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2001-08-14 14:43:46 (UTC)


Climate change linked to anomaly in Earth's orbit
Posted: April 16, 2001

About 23 million years ago, a huge ice sheet spread over
Antarctica, temporarily reversing a general trend of global
warming and decreasing ice volume. Now a team of
researchers has discovered that this climatic blip at the
boundary between the Oligocene and Miocene epochs
corresponded with a rare combination of events in the
pattern of Earth's orbit around the Sun.

In a paper published in the April 13 issue of the journal
Science, the researchers show that the transient glaciation
and other climatic variations during a period from about 20
to 25.5 million years ago correspond with variations in
Earth's orbit known as Milankovitch cycles. Although the
concept of such relationships is not new, some of the
results were surprising, said James Zachos, a professor of
Earth sciences at the University of California, Santa Cruz,
and lead author of the paper.

"When we began examining the temporal relationship of the
orbital oscillations relative to the oscillations in the
climate record, we never suspected that the transient
glaciation at 23 million years ago had anything to do with
orbital anomalies," Zachos said.

The astrophysicist Milutin Milankovitch first proposed that
cyclical variations in certain elements of Earth-Sun
geometry can cause major changes in Earth's climate. The
main variables are eccentricity, obliquity, and precession.
Eccentricity refers to the changing shape of Earth's orbit
around the Sun, which varies from nearly circular to
elliptical over a cycle of about 100,000 years. Obliquity
refers to the angle at which Earth's axis is tilted with
respect to the plane of its orbit, varying between 22.1
degrees and 24.5 degrees over a 41,000-year cycle. And
precession is the gradual change in the direction Earth's
axis is pointing, which completes a cycle every 21,000

"Because there are several components of orbital
variability, each with lower frequency components of
amplitude modulation, there is the potential for unusual
interactions between them on long timescales of tens of
millions of years," Zachos said. "What we found at 23
million years ago is a rare congruence of a low point in
Earth's eccentricity and a period of minimal variation in

The result of this rare congruence was a period of about
200,000 years when there was unusually low variability in
the planet's climate, with reduced extremes of seasonal
warmth and coldness. Earth's orbit was nearly circular, so
its distance from the Sun stayed about the same throughout
the year. In addition, the tilt of Earth's axis, which
gives rise to the seasons, varied less than usual. In other
words, the tilt doesn't always vary between the same
extremes in its 41,000-year cycles; the obliquity cycle
itself varies in amplitude over a longer period of about
1.25 million years. Similarly, the eccentricity cycle peaks
every 400,000 years.

The combination of a low-amplitude "node" in the obliquity
cycle and a minimum in eccentricity would have caused only
several degrees difference in summer temperatures at the
poles, but it was probably enough to allow the Antarctic
ice sheet to expand, Zachos said.

Zachos's collaborators on the paper were Nicholas
Shackleton and Heiko Palike of Cambridge University, Justin
Revenaugh of UC Santa Cruz, and Benjamin Flower of the
University of South Florida.

The researchers obtained detailed climate records for the
late Oligocene and early Miocene by analyzing sediment
cores drilled out of the ocean floor. Cutting through
layers of sediments laid down over millions of years, such
cores contain a chronological record of past climates
written in the chemistry of fossilized shells left behind
by tiny marine organisms. Oxygen isotopes in the shells,
for example, reflect ocean water temperatures and the
amount of ice trapped in glaciers.

In the 1970s, scientists using these techniques obtained
the first good evidence in support of Milankovitch's
theory, almost 50 years after he had proposed it. According
to Zachos, researchers are still trying to get a handle on
the relationships between climate cycles and orbital
variations. Since most of the research has focused on the
past 5 million years, the new paper is valuable because it
looks at a more distant window in time when conditions on
the planet were different.

In the period they examined, the late Oligocene and early
Miocene, Zachos and his collaborators found evidence of
several climate cycles with frequencies corresponding to
the Milankovitch cycles. But the correspondence of the
orbital anomaly with the transient glaciation event at the
boundary between the two epochs is especially interesting,
Zachos said. The climate system seems to have undergone a
fundamental shift at this boundary, which also marks a
major break in the paleontologic record.

"I'm not sure everyone will be convinced that the orbital
anomaly alone is responsible," Zachos said. "But the
congruence of those orbital cycles is a very rare event,
and the fact that it exactly corresponds with this rare
climatic event is compelling."

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