Long before humans burned anything, Earth’s climate swung between ice ages and warm interglacials. The pacing came from three slow changes in how our planet travels around the Sun. Knowing them matters for two reasons: they explain natural climate change, and they explain why current warming cannot be natural.
📚 What you need to know
Milankovitch cycles are three long-term changes in Earth’s position and movement relative to the Sun.
They alter the amount and distribution of solar radiation Earth receives, over tens to hundreds of thousands of years.
Eccentricity — the shape of the orbit, circular to elliptical, about every 100,000 years.
Obliquity — axial tilt varying between about 22.5° and 24.5°, about every 41,000 years.
Precession — the wobble of the axis, about every 26,000 years, shifting the timing of the seasons.
The cycles alone are small; positive feedback loops involving ice albedo and carbon dioxide amplify them into full glacials and interglacials.
The three cycles
Eccentricity changes how much radiation arrives in total; obliquity and precession mostly change where and when it arrives. Seasons, not annual averages, do most of the work.
1. Eccentricity — the shape of the orbit
Earth’s orbit shifts between more circular and more elliptical over a cycle of about 100,000 years.
A more elliptical orbit means Earth’s distance from the Sun varies more, so there are bigger differences in the radiation received at different times of year.
A more circular orbit keeps the distance steadier, giving smaller seasonal differences.
Climate impact: low eccentricity gives milder seasons. Milder summers mean less ice melts, so ice sheets can slowly build up in polar regions. If the tilt is also small, the poles receive less direct sunlight and it becomes even harder for ice to melt. Snow and ice accumulate, and a glacial period can begin.
2. Obliquity — the tilt of the axis
The angle of Earth’s axis relative to its orbit changes between about 22.5° and 24.5° over roughly 41,000 years.
Greater tilt intensifies the seasons: warmer summers, colder winters. Warmer summers reduce the build-up of polar ice because more of it melts.
Lower tilt reduces seasonal contrast, allowing ice to persist through the summer at the poles and contributing to glaciation.
The counter-intuitive bit: colder winters do not build ice sheets. Ice sheets grow when summers are too cool to melt the previous winter’s snow. That is why lower tilt and milder seasons favour glaciation.
3. Precession — the wobble of the axis
Earth’s rotational axis wobbles like a slowing spinning top, over about 26,000 years. As it does, the position of the North Star gradually changes.
Precession affects the seasonal distribution of solar radiation rather than the total amount.
Whether a hemisphere has warmer or cooler summers depends on where Earth sits in its orbit when that season occurs.
Why small nudges become ice ages
The orbital changes alone are too weak to freeze continents. What turns a nudge into an ice age is positive feedback.
🧩 Two amplifying loops
The ice-albedo loop. Reduced solar radiation causes cooling and glaciation. Snow and ice are highly reflective, so they bounce more sunlight back to space (the albedo effect), which causes further cooling, which grows more ice.
The carbon dioxide loop. Cooler temperatures mean the atmosphere holds less carbon dioxide, and oceans and soils absorb more of it. Lower carbon dioxide weakens the greenhouse effect and enhances the cooling. Running the other way, warming releases more carbon dioxide from soils, oceans and plant life, enhancing the warming and pushing towards an interglacial.
Note the direction of causation here: in the Milankovitch story, carbon dioxide is a feedback that follows temperature. In modern climate change it is the driver that leads temperature. Same gas, opposite role — and examiners love that distinction.
Real-world examples
Period
When
What happened
Last glacial maximum
About 20,000 years ago
Eccentricity, axial tilt and precession aligned to favour cooling, producing extensive ice sheets over parts of North America and Europe
Current interglacial (the Holocene)
Began about 11,700 years ago
A relatively stable, warmer climate — the conditions in which agriculture and human civilisation developed
EXAM PRACTICE
Explain why Milankovitch cycles cannot account for the global warming observed since about 1850. [4]
Point 1: the timescales do not match
The cycles operate over tens of thousands of years; recent warming has occurred over roughly 150 years.
Point 2: the rate is wrong
Orbital forcing produces slow, gradual change. Observed warming is unprecedentedly rapid.
Point 3: the direction is wrong
Current orbital conditions do not favour the warming being observed.
Point 4: there is a better explanation
The rise matches the increase in greenhouse gas concentrations from fossil fuel combustion, deforestation and agriculture.
Right kind of mechanism, wrong timescale, wrong rate and wrong directionThis question, or a version of it, appears constantly. Learn the four points as a set.
💡 Exam tip
Learn all three names, timescales and one climate effect each. That structure answers most short questions.
Say the cycles change the distribution of solar radiation as well as the amount — precession changes almost nothing about the total.
Always mention feedback loops. Without them the cycles are too weak to explain ice ages.
Use albedo by name when describing the ice feedback.
Finish any Milankovitch answer with the point that they do not explain recent rapid warming.
⚠️ Common mix-up
Swapping obliquity and precession. Obliquity is the angle of tilt; precession is the wobble of the axis direction.
Muddling the timescales. 100,000 / 41,000 / 26,000 — in that order for eccentricity, obliquity, precession.
Thinking colder winters cause glaciation. Cool summers that fail to melt the snow are what matter.
Using the cycles to argue current warming is natural. The timescale rules that out.
Forgetting carbon dioxide’s role as a feedback here. It amplifies the orbital signal rather than starting it.
Up next: Hothouse Earth (HL) — what happens if feedback loops take over and warming starts driving itself.
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