ICE CORES : Forecast - Ice Age?

In the 19th century, European geologists became convinced that sometime in the distant past great glaciers covered portions of the Northern Hemisphere. The evidence partly lay in big boulders, called "erratics," that somehow had been carried great distances from their origins, and in deep scratches on rock surfaces. At first scientists didn't know when the ice age began, when it ended, and what caused it. Gradually scientists have answered all of these questions.

It turns out that there wasn't just one ice age in the past but many. They began occurring about three million years ago. Ever since then, there has been an ice age about once every hundred thousand years. Scientists divide the cycle into cold periods, or "glacials" and warm periods, or "interglacials." Since our own lives, and all recorded history, have taken place during an interglacial, we tend to think of Earth's present warm period as what the climate is like. In fact, glacials, which last about 90,000 years are much longer than interglacials, which generally last only about 10,000 years.

During past glacial periods, three huge ice sheets occupied much of the land in the Northern Hemisphere one in North America that covered most of Canada and much of the northern U.S. and one that covered most of Northern Europe and parts of Asia. The Greenland ice sheet was largely the same.

Scientists are in general agreement that ice ages are caused by three aspects of Earth's orbit that change in a regular pattern. The theory, known as the Milankovitch effect was first worked out in detail in the early 19th century by the Serbian mathematician Mulantin Milankovitch. First, Earth's orbit is not round, but slightly elliptical. The out-of-roundness of the orbit changes gradually over thousands of years. Another change that takes place in Earth's orbit is the degree of the tilt of the planet. Earth's axis is not perpendicular to the plane of its orbit around the sun, but inclined slightly. When this "inclination" changes the patterns of warming and cooling of the planet also change. Finally, the relationship between the tilt and the location of the planet in space changes over thousands of years. Sometimes the northern hemisphere is titled toward the sun when the Earth is closet to the sun and sometimes it is titled away. Though there are still many questions to resolve, scientists say the changes in these three parameters explain in large part the comings and goings of glaciers.

	Tropical Ice Cores Ice samples are taken from snow-capped mountains in the tropics. See the lab where one team studies tropical ice cores. Lonnie Thompson A leading scientist in tropical ice core research explains the findings of his work.