One of a world-wide series of ice ages sets in. Thick ice sheets expand over vast areas of Earth. What causes these ice ages is not completely understood. One theory attributes the ice ages to fluctuations in Earth's orbit, which affect the delivery of solar radiation to Earth.
Temperature swings can be amplified by other factors. If solar radiation decreases, for example, Earth cools and glaciers expand. Since glaciers are reflective, the expansion reduces the amount of heat absorbed by Earth. Thus, temperatures spiral down.
The life system of Earth is closely coupled to climate change. Since the origin of life, microbes have played a major role in the carbon cycle. Ocean-floor and ice-core drilling shows precision synchrony between climate change and atmospheric carbon dioxide. This coupling affects the magnitude of climate change.