioxide levels to a warmer world comes from the coldest places on earth. Samples of ancient air extracted from deep inside the Antarctic and Greenland ice caps make it clear that CO2 is scarce in the atmosphere during ice ages and relatively abundant during warmer interglacial periods — like the one we're in now.

The relationship between CO2 and climate is clear going back about 800,000 years. Before that, however, it gets murkier. That's largely because ice and air that old haven't yet been found. So scientists rely instead on indirect measurements — and these have led to a climate mystery: some episodes of past warming, including a planetary heat wave about 15 million years ago and another about 3.5 million years ago, seem to have happened without a rise in CO2. No one quite understands why. Maybe other greenhouse gases were the cause — methane, for example. Or maybe it had to do with changes in ocean circulation.

But according to a new study just published in Science, there may not be any mystery after all. By looking at the chemistry of fossilized foraminifera — tiny sea creatures no bigger than a grain of sand — a team led by Aradhna Tripati, of University of the California, Los Angeles, has detected a significant CO2 bump during both warming episodes.

These scientists aren't the first to look at the chemistry of foraminifera; the fossils are abundant in ancient ocean sediments, so they're a particularly good tracer of the past. But they used a new technique to measure CO2: looking at how much of the element boron was present in the foraminifera's shells. When there's lots of CO2 in the air, there's also more in the top layers of seawater, where the relevant species of foraminifera live. That makes the water more acidic, which in turn makes the tiny animals incorporate less boron into their shells as they grow.

But while other scientists take the new result seriously, they're not quite ready to buy into it completely. "It's a really worthwhile and bold effort to understand a period we have a hard time explaining," says Ed Boyle, a professor of ocean geochemistry at MIT. "My cautious view is that this looks promising right now, but I've been studying chemical tracers in foraminifera for pretty much my whole career, and there are often unexpected twists and turns." It is, he says, "the kind of thing where they may turn out to be right, and we'll look back in five years and say, 'that was when it all began to come together.'"