NEWS RELEASE, 7/11/97 Astronomical theory offers new explanation for ice ages

by Jeffery Kahn, LBNL

Berkeley -- Recent ice ages -- ten periods of glaciation in the past million years -- are caused by changes in the tilt of the Earth's orbit, according to research published in today's (July 11) issue of Science magazine. The new analysis also presents strong evidence that another long prevailing theory does not account for these ice ages. Researchers Richard A. Muller of the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley, and Gordon J. MacDonald of the International Institute for Applied Systems Analysis, Austria, are co-authors of the Science article. Muller and MacDonald report that cyclical changes in the location of the Earth's orbit cause differing quantities of extraterrestrial debris to come into the Earth's atmosphere. This, in turn, results in variations of climate on the planet. Said MacDonald, "As the Earth moves up and down in the plane of the solar system, it runs into various amounts of debris, dust and meteoroids. Our work was an outgrowth of investigations of larger impacts, such as the comet or asteroid that killed the dinosaurs. However, meteoroids and dust are much smaller and more spread-out over time." Muller, a professor of physics at UC Berkeley, notes that this new research has important implications for the understanding of the present climate, and for predictions of future climate. "As far as we know," he said, "none of the present climate models include the effects of dust and meteors. And yet our data suggests that such accretion played the dominant role in the climate for the last million years. If we wish to make accurate predictions, we must understand the role played by such material." (Check out Muller's web pages for a history of his work on the theory of glacial cycles.) Despite the current relatively warm climate on Earth, regular recurring epochs of glaciation have dominated the planet for the past million years. Ten times, glaciers have advanced and then retreated with the duration of retreat (and corresponding warmth) frequently lasting not more than 10,000 years. The Earth has been in a warm period for about 10,000 years now. In the paper in Science, the researchers compared the geological record to the climactic cycles that would result from their theory and to that of the competing theory, first published in 1912 by Serbian scientist Milutin Milankovitch. Using a geological fingerprinting technique, Muller and MacDonald found that the climactic changes recorded in the rocks matched their theory but not that of Milankovitch. Milankovitch said the ice ages are caused by variations in sunlight hitting the continents. In his theory, the ice ages are linked to "eccentricity," a very gradual, cyclic change in the shape of the Earth's egg-shaped orbit around the sun that completes a cycle roughly every 100,000 years. Eccentricity changes the Earth's average annual distance from the sun and slightly alters the amount of sunlight hitting the Earth. To visualize the different astronomical cycle that Muller and MacDonald have found to match that of the climatic record, imagine a flat plane with the sun in the center and nine planets circling close to the plane. In fact, all the planets orbit the sun close to such a fixed orbital plane. The Earth's orbit slowly tilts out of this plane and then returns. As Muller first calculated in 1993, the cycle of tilt repeats every 100,000 years. In their Science paper, Muller and MacDonald examine the geological record of the past million years to see which of the two 100,000-year cycles (eccentricity or tilt) matched the data. They applied a technique called spectral analysis to ocean sediments taken from eight locations around the world, examining the oxygen-18 composition. This isotope is generally accepted to reflect the percentage of the Earth's water frozen in ice. Muller and MacDonald's analysis yields "spectral fingerprints" which can be compared to the predictions of the two theories. Their analysis shows a clear pattern: The fingerprints of the ice ages show a single dominant feature, a peak with a period of 100,000 years. This precisely matches their theory. The fingerprints do not match the expected trio of peaks predicted by the Milankovitch theory. Said Muller, "The mechanism proposed by Milankovitch could be adjusted to explain the cycles of glaciation that occurred prior to one million years ago. However, for the past million years the glacial record is an excellent match to the cycle of tilt." Berkeley Lab conducts unclassified scientific research for the U.S. Department of Energy. It is located in Berkeley, California and is managed by the University of California.