Ice ages are periods of time when large continental ice sheets cover much of the northern hemisphere, and they are characterized by a large drop in global temperatures. At least five major ice ages have occurred throughout Earths history, and the most recent one began approximately 3 million years ago and continues today. Currently, we are in a warm interglacial that began about 11,000 years ago, and the last period of glaciation, which is often informally called the "Ice Age," peaked about 20,000 years ago.

There are many factors that contribute to climate variations, including changes in ocean and atmosphere circulation patterns, varying concentrations of atmospheric carbon dioxide, and even volcanic eruptions. One significant trigger in initiating ice ages is the changing positions of Earths ever-moving continents, which affect ocean and atmospheric circulation patterns. When plate-tectonic movement causes continents to be arranged such that warm water flow from the equator to the poles is blocked or reduced, ice sheets may arise and set another ice age in motion.

Changes in atmospheric greenhouse gas concentrations, particularly carbon dioxide, play a large role in the development of cold conditions during ice ages and warm conditions during interglacial periods. However, CO2 is not the immediate cause of ice ages; rather, it serves as a feedback to amplify changes initiated by orbital variations. There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect.