Milankovitch cycles

Earth's orbit and orientation change gradually under gravitational influences within the Solar System. Eccentricity describes changes in the orbit's shape, from nearly circular to more elliptical; its principal climatic cycle is about 100,000 years. Obliquity is the tilt of Earth's rotational axis relative to its orbital plane; it varies between roughly 22.1 and 24.5 degrees on a cycle of about 41,000 years. Axial precession is the slow change in the direction of the rotation axis, which shifts the timing of the seasons relative to Earth's closest and farthest orbital points. Its combined climatic cycle averages about 23,000 years. These processes change the seasonal and latitudinal pattern of insolation—solar energy reaching the top of the atmosphere—rather than simply switching the planet's total annual energy supply by a large amount.

The orbital variations can reinforce or counteract one another, changing summer sunlight at high northern latitudes and affecting whether winter snow survives through summer. Persistent snow and ice can expand into ice sheets; their reflectivity then increases cooling, while feedbacks involving oceans, greenhouse gases and ice sheets amplify or modify the initial orbital forcing. In 1976, J. D. Hays, John Imbrie and Nicholas Shackleton reported that climate indicators in Southern Hemisphere ocean sediments over roughly 450,000 years contained prominent cycles corresponding to precession, obliquity and eccentricity. Subsequent evidence from marine sediments and polar ice cores supports a connection between orbital forcing and glacial–interglacial cycles. The timing and strength of individual ice-age cycles are not a simple clockwork response: interactions within the climate system shape the outcome.

Milankovitch variations unfold over geological timescales. They help explain long-term glacial cycles but cannot account for the speed or direction of present-day warming. Measurements show that the current orbital configuration would, by itself, tend toward gradual long-term cooling rather than the observed rapid warming. Modern climate change is driven predominantly by human-caused increases in heat-trapping greenhouse gases. Orbital cycles remain relevant to understanding Earth's climate history and very long-range climate evolution, but their effects must be distinguished from modern human forcing.