Orbital eccentricity

Changes in nature of Earth’s orbit tend to drive glacial cycles  

By Ishita Das
Last Updated: Saturday 04 July 2015

imageThe world is currently experiencing the Ice Age called the Quarternary period.

There have been five ice ages since the planet was born. Each of them was characterized by periods of lower temperatures and glacier formation, interspersed with periods of higher temperature. The glacial periods used to occur roughly at periodicity of 41,000 years 0.8 million years ago and now, every 100,000 years.

Some astronomical phenomena also occur with similar periodicities, namely, changes in obliquity and eccentricity of the Earth’s orbit. Obliquity (41,000 years) is the tilt of the Earth’s axis and cycles between 22° to 24°. It is currently at about 23° 30’ and decreasing. Eccentricity describes the shape of the Earth’s orbit around the sun and it is influenced by changes due to gravitational interactions with other planets. It varies from 0.0034 to 0.055 every 100,000 years and is close to 0.016 now, making the orbit almost circular.

There may or may not be a cause-effect relationship between the astronomical phenomenon and glacial cycles. Nevertheless, it is being rigorously researched. In some experiments, the search begins with collecting fossilized “foraminifera” protozoans, present in sediments of the ocean beds. The specimens are analyzed for the presence of a special isotope of oxygen, Δ18. The level of Δ18. oxygen is in tune with the temperature of seawater in which the organism lived, allowing a peek at the ice ages of long ago. It is similar to carbon dating used to determine the age of fossils.

Lorraine Lisiecki of the University of California in Santa Barbara, usa, used data on levels of Δ18 oxygen in sediments containing foraminifera, collected from about 47 locations. The oldest data was 1.2 million years old. The idea was to correlate the periodicity of glacial cycles to changes in earth’s eccentricity. Previous analyses, looking for such a relationship had been unable to find a statistically sound argument for that.

Lisiecki used a different type of analysis on her set of data to arrive at a significant correlation. She was able to show a link between occurrence of the largest glacial cycles and weakest changes in orbital eccentricity. The negative correlation between the two is unusual and tricky to explain in a cause-effect relationship. Hence, Lisiecki theorizes a complex interplay of various factors including eccentricity, obliquity and the climate system in its entirety.

Some do not believe such an inverse correlation exists. Richard Muller, professor at the University of California, in Berkeley had previously overruled a statistically significant link between eccentricity and glacial cycles. “Different kinds of analyses test different aspects. If the theory fails even one of them, it is enough to throw serious doubt. Lisiecki avoided the analysis that showed the contradictions,” he said.

Muller and Lisiecki agree on one point—astronomical phenomena do drive glacial cycles via complex interactions with the climate system. The study was published in the April issue of Nature Geoscience.

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