In the two previous posts on Milankovitch cycles — Eccentricity and Obliquity — we looked at the changing shape of Earth’s orbit around the Sun, and the changing tilt of Earth’s axis respectively. In this post we look at the other axial cycle: precession. In addition to changes in the tilt of the axis relative to the plane of our orbit, the direction that the axis is pointing in space changes over time. Over a period of about 26,000 years, it draws a circle on the star field. Presently the north end of the axis is pointing almost directly at Polaris, the pole star. About 13,000 years ago, near the end of the last great glaciation, it was pointing at the star Vega. The spinning planet is wobbling like a spinning top.
The effect of precession on climate is to continuously change where on our orbit the seasons occur. You can imagine that if we were on the opposite side of our wobble’s circle, then our summers and winters would be on the opposite sides of our orbit. Instead of winter in the northern hemisphere occuring during the part of the orbit when we are closest to the Sun, it would be at the farthest away. Northern hemisphere winters would be colder, but summers would be warmer, perhaps preventing the accumulation of ice and snow from season to season. Precession doesn’t affect the tilt of the axis, only where it’s pointing, so we still have seasons and they’re still affected by the obliquity cycle.
Now we have three cycles interacting: eccentricity at about 100,000 years, obliquity at about 41,000 years, and precession at about 26,000 years. You can see how complex this interaction is, and appreciate the work it took for Milankovitch to untangle it. He was doubted for a time, but these astronomical cycles are now accepted as important drivers of Earth’s climate. They came together to pull us out of the last glaciation, and now they are conspiring to push us into the next one. If we follow the established cycles, we should see a large part of the northern hemisphere’s continents covered in gigantic glaciers in the future.
Next up: some bonus extras.