According to the rock record, until about a million years ago the climate followed a neat 41,000-year cycle of ups and downs. Then, abruptly, that cycle stopped operating, and another one — 100,000 years long — took over. Why? Some scientists speculated that a cosmic catastrophe had reset the climatic clock. Others simply shrugged. Now two paleoclimatologists — Steven Clemens, at Brown University in Rhode Island, and Ralf Tiedemann, at the University of Kiel, Germany — have shown that people have been asking the wrong question. The 100,000-year cycle was there, unnoticed, all along; the real mystery is not where it came from, but what turned up the volume.
The 41,000-year cycle was no mystery at all. That’s the time it takes the tilt of Earth’s axis to wobble from 22 degrees to 25 degrees from the vertical and back again. Those changes in tilt, or obliquity, affect the amount of sun each region of the planet gets, especially near the poles. And that, in turn, affects the climate.
The 100,000-year climate cycle was harder to explain. Astrophysicists had found three orbital cycles — 95,000 years, 124,000 years, and 404,000 years long — which, in combination, cause Earth’s orbit to stretch from nearly circular to slightly elliptical and back again about every 100,000 years. The shape of the orbit (which physicists call, charmingly, “eccentricity”) determines how close Earth gets to the sun. In principle, that could affect the climate. But while cycles such as the 41,000-year rhythm change the amount of radiation from the sun by several percent, the eccentricity cycle changes it less than a tenth of a percent — too little, climatologists thought, to have much effect.
Clemens and Tiedemann’s results may change that view. The pair studied a 460-foot-long cylinder of mud bored from the ocean floor near the Cape Verde Islands, off the northwest coast of Africa. The mud was deposited continuously between 5.2 million and 1.2 million years ago. Much of it was made up of the remains of foraminifera — tiny one-celled organisms whose heaped-up shells record the vagaries of Earth’s changing climate.
It works like this: The shells are made of calcium carbonate, which is made, in part, of oxygen atoms absorbed from seawater. A normal oxygen atom has 16 particles in its nucleus. But a small proportion of oxygen atoms have 18. Water with O-16 is lighter than water with O-18, so it evaporates more easily. Usually the O-16 water rains back down and returns to the ocean. But when the climate turns cold, a lot of the water gets trapped on land as ice and snow, leaving the O-18 water in the ocean. And when there’s a lot of O-18 in the ocean, there’s a lot of O-18 in the forams’ shells.
Clemens and Tiedemann measured the O-18 in foram skeletons to see how it varied over the 4-million-year time span of the sediment core. To tease apart the several climate patterns that might show up in the core, they used a mathematical tool known as a Fourier transform. Just as a prism splits a beam of light into different colors, a Fourier transform takes a jumble of cycles and splits it into bands representing the underlying time periods The researchers found highs and lows that, as expected, matched well-known orbital shifts, including the 41,000-year cycle. But three other patterns showed up as well. They were at 95,000 years, 124,000 years, and 404,000 years — just the ones predicted by eccentricity cycles.
They came through loud and clear, even though the core spanned a time before the mysterious climate switchover. The tiny 0.1-percent difference in solar radiation seems to have made a difference after all. The 100,000-year cycle was ticking away much more than a million years ago; it was just too faint to be detected.
Why it got stronger is anybody’s guess. Rich Muller, a physicist at Lawrence Berkeley National Laboratory in California, thinks that orbital change is too feeble to do the job and that Clemens and Tiedemann’s evidence is an artifact of data processing. “The 100,000-year cycle of the ice ages could not possibly be explained by eccentricity,” Muller says. Instead, he believes that a million years ago, two asteroids collided, creating a large dust cloud. The 100,000-year climate cycle is caused by the Earth’s periodic passage into and out of that dust cloud.
Most other scientists reject Muller’s theory, but no one yet has convincingly explained why the 100,000-year eccentricity cycle suddenly grew powerful enough to overwhelm the 41,000-year obliquity cycle. “The thing that Muller’s right about is that we don’t know how eccentricity works,” says David Thomson, a mathematician at Bell Labs in Murray Hill, New Jersey, who has worked extensively with ocean sediment cores. “The few explanations I’ve heard seem contrived.”