Milankovitch Cycles: Eccentricity, Obliquity, Precession and the Ice Ages

The Milankovitch cycles are the three slow, regular variations in the earth’s orbit and axis that pace the ice ages. They matter because they redistribute sunlight across latitude and season without changing the total amount the earth receives by very much. What they alter is where and when the sunlight falls, and that turns out to be enough to grow or melt a continental ice sheet.

Milutin Milanković computed them in the 1920s and 1930s, decades before there was any way to test them. The test came in the 1970s, when deep-sea sediment cores were analysed for the same periods and found them.

Eccentricity: the Shape of the Orbit

Eccentricity is the departure of the earth’s orbit from a circle. It varies between nearly circular and mildly elliptical over cycles of roughly 100,000 and 400,000 years.

When the orbit is more elliptical the difference between perihelion and aphelion distance grows, so the seasonal contrast sharpens in one hemisphere and softens in the other. Eccentricity is the only one of the three that changes the annual total of energy the earth receives, and even then by well under 1 per cent.

Obliquity: the Angle of Tilt

Obliquity is the angle of the axial tilt, currently 23.5 degrees, varying between about 22.1 and 24.5 degrees on a 41,000-year cycle.

A larger tilt gives hotter summers and colder winters in both hemispheres, because the poles lean further toward and away from the sun. A smaller tilt evens the seasons out. Obliquity acts most strongly on the high latitudes, which is exactly where ice sheets form, so it has an effect on glaciation out of proportion to its size.

Precession: Two Motions Usually Collapsed Into One

Precession has two components, and treating them as one is the commonest error on this topic.

• Axial precession is the slow conical wobble of the rotation axis, like a spinning top, completing a circuit in roughly 26,000 years. It changes which star sits near the celestial pole and shifts the calendar date of each solstice against the stars.
• Apsidal precession is the slow rotation of the orbital ellipse itself, so the position of perihelion drifts around the orbit.

The two combine into climatic precession, on an effective cycle of about 19,000 to 23,000 years, which determines the season at which the earth is closest to the sun. At present perihelion falls in early January, so the northern hemisphere has its winter when the earth is nearest, which moderates the northern seasonal range. In about 11,000 years the reverse will hold.

Why the Cycles Produce Glaciations

The mechanism is not global cooling, and this is the part that separates an explanation from a recitation. What matters is summer insolation at high northern latitudes, conventionally measured at 65 degrees north, where the great continental ice sheets of North America and Eurasia grow.

An ice sheet grows when the previous winter’s snow survives the summer. The question that decides a glaciation is therefore not how cold the winter was but whether the summer was weak enough to leave snow on the ground in September.

When obliquity is low, precession puts northern summer at aphelion, and eccentricity is high enough to deepen that effect, northern summers are weak. Snow persists, the surface albedo rises, more sunlight is reflected, and the ice-albedo feedback carries the cooling considerably further than the orbital forcing alone would.

The southern hemisphere cannot do this. At the equivalent latitudes there is ocean where the north has continent, and water will not hold a summer snowpack.

The Evidence

Oxygen isotope ratios in deep-sea sediment cores and in the Greenland and Antarctic ice cores record global ice volume through the Quaternary. Spectral analysis of those records contains peaks at close to 100,000, 41,000 and 23,000 years, which are the three Milankovitch periods.

That periods calculated from celestial mechanics turn up in a mud core is among the strongest results in palaeoclimatology, and it is why the cycles are treated as the pacemaker of the ice ages. The 1976 paper by Hays, Imbrie and Shackleton, titled Variations in the Earth’s Orbit: Pacemaker of the Ice Ages, is the reference.

The 100,000-Year Problem

The honest qualification, and the thing that distinguishes an informed treatment, is this. The dominant rhythm of the last 800,000 years is the 100,000-year cycle, and eccentricity is the weakest of the three forcings. The strongest signal in the record corresponds to the weakest cause.

That mismatch is the 100,000-year problem, and it is unresolved. The leading explanations involve non-linear response in the ice sheets and in the carbon cycle rather than a stronger orbital signal: once an ice sheet is large it has its own dynamics of collapse, and atmospheric carbon dioxide falls and rises with the cycles, amplifying them.

The defensible position is therefore that the orbital cycles set the timing of glaciations while something internal to the earth system sets their amplitude.

Why This Does Not Explain Present Warming

The cycles are also the reason current warming cannot be attributed to orbital variation. Orbital forcing over the past century is very small, and on its own it points slightly toward cooling rather than warming. The observed rate of change is also wrong by orders of magnitude: Milankovitch forcing operates over tens of thousands of years, and the present change has occurred within a century and a half.

Practice Questions

Prelims

1. The obliquity of the earth’s axis varies between approximately

• (a) 21.5 and 22.5 degrees
• (b) 22.1 and 24.5 degrees
• (c) 23.0 and 25.0 degrees
• (d) 20.0 and 26.0 degrees

Answer: (b)

2. The effective period of climatic precession is about

• (a) 11,000 years
• (b) 19,000 to 23,000 years
• (c) 41,000 years
• (d) 100,000 years

Answer: (b)

3. Which Milankovitch cycle alters the total annual insolation received by the earth?

• (a) Obliquity
• (b) Axial precession
• (c) Eccentricity
• (d) Apsidal precession

Answer: (c)

4. Ice sheet growth is most sensitive to

• (a) winter temperature at low latitudes
• (b) summer insolation at high northern latitudes
• (c) annual mean global temperature
• (d) southern hemisphere summer insolation

Answer: (b)

5. The 100,000-year problem refers to the fact that

• (a) the cycle has no astronomical basis
• (b) the strongest climate signal matches the weakest forcing
• (c) ice cores do not record it
• (d) it was discovered after Milanković’s death

Answer: (b)

Mains

1. Explain the three Milankovitch cycles and the periods over which each operates. (10 marks)
2. Why is summer insolation at 65 degrees north the critical variable for glaciation? (10 marks)
3. Distinguish between axial precession, apsidal precession and climatic precession. (10 marks)
4. “The orbital cycles set the timing of glaciations but not their amplitude.” Examine this statement with reference to the 100,000-year problem. (15 marks)
5. Assess the evidence from deep-sea and ice cores for the astronomical theory of the ice ages. (20 marks)