Ice ages and Milankovitch cycles.

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An oval orbit

Twenty thousand years ago ice sheets up to three kilometres thick covered most of Canada and northern Europe. Within about ten thousand years most of that ice melted. That was only the latest of many ice ages: Antarctic ice cores show cold spells of about $100{,}000$⁠ years, broken by short warm ones, the interglacials. We live in one.

The Serbian engineer Milutin Milankovitch spent decades calculating, by hand, how the pull of the other planets slowly changes the Earth’s orbit and spin. The first change: eccentricity, how oval the orbit is. It drifts between almost a perfect circle and about $0.06$⁠, over cycles of about $100{,}000$⁠ and $405{,}000$⁠ years. In the Scene the Sun sits at one focus of a true ellipse.

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Closest in winter

The second change: axial tilt, which swings between about 22.1° and 24.5° and back every $41{,}000$⁠ years. Today it is 23.44°. In the Unit on the Earth, the Moon and the Sun we saw that this tilt makes the seasons.

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The most oval orbit

The more oval the orbit, the bigger the difference between the point closest to the Sun, perihelion, and the farthest, aphelion.

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Northern summers

The third change: precession, the slow wobble of the axis, like a spinning top’s, which moves perihelion all the way round the calendar every $19{,}000$⁠ to $24{,}000$⁠ years. Today the Earth is closest in early January.

Milankovitch’s key idea: ice sheets do not grow because winters are cold. They grow when summers are too weak to melt the winter’s snow. The north is what matters, because that is where the big continents reach close to the pole. So the yardstick is midsummer sunlight at a high northern latitude, 65° north, and the Scene draws the northern ice cap by it.

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A weak summer

The three cycles push northern summer sunlight each its own way. Precession is locked at 270°: the closest point in the northern winter.

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The record

The chart beside the Scene adds up the three cycles, through the same sunlight calculation, into summer sunlight at 65° north for each moment of the last 400,000 years. Below it runs the temperature from Antarctic ice cores, with the interglacials shaded in orange. In 1976 James Hays, John Imbrie and Nicholas Shackleton found the $23{,}000$⁠, $41{,}000$⁠ and $100{,}000$⁠-year rhythms in the chemistry of ocean-floor mud.

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Farthest out

When the orbit is at its most oval, the Earth comes nearer the Sun on one side and goes farther from it on the other.