Earth meanders a bit in her slow walk around the sun, and scientists report in today's issue of Science that a particularly unusual orbit our planet took about 23 million years ago may have caused a sudden dip in global temperatures. To reach this conclusion, they considered three recurring orbital variations known as Milankovitch cycles. The first 100,000-year cycle involves eccentricity, or variations in the shape of Earth's orbit from a near perfect circle to an ellipse and back. The second cycle describes obliquity, or changes in the tilt of Earth's axis, which varies between 22.1 and 24.5 degrees over 41,000 years. The third cycle tracks precession, or slight wobbles in Earth's axis that repeat every 21,000 years.

"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," James Zachos, a professor at the University of California, Santa Cruz and lead author of the paper, explains. In other words, the earth's orbit was almost circular and, at the same time, its axis tilted less, leading to fewer seasonal variations and less extreme weather conditions for a period of about 200,000 years.

The researchers suggest that such conditions would have made a difference of several degrees in summer temperatures at the poles¿enough, they think, to have allowed the Antarctic ice sheet to expand. They looked to sediment cores from the ocean floor for a detailed climate record, and found a correlation between climatic changes and the Milankovitch cycles. "I'm not sure everyone will be convinced that the orbital anomaly alone is responsible," Zachos says, "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."