The summer solstice is a time when the Earth’s pole tilts closest to the sun in its yearly cycle. The solstice, equinox, and lunar cycles are all familiar to most.
But one scientist, over a hundred years ago, theorized that three of Earth’s longer cycles could explain long-term climate changes. These are called the Milankovitch cycles, after Serbian scientist Milutin Milankovitch.
First, he looked at Earth’s elliptical orbit, which is on a 100,000 year cycle. When Earth is closest to the sun it receives about a quarter more radiation than when it’s at its furthest point. Next is the varying angle of Earth’s tilt which follows a 41,000 year cycle. The greater the tilt the more extreme the seasons, favoring periods of melting ice sheets.
The last factor is the direction Earth’s axis points, which is on a 26,000 year cycle. Because of the gravity of the sun and moon, Earth bulges at the equator and that causes it to wobbles as it rotates, like a top spinning off-center. This cycle results in seasonal contrasts that are more extreme in one hemisphere and less in the other.
Milankovitch combined all three cycles into a mathematical model. He understood that collectively they drive Earth’s long-term climate, triggering the start and end of Ice Ages. Though these cycles don’t explain all of Earth’s climatic changes, or the modern period of human-caused warming, they laid a foundation that scientists still use today.
This Earth Note was written by Carrie Calisay Cannon and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.
