The Rhythm of the Ice
For the past 2.6 million years, Earth has oscillated between glacial and interglacial states in a remarkable rhythm. Continental ice sheets grew to cover Canada and Scandinavia under kilometers of ice, then retreated in relatively rapid deglaciations. This simulation models the core dynamics: orbital forcing provides the pacemaker, while CO₂ and albedo feedbacks amplify the signal into the dramatic glacial-interglacial swings preserved in ocean sediment and ice-core records.
Feedback Amplification
Milankovitch orbital forcing alone is too weak to explain the magnitude of ice ages — insolation changes of a few percent cannot directly produce 5°C global cooling. The answer lies in positive feedbacks: growing ice sheets reflect more sunlight (albedo feedback), cold oceans absorb more CO₂ (greenhouse feedback), and expanded ice coverage shifts atmospheric circulation patterns. Together, these feedbacks amplify the orbital signal by a factor of 3-5, turning subtle insolation changes into continental glaciation.
The Sawtooth Pattern
Glacial cycles are strikingly asymmetric: ice sheets build slowly over ~80,000 years but collapse in just ~10,000 years. This sawtooth shape reflects a fundamental asymmetry in the physics. Ice growth is rate-limited by snowfall, a relatively slow process. But ice collapse triggers cascading feedbacks — rising CO₂, falling albedo, marine ice-sheet instability, meltwater pulses — that accelerate deglaciation. Adjust the feedback strengths to see how different combinations produce more or less asymmetric cycles.
Sea Level and Human History
The practical consequences of glacial cycles are immense. At the Last Glacial Maximum, sea level stood 130 m below present, exposing vast continental shelves. The Bering land bridge connected Asia to North America, enabling human migration into the Americas. As ice sheets melted, sea level rose rapidly — sometimes meters per century — flooding coastal areas and reshaping human geography. Understanding these natural cycles provides essential context for projected future sea-level rise from anthropogenic warming.