What “ice age” actually means
In everyday speech, an ice age means a time when woolly mammoths roamed and glaciers covered much of the Northern Hemisphere. Geologists use the term more broadly. An ice age is a long interval, often millions of years, when ice sheets exist at the poles. By that definition, Earth is still in an ice age today, because Greenland and Antarctica hold large ice sheets.
Within an ice age, the climate swings between colder glacial periods, when ice sheets expand into lower latitudes, and warmer interglacial periods, when they retreat. The last glacial period peaked roughly 20,000 years ago, and the current interglacial, called the Holocene, began roughly 11,700 years ago. When people talk about “the Ice Age,” they usually mean that last glacial period.
Earth has not always had ice at the poles. For long stretches of its history the planet was much warmer, and ice ages are the exception, not the rule. Geologic evidence points to several major ice ages in the distant past, some of them extreme, though the details of the oldest ones are uncertain.
How scientists know
The evidence comes from several sources. Ice cores from Greenland and Antarctica contain layers of ice that trap tiny bubbles of ancient air and record temperature-related chemical signals. Ocean sediments hold shells whose chemistry reflects past temperatures and ice volume. On land, glaciers leave behind scratched bedrock, scoured valleys and piles of debris called moraines. Together, these records show repeated advances and retreats of ice. Dating them draws on techniques like radiometric dating, and changes in sea level tied to ice volume are also recorded in coastlines.
Orbital cycles
The most widely accepted explanation for the timing of glacial and interglacial swings involves slow, regular changes in Earth’s orbit and orientation. These are often called Milankovitch cycles, after the scientist who developed the idea. There are three main ones.
- Eccentricity is the shape of Earth’s orbit, which shifts from nearly circular to slightly more elongated over cycles of roughly 100,000 years and longer.
- Axial tilt, or obliquity, is the angle of Earth’s axis relative to its orbit. It varies between roughly 22 and 24.5 degrees over about 41,000 years. A greater tilt makes seasons more extreme.
- Precession is a wobble in the direction the axis points, which shifts which season occurs when Earth is closest to the Sun. Its main periods are roughly 19,000 to 23,000 years.
These cycles do not change the total sunlight Earth receives by very much. What matters more is how sunlight is distributed over seasons and latitudes. A common idea is that cooler summers at high northern latitudes may allow winter snow to persist from year to year, building up into ice sheets. The orbital timing matches the rhythm seen in ice and sediment records quite well, though the mechanism for turning small changes in sunlight into large climate swings is still being studied. Notably, the roughly 100,000-year pacing of recent glacial cycles is harder to explain than the others.
Feedbacks that amplify change
Small orbital nudges are generally thought to be amplified by feedbacks inside the climate system.
Ice and reflectivity. Snow and ice reflect much more sunlight than dark ground or open water. As ice spreads, more sunlight is reflected, cooling the planet further. When ice retreats, the effect works in reverse.
Greenhouse gases. Ice core records show that carbon dioxide and methane levels were lower during glacial periods and higher during interglacials, moving broadly in step with temperature. This links to the greenhouse effect, in which certain gases trap heat. The deep ocean seems to have stored more carbon during cold periods, but the full set of reasons is still debated. In this case, greenhouse gases are generally seen as an amplifier of orbital changes rather than the original trigger.
Ocean circulation. Changes in how the ocean moves heat can also shift climate. This is one reason ocean currents matter so much for how regions warm or cool.
Other factors on longer timescales
Over millions of years, the arrangement of continents plays a role. When landmasses sit near the poles, ice sheets can build up on them, and continental positions steer ocean flow. The uplift of large mountain ranges can change atmospheric circulation and affect the rate at which rock weathering removes carbon dioxide from the air. Volcanic activity adds carbon dioxide, so changes in the balance between volcanic input and removal by weathering are thought to matter over very long periods. These slow processes tie directly to plate tectonics, which rearranges continents and mountain belts.
Some very large ice ages, such as those some researchers describe as “Snowball Earth” events, remain the subject of ongoing debate about how extensive the ice really was.
Effects of ice ages
During glacial periods, vast amounts of water were locked in ice sheets, so global sea level was far lower, by on the order of 100 meters or more at the peak. Coastlines were in different places, land bridges appeared in some areas, and the weight of ice pressed down the crust beneath it, which then slowly rebounds after the ice melts. That rebound is still measurable in places.
Climate belts shifted, and many ecosystems moved with them. Glaciers carved features like U-shaped valleys and fjords that are visible today. The Grand Canyon, by contrast, was carved mainly by river erosion rather than glaciers, a reminder that not every dramatic landscape has the same origin.
What this means for today
Orbital cycles are slow, operating over tens of thousands of years, so they do not explain recent warming over decades. The current rise in global temperature is attributed mainly to human-caused increases in greenhouse gases, as described in discussions of climate change drivers. The natural cycles are valuable mainly because they show how sensitive the climate system can be to relatively small changes and how feedbacks can amplify them. Scientists continue to refine the details, and some questions, including exactly when the next glacial period would otherwise begin, remain open.