[jinx]

And there's lots to consider beyond green house gasses...

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 years.

"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 obliquity."



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.