Passage IV

NATURAL SCIENCE: This passage is adapted from the article "What Causes Ice Ages?" by Traci Watson (©2002 by U.S. News & World Report).

The Cenozoic era, the most recent of the four major subdivisions of geologic history, began about sixty-five million years ago and continues to the present.

Scientists have long known about the giant ice sheets that the Cenozoic Ice Age ushered in. Even in the mid-19th century they knew that glaciers had repeatedly raked swaths of Europe and North America in the not so distant past. Yet despite the efforts of marine geologists, atmospheric chemists, oceanographers, and more, no one knows what caused the ice ages.

If they tried, scientists could hardly invent a more difficult mystery to crack. Most of the events in question took place tens or hundreds of millions of years ago. The shift in the Earth's climate was subtle: The planet is only 10 degrees cooler today than it was in the tropical period just before the Cenozoic.

Sixty million years ago, the planet started to cool in earnest. Twenty-five million years later, Antarctica was buried under a thick sheet of ice; 18 million years after that, glaciers overran Greenland. Last of all ice sheets invaded North America and Europe, retreated, advanced, retreated, and so on. Today, the planet is enjoying an "interglacial," one of the slightly warmer periods when the glaciers slink back to the poles.

The cause of the ice ages is a remarkably consensus-free scientific topic, but on one point most researchers agree: The cooling that started 60 million years ago was caused by a drop in carbon dioxide in the atmosphere. Carbon dioxide is a so-called greenhouse gas. It traps sunlight close to the Earth, raising the planet's temperature. Less carbon dioxide in the atmosphere means colder weather.

But why did carbon dioxide levels fall? The leading theory is because two of the Earth's tectonic plates collided, forming the Himalayas.

Marine geologist Maureen Raymo, now at the Massachusetts Institute of Technology, worked out a scenario that is widely cited in the scientific literature—both favorably and not. Raymo noted that theHimalayas are taller than any other mountain range on Earth and that they formed at nearly the same time as the start of the ice ages. Scientists realize that carbon dioxide from the atmosphere combines with rain to make an acid that erodes rock; in the process, minerals such as calcium silicate in the rocks react with carbon dioxide, removing the gas from the atmosphere. Therefore, erosion on slopes as vast as the Himalayas', says Raymo, could reduce carbon dioxide levels enough to give the ice ages a push.

Raymo acknowledges that her idea has holes. Weathering on the scale she theorizes would devour so much carbon dioxide that soon there wouldn't be any left at all. Other scientists point to an opposite problem: The kind of minerals in the Himalayan rocks rarely consume carbon dioxide when they erode. The mystery of Cenozoic cooling has encouraged researchers to propose many other candidate culprits. One recent scientific paper theorizes that the key factor is the tons of sediment shed by the mountains every year. The sediment runs into the Indian Ocean and buries large amounts of marine plankton and other plants .Entombed along with the plants is their carbon dioxide, meaning less carbon dioxide in the atmosphere and therefore less warming. Solutions championed by other scientists include the evolution of grasses, which happened at roughly the right time and would have stored large amounts of carbon dioxide in the soil; and weathering of the mountains in either New Guinea br Siberia.

There is equally great bafflement over the much more recent Northern Hemisphere glaciations. For most of the Cenozoic Ice Age, the polar regions were the only place to suffer heavy invasions of ice. But 2.5 million Y.ears ago, glaciers moved into Europe and North America and have paid regular visits since. Scientists can explain the timing of these local ice ages. As the Earth's orbit slowly changes, so does the pattern of the sunlight .falling on a given spot on the globe, triggering the glaciers' advance and retreat. The problem is that the orbital changes are not big enough to make or to melt ice sheets.

What ice age scholars need are cold, hard data. For example, there are no direct measurements of atmospheric carbon dioxide beyond 200,000 years ago. Carbon dioxide levels are revealed by long cylinders of Ice extracted from Antarctica. But there is no ice old enough to reveal the composition of the atmosphere millions of years ago.

"Information is pouring in now, and it is not converging on a simple explanation," says Isaac Winograd, a hydrologist at the U.S. Geological Survey.

Until scientists can explain changes in Earth's climatic past, they cannot be sure they aren't overlooking factors that could either counteract a human-made warming—or amplify it.