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Earth Notes: Milankovitch Cycles

The bottom three-quarters of the image is filled with a view of the planet Earth, mostly in grays and blues, with lots of clouds. At the top, set in the blackness of space, is the bright yellow sun.
NASA
Earth as seen by astronauts aboard the International Space Station in 2003.

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.

Carrie Calisay Cannon is a member of the Kiowa Tribe of Oklahoma, and also of Oglala Lakota and German ancestry. She has a B.S. in Wildlife Biology and an M.S. in Resource Management. If you wish to connect with Carrie you will need a fast horse; by weekday she fills her days as a full-time Ethnobotanist with the Hualapai Indian Tribe of the Grand Canyon of Arizona, by weekend she is a lapidary and silversmith artist who enjoys chasing the beautiful as she creates Native southwestern turquoise jewelry.
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