Milanković cycles
Why do ice ages come and go?
Three slow changes in Earth’s orbit set the rhythm. They shift how much sunlight reaches the far north each summer, and that has been enough to build ice sheets kilometres thick and melt them again.
01 · The idea
Summer decides whether ice grows
Snow falls every winter in the far north. Whether it turns into ice depends on how much of it survives the summer.
- 1
The orbit shifts slowly
Earth’s distance, tilt and season timing change over tens of thousands of years.
- 2
Summer sunlight at 65°N changes
That latitude crosses northern Canada, Scandinavia and Siberia, where the big ice sheets grew.
- 3
Ice and carbon dioxide add to it
White ice reflects sunlight and CO₂ levels fall, so a small push becomes a large one.
Ice covers Greenland and Antarctica. Northern Canada and Europe are free of it.
Go deeper
The number in the corner is the daily average sunlight at the top of the atmosphere on the summer solstice at 65°N. Scientists call incoming solar energy insolation. It is the usual yardstick for ice because melting in summer is what limits an ice sheet.
The ice on the globe is drawn by hand to follow the measured record. It shows how much ice there was, not its exact outline.
Stretch the orbit
Earth’s orbit is almost a circle. Over about 100,000 years it stretches a little, then relaxes. Drag the scene or move the slider.
Earth swings between 147.1 and 152.1 million km from the Sun. Sunlight is 7% stronger at the near end.
What it does
A stretched orbit puts Earth closer to the Sun at one time of year than at another. Today the gap is 5 million km. At the most stretched it is 17 million km, and the nearest sunlight is 26% stronger than the farthest.
On its own, a stretched orbit hardly changes the sunlight Earth gets over a whole year (under 0.2%). What it does is give the wobble something to work with.
Go deeper
The Sun sits at one focus of the ellipse, not at its centre. The scene draws the stretch five times larger than it is, because the real orbit would look like a circle. Choose “True shape” under the scene to see it as it is.
Eccentricity has more than one rhythm. The record shows strong pacing near 100,000 and 405,000 years, so “100,000 years” is a handy label, not an exact clock.
Lean the axis
Earth’s axis leans 23.4° from upright. Over 41,000 years it swings between 22.1° and 24.5°. More lean means stronger seasons, most of all near the poles.
At midsummer the Sun stays up for 21.1 hours at 65°N and climbs 48.4° above the horizon.
What it does
At 22.1° the Sun stays up 20.1 hours on a midsummer day at 65°N. At 24.5° it stays up 22.4 hours and climbs higher. That is 457 W/m² against 496 W/m².
Less lean gives cooler northern summers, and cool summers are when snow survives.
Go deeper
Tilt, not distance, makes the seasons. Earth is closest to the Sun in early January, when it is winter in the north.
The dashed grey line in the scene is today’s tilt. The angle is measured from the line that stands upright on the orbit’s plane.
Wobble the axis
Earth’s axis slowly turns, like the axis of a spinning top. The orbit turns too. Together they move the seasons around the orbit. Drag to turn the axis.
Northern summer falls near the farthest point of the orbit, 152.0 million km from the Sun.
What it does
Today northern summer falls on the far side of the orbit. In about 11,000 years it falls on the near side, and midsummer sunlight at 65°N rises from 478 to about 510 W/m².
That swap matters more when the orbit is stretched. At the most stretched orbit it changes midsummer sunlight by 115 W/m².
Go deeper
The axis alone takes about 25,800 years to turn once. The ellipse turns as well, so the seasons meet the orbit’s near point again after about 21,000 years on average. The climate record has peaks near 19,000 and 23,000 years.
The two hemispheres pull in opposite directions. When northern summer is near the Sun, southern summer is far from it.
05 · Putting it together
Run the clock
All three motions now run at once, using the orbit calculated by Jacques Laskar’s team (La2004). Press play and 800,000 years pass in about a minute. The measured ice record is the bottom line.
What to look for
The ice line follows the sunlight line, but later. Ice builds slowly over tens of thousands of years and goes fast, in around ten thousand.
Milutin Milanković worked this out by hand over about thirty years and published it in 1941. Nobody could check it until 1976, when seabed sediment showed the same rhythms. He had died 18 years earlier.
Go deeper
The strongest rhythm in the ice record is 100,000 years, yet the orbit’s direct push at that period is weak. The ice sheets, oceans and carbon dioxide must be amplifying it, and how exactly is still debated.
Past 0 on the clock the orbit keeps going and the ice line stops: nothing here predicts future ice. For the next 50,000 years the orbit is unusually calm, and midsummer sunlight at 65°N stays between 475 and 503 W/m².
06 · Recap
The short version
- 1
Stretch sets how far apart Earth’s near and far points are, and so how much the wobble can matter.
- 2
Lean gives high-latitude summers more sun when it grows and less when it shrinks.
- 3
Wobble decides whether northern summer falls near the Sun or far from it.
Together they swing midsummer sunlight at 65°N between 431 and 552 W/m². Ice answers slowly: it takes tens of thousands of years to build and about ten thousand to melt.
What about today’s warming?
The orbit changes far too slowly to matter in a human lifetime. Today’s warming comes mainly from greenhouse gases released by human activity.
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