The long-running mystery of what causes Earth's climate to fluctuate between ice ages and warm periods, which most scientists thought had finally been solved in the 1970s, has been reopened with a new discovery announced in Friday's issue of the journal Science.
The generally accepted explanation linked the ice ages to changes in the amount of sunlight absorbed by the Earth _ changes caused by periodic variations in the shape of Earth's orbit and the tilt of its axis. Those cycles were said to match the timing of the advance and retreat of ice ages as deduced from sea-floor sediments.
The new evidence _ a column of mineral deposits found in Nevada and said to be more accurately dated than the sea-floor sediments _ indicates that ice ages have not recurred at regular intervals and that, in some cases, the ice began melting away thousands of years before orbital changes would have increased the amount of sunlight. In one case, the new evidence shows an ice age came and left about 400,000 years ago _ during a time when the old theory said there should have been no appreciable climate change.
"We're saying the amount of insolation (incoming sunlight) is not a key factor, and that flies in the face of what a lot of people have been saying," said Isaac Winograd of the U.S. Geological Survey in Reston. Winograd is the leader of a large team that found and interpreted the new evidence.
Winograd said although changes in sunlight intensity probably play a role, climatic fluctuations must be governed by a complex web of interactions among many factors that can change including ocean currents, jet streams, cloud cover, and dust and greenhouse gases in the atmosphere.
The older theory was first proposed in the 1920s by the Serbian astronomer Milutin Milankovitch. He examined physical phenomena that govern the motion of bodies in space and found they could have caused significant, regular variations in the amount of sunlight absorbed by the Earth.