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.
If they're right, it could be pretty bad news, even for those who already worry about rising CO2. It's generally agreed that during the earlier warm period, known as the Miocene Climatic Optimum, which occurred 15 million years ago, the global temperature was high enough to make sea levels between 80 ft. and 130 ft. higher than they are today. According to the new study, CO2 levels in the atmosphere at that time hovered at from 390 to 430 parts per million (p.p.m.). Today's CO2 level: 387 p.p.m. and rising.
Of course, an increase in CO2 (or any other heat-trapping greenhouse gas) can't lead to that kind of sea-level rise unless the CO2 level stays high for a while. The latest projections suggest a rise of 6 ft. at most by 2100, even if CO2 continues to increase at the current rate. But the new study implies that failing to tamp down emissions could eventually lead to a disaster worse than most climate Cassandras have dreamed of.
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.
It's reasonable, in theory, and Tripati and her colleagues made sure to use two species of foraminifera that are still around ("You can grow them in the lab," she says), just in case the effect varied from one species to another. Beyond that, they compared their own foraminifera-based CO2 estimates for the past 800,000 years with the measurements from the ice caps and, says Tripati, "they matched to within 20 p.p.m." That makes her and her colleagues confident that the older measurements are valid as well.
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.'"
Lemonick is the senior science writer at Climate Central.