The Earth’s orbit is a slightly stretched out circle called an ellipse, with the Sun just off centre. This means that our planet is closer to the Sun at some times of year than at others. Over long periods of time, our orbit is slightly influenced by other planets in the Solar System, particularly Venus because it passes relatively close to Earth and Jupiter because it’s so massive. The gravitational pull of these planets gradually changes the eccentricity of Earth’s orbit, making it slightly more or slightly less stretched out. This cycle takes around 100,000 years and changes the distance between the Earth and the Sun at different times of the year.

The Earth spins around an invisible line running from the North Pole to the South Pole, called the axis of rotation, causing the 24-hour variations in sunlight that give us day and night. The axis is slightly tilted, so that the northern hemisphere leans away from the Sun for half the year and towards the Sun for the other half, creating the seasons. Because of the gravitational pull of the Moon, the tilt of the Earth’s axis moves gradually back and forth every 41,000 years or so, changing the angle of the tilt. This cycle affects the seasons – differences between winter and summer are most pronounced when the tilt of the Earth’s axis is greatest.

The Earth isn’t a perfect sphere – it has a bulge around the equator, giving it a slightly squashed shape. The Sun and Moon both exert a gravitational pull on the Earth’s equatorial bulge, causing the planet’s poles to wobble round and round over a period of about 19,000 to 24,000 years, an effect known as precession. This cycle has an influence on our planet’s seasons. Currently, the Earth is closest to the Sun in January, giving the northern hemisphere slightly shorter, warmer winters and longer, cooler summers than the southern hemisphere. But in about 13,000 years the situation will be reversed, giving the southern hemisphere warmer winters and cooler summers.

Orbital cycles change the distribution of sunlight across the Earth’s surface. Scientists think that when these cycles combine to reduce summer sunlight in the Arctic, the resulting build-up of ice can trigger positive feedbacks leading to ice ages: white ice and snow spreading over darker land or water increases the amount of sunlight reflected from the surface, cooling the planet. During a cooling phase, greenhouse gases are locked up in ice and frozen ground and absorbed by the oceans, weakening the greenhouse effect and causing further cooling. Over thousands of years, ice spreads across large areas of Europe and North America. The positive feedbacks work in reverse to melt the ice when the orbit shifts back.

Because the various orbital changes happen over different lengths of time, there isn’t a single regular cycle. Instead, scientists must calculate the combined effects of the various cycles. Sometimes the different cycles are out of step, so the overall effect is insignificant. But at other times they act to reinforce each other, strengthening their combined effect. The last ice age ended about 12,000 years ago, since when global temperatures have remained relatively stable. Scientists have calculated that the present state of the different orbital cycles means they won’t have any significant effect for at least 30,000 years, when another cooling cycle may begin.

Scientists examining ice core data have observed that levels of carbon dioxide (CO₂) in the atmosphere changed almost perfectly in step with temperature during the ice age cycles. But the CO₂ changes lagged behind the temperature changes by a few hundred years. This is because the temperature changes caused by orbital shifts and ice feedbacks eventually led to changes in the carbon cycle. Falling temperatures caused CO₂ to be dissolved in the oceans and other carbon sinks, while rising temperatures at the end of ice ages caused CO₂ to be released. CO₂ is a greenhouse gas, so decreasing CO₂ levels caused further decreases in temperature and vice versa. The result is that global average temperature and atmospheric CO₂ levels are closely linked.

• Koshland Science Museum: Orbital Variations
• NASA: Milutin Milankovitch
• IPCC FAQ: What Caused the Ice Ages?