What counts as an ice age
In the strict geological sense, an ice age is a long interval, lasting millions of years, when Earth has permanent ice sheets at the poles. By that definition, we are living in an ice age right now, because Greenland and Antarctica are covered in ice. In everyday speech, “the Ice Age” usually means the most recent cold period, when huge ice sheets also covered large parts of North America and northern Europe.
Within an ice age, the climate swings between colder intervals called glacials, when ice sheets expand, and warmer intervals called interglacials, when they shrink. The Holocene, the warm interval that began roughly 11,700 years ago, is the current interglacial.
Ice ages in Earth’s history
Earth’s climate has varied a great deal over its history of about 4.5 billion years. Geologists have found evidence of several major glaciations, generally in the form of rocks scraped and carried by ice, which are called glacial deposits:
- A glaciation roughly 2.4 billion years ago, among the oldest known.
- A series of glaciations in the Cryogenian period, roughly 700 million years ago, which some scientists think may have covered much or all of the planet, in what is sometimes called “snowball Earth.” Researchers still debate how complete the ice cover was.
- A late Paleozoic ice age, around 300 million years ago.
- The Quaternary ice age, which began roughly 2.6 million years ago and continues to the present.
Dates for the older events are less certain than for recent ones. For most of the time dinosaurs were alive, the climate was warm and there were no large polar ice sheets, though that, too, varied.
The recent glacial cycles
During the last couple of million years, ice sheets have advanced and retreated many times. In the more recent part of that record, the main rhythm has been about 100,000 years between glacial peaks, with shorter cycles superimposed. The most recent glacial maximum, when ice was at its greatest extent, was about 20,000 years ago. At that time, sea level was on the order of 120 meters lower than now, because so much water was locked up in ice, and land now underwater, such as shallow shelves, was dry.
Warming then began, and much of the large northern ice sheets had melted within the following several thousand years. Human beings lived through the whole period; our ancestors had already spread widely, and the climate shaped where they could live.
How scientists know
Evidence comes from many sources. Ice cores drilled from Greenland and Antarctica trap tiny bubbles of ancient air and layers of snow whose chemistry records past temperature. Deep-sea sediments contain the shells of tiny organisms, and the proportions of different atomic isotopes of oxygen in those shells reflect both temperature and how much water was stored as ice. Glaciers also leave landforms such as moraines, U-shaped valleys and scattered boulders far from their source rock. Pollen records, ancient shorelines and cave deposits add more detail.
What drives the cycles
No single cause explains everything. Scientists link several factors that operate on different timescales.
Changes in Earth’s orbit
In the 1900s the Serbian scientist Milutin Milanković worked out how slow, regular changes in Earth’s orbit alter how sunlight is distributed across latitudes and seasons. Three are usually named:
- Eccentricity, the shape of Earth’s orbit, which varies over cycles of roughly 100,000 years.
- Obliquity, the tilt of Earth’s axis, which changes slightly over about 41,000 years.
- Precession, the wobble of the axis, which shifts the timing of seasons over about 23,000 years.
These changes hardly alter the total sunlight Earth receives in a year, but they change how much falls on northern high latitudes in summer. If summers there are cool enough that winter snow does not fully melt, ice can build up. Evidence shows that glacial timing matches these cycles quite closely, which is strong support for the idea as a pacemaker.
Feedbacks that amplify the change
The orbital changes alone seem too small to explain the size of the swings, so scientists look at feedbacks. Ice and snow are bright and reflect sunlight, so more ice means more cooling. In addition, carbon dioxide levels in ice cores were lower during glacial periods and higher during interglacials. As the oceans cooled they could hold more of the gas, and lower levels weakened the greenhouse effect, further cooling the planet. Researchers continue to study exactly how the ocean, the biosphere and ice sheets interact in this process.
Slow changes in the continents and ocean
Over millions of years, plate tectonics changes the arrangement of continents, which affects ocean currents and how much land sits near the poles. The rise of large mountain ranges can change atmospheric circulation and the rate at which rocks weather and absorb carbon dioxide. These slow changes may help explain why ice ages begin and end on million-year scales.
Other contributors
Volcanic eruptions can produce short-term cooling, and changes in the Sun’s output may play a small role, but these are not generally considered the main drivers of the glacial rhythm.
Ice ages and the present climate
The natural causes of past cycles do not by themselves explain the rapid warming measured over the last century, which scientists attribute mainly to human activity. Past climate change is an important source of evidence about how sensitive the climate is, which helps when assessing popular climate claims. Large animal populations adapted to the cold, and changes in climate have been one of several factors scientists consider when studying species extinction near the end of the last glaciation, though the relative roles of climate and human hunting are still debated.