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 the 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 Gordon J. MacDonald of the International
Institute for Applied Systems Analysis, Austria, are co-authors of the
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 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
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
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.