Scientists have long studied how variations in Earth's orbit relate to ice ages, cycles of glacier building and retreat, and even mass extinctions. New research has uncovered some surprising relationships among all these things.
About 23 million years ago, a huge ice sheet spread over Antarctica, temporarily reversing a general trend of global warming, decreasing ice volume and ushering in a generally calm climatic period. The new study suggests this period corresponded with a rare combination of events in the pattern of Earth's trek around the Sun.
The idea that cyclical variations in Earth's orbit can cause major climate changes was first proposed by astrophysicist Milutin Milankovitch. 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. Precession is the gradual change in the direction Earth's axis is pointing, a 21,000-year cycle during which the axis carves out an imaginary cone shape.
"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 obliquity," said James Zachos, a professor of Earth sciences at the University of California, Santa Cruz.
The result 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, say Zachos and his colleagues.
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.
When the researchers began the study, "we never suspected that the transient glaciation at 23 million years ago had anything to do with orbital anomalies," Zachos said.
Zachos worked with Nicholas Shackleton and Heiko Pälike of Cambridge University, Justin Revenaugh of UC Santa Cruz, and Benjamin Flower of the University of South Florida. Their findings are discussed in the April 13 issue of the journal Science.
The researchers obtained detailed climate records 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.
"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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