Definition
Milankovitch cycle
Milankovitch cycles are long-term variations in Earth’s orbital geometry and axial behaviour that alter solar radiation distribution and drive natural climate cycles such as ice ages and interglacial warm periods.
- Milankovitch cycles are long-term, predictable variations in Earth’s movement and orientation in space.
- These cycles alter how much solar radiation (insolation) reaches Earth and how it is distributed across latitudes and seasons.
- They operate over tens to hundreds of thousands of years, driving natural shifts between glacial and interglacial periods.
- There are three primary cycles:
- Eccentricity (shape of Earth’s orbit)
- Obliquity (axial tilt)
- Precession (axial wobble)
- Together, they determine changes in Earth’s energy budget, influencing ice sheet expansion, carbon dioxide concentrations, and global climate patterns
These cycles play a crucial role in glacial and interglacial periods, but they do not account for the current rapid warming caused by human activities.
1. Eccentricity - Changes in the Shape of Earth’s Orbit
- Earth’s orbit shifts between more circular and more elliptical shapes.
- This cycle lasts approximately 96,000–100,000 years.
- When the orbit is more elliptical, Earth’s distance from the Sun varies more widely during the year.
- This means that the difference between the closest point (perihelion) and farthest point (aphelion) creates greater seasonal variations in insolation.
- When the orbit is more circular, the solar radiation received is more evenly distributed throughout the year.
Climate Impacts of Eccentricity
- A more elliptical orbit intensifies seasonal contrasts, especially in the Northern Hemisphere.
- A more circular orbit reduces these seasonal differences, which can allow ice sheets to grow if other conditions also favour cooling.
- When eccentricity aligns with lower tilt and certain precession conditions, glaciers expand over thousands of years.
- Think of eccentricity as changing the shape of a racetrack.
- A more circular track keeps the runner (Earth) at a consistent distance from the Sun.
- A stretched track causes more variation in distance and energy received.
During the last glacial maximum (about 20,000 years ago), Earth's orbit was closer to circular, contributing to long-term cooling.
2. Obliquity - Changes in the Tilt of Earth’s Axis
Definition
Obliquity
Obliquity refers to the angle of Earth’s axial tilt, which determines how strongly sunlight is concentrated at different latitudes.
- Earth’s axial tilt currently sits at 23.5°, but over a 41,000-year cycle, it varies between about 21.5° and 24.5°.
- The tilt controls the intensity of seasons, because it determines how directly sunlight strikes different latitudes.
Climate Impacts of Obliquity
- A greater tilt (closer to 24.5°):
- Stronger seasonal differences
- Hotter summers and colder winters
- Warmer summers melt more ice which promotes interglacial periods
- A smaller tilt (closer to 21.5°):
- Weaker seasonal contrasts
- Cooler summers allow ice to persist
- Favourable conditions for glaciation
- Higher tilt → stronger seasons → more melting → interglacial periods
- Lower tilt → weaker seasons → more snow accumulation → glacial periods
