For the last million years, great ice sheets have grown over North America and Europe and melted away again, over and over. The pacemaker is Earth's orbit, and the feedbacks you just met turn its gentle nudges into ice ages.
Key idea
Three slow cycles change how sunlight is spread over Earth's seasons and latitudes (the Milankovitch cycles): the shape of the orbit (eccentricity, about 100,000 and 400,000 years), the tilt of the axis (between about 22.1° and 24.5°, every 41,000 years) and the direction the axis points (precession, which shifts the date of closest approach to the Sun, about every 19,000 to 23,000 years). Ice ages have come and gone about every 100,000 years for the last 800,000 years.
Why it works
The cycles barely change the total sunlight Earth gets in a year, but they shift it between seasons and latitudes. What matters most is summer sunlight far north, near 65° N, where the great ice sheets grow: when northern summers are weak (less tilt, and the far point of the orbit falling in northern summer), winter snow survives the summer and ice builds up year after year. Feedbacks then amplify the change: growing ice reflects more sunlight, and the colder ocean takes up carbon dioxide, which cools Earth further. Ice-core records show carbon dioxide and temperature rising and falling together through every cycle.
Worked example: Closest and farthest
Today Earth is about 147.1 million km from the Sun in early January and 152.1 million km in early July. How much stronger is the sunlight in January?
- Sunlight weakens as 1/distance², so the ratio is $(152.1/147.1)^2$.
- $152.1/147.1 \approx 1.034$, and $1.034^2 \approx 1.07$: about 7% stronger in January.
- The seasons are still set by the tilt, not the distance: January is winter in the north.
Answer: About 7% stronger
Worked example: Counting cycles
How many tilt cycles (41,000 years each) fit into one 100,000-year ice-age cycle?
- $100{,}000 \div 41{,}000 \approx 2.4$ cycles.
- The cycles overlap and combine, which is why ice ages are irregular in shape.
Answer: About 2.4
Common mistake
Thinking orbital cycles explain today's warming. They act over tens of thousands of years, and the orbital pattern today would, on its own, cool Earth very slowly. The warming since 1900 has happened in about a century and is matched by the measured rise in greenhouse gases. Check the time scales: about 5 °C of warming at the end of the last ice age took roughly 10,000 years; about 1.1 °C of warming since the late 1800s has taken about 150.