An ice age is a long stretch of time, lasting tens of millions of years, when large ice sheets cover portions of Earth’s continents. Within each ice age, temperatures swing back and forth between colder phases called glacial periods, when ice expands dramatically, and warmer phases called interglacial periods, when much of that ice retreats. Earth is technically inside an ice age right now. The current one began about 2.6 million years ago, and we’re living in one of its warmer interglacial windows, which started roughly 11,700 years ago.
Glacial vs. Interglacial Periods
The distinction between an ice age and a glacial period trips up a lot of people. An ice age is the entire multi-million-year era when permanent ice sheets exist on land. A glacial period is a colder chapter within that era, when those ice sheets push outward and temperatures drop sharply. An interglacial period is a warmer chapter when the ice pulls back. Our current warm phase, known as the Holocene, is an interglacial period sitting inside the larger Quaternary Ice Age.
At the peak of the last glacial period, roughly 26,000 to 20,000 years ago, glaciers covered about 25% of Earth’s land surface. Today, glaciers and ice sheets still cover about 11%, mostly in Antarctica and Greenland. So even in a “warm” interglacial, a significant portion of the planet remains frozen.
What Causes Ice Ages
The single most important trigger for glacial and interglacial cycles is the way Earth’s orbit around the Sun slowly shifts over thousands of years. A Serbian scientist named Milutin Milankovitch worked out the math about a century ago, and the three orbital shifts now carry his name.
The first is eccentricity: Earth’s orbit isn’t a perfect circle, and over long cycles it stretches more oval or rounds out, changing how far we sit from the Sun at different points in the year. The second is obliquity, the tilt of Earth’s axis, which currently sits at about 23.4 degrees but has varied between 22.1 and 24.5 degrees over the last million years. When the tilt is smaller, summers become cooler and winters milder. That sounds pleasant, but cooler summers mean less snow melts each year, allowing ice to slowly accumulate at high latitudes. The third is precession, a slow wobble of Earth’s rotational axis that completes a full cycle roughly every 26,000 years, shifting which hemisphere gets more intense sunlight during its summer.
Together, these three cycles can vary the amount of solar energy hitting Earth’s mid-latitudes by up to 25%. Once ice begins building, it reflects more sunlight back into space, cooling the planet further in a feedback loop that deepens the freeze.
Major Ice Ages in Earth’s History
Earth has experienced several major ice ages stretching back billions of years, separated by long warm intervals of roughly 150 million years.
- Huronian (2.4 to 2.1 billion years ago): Among the earliest ice ages found in the geological record, occurring in several pulses interspersed with warmer stretches.
- Cryogenian (roughly 720 to 635 million years ago): Earth fell into at least two deep freezes during this period. The Sturtian glaciation lasted from about 720 to 660 million years ago, and the Marinoan from about 640 to 635 million years ago. Some evidence suggests ice extended nearly to the equator, earning these events the nickname “Snowball Earth.”
- Late Paleozoic (roughly 300 to 250 million years ago): A major glaciation that covered large parts of the southern supercontinent Gondwana.
- Quaternary (2.6 million years ago to present): The ice age we’re living in. It includes the Pleistocene glacial cycles and our current interglacial period.
How Ice Ages Reshape Geography
During glacial peaks, so much water gets locked up in continental ice sheets that sea levels plunge. At the height of the last glacial period, global sea level sat about 130 meters (425 feet) lower than it does today. That drop exposed vast stretches of seafloor, creating land bridges between continents that are now separated by water.
The most famous example is the Bering Land Bridge, which connected Asia to North America. Recent research shows it didn’t fully emerge until around 35,700 years ago, and it remained passable until rising seas flooded it again between 13,000 and 11,000 years ago. During that window, humans and animals crossed between the continents, permanently reshaping the populations of both.
Glaciers also leave physical fingerprints on the landscape. Moraines, which are ridges of sediment bulldozed into place by advancing ice, mark the farthest reach of ancient glaciers across the midwestern United States and northern Europe. Terminal moraines show where a glacier stopped and began retreating. Rolling, hummocky terrain left behind by melting ice sheets defines the topography of states like Indiana and Wisconsin today.
Life During the Last Glacial Period
The ice age world supported a range of enormous animals, often called megafauna, that have no modern equivalent outside of Africa. Woolly mammoths, giant ground sloths, saber-toothed cats, and ice age horses roamed grasslands known as the mammoth steppe, which stretched across northern Europe, Siberia, and into North America. These communities were far more diverse and densely populated than the sparse tundra wildlife living in those same regions today.
Survival required constant adaptation. Megafauna species like mammoths, horses, and bison experienced boom-and-bust population cycles as they tracked rapid climate shifts, sometimes dispersing across continental distances to find suitable habitat. It was, in effect, a game of musical chairs played over thousands of miles.
The end of the last glacial period proved fatal for many of these species. As temperatures rose, the dry grasslands they depended on were replaced by wet, acidic tundra covered in spreading peat. This degraded the quality of their grazing range. At the same time, rising sea levels flooded the land bridges that had allowed populations to migrate between continents. Cut off from new habitat and unable to cross the increasingly soggy landscape, many megafauna populations collapsed within roughly a thousand years of these changes taking hold.
When the Next Glacial Period Might Arrive
Under natural orbital conditions alone, the next glacial period would be expected to begin roughly 50,000 years from now. But human carbon dioxide emissions are likely to push that timeline back significantly. CO2 stays in the atmosphere for thousands of years, and its warming effect works against the slow orbital cooling that would otherwise nudge Earth toward the next freeze.
Modeling studies suggest that the carbon already released by human activity (around 500 billion metric tons) probably isn’t enough on its own to delay the next glacial onset. But if cumulative emissions double to around 1,000 billion metric tons, which current trajectories make plausible, the next glacial period could be postponed by an additional 50,000 years, effectively pushing it to roughly 100,000 years from now. In geological terms, human activity may be rewriting the planet’s deep climate schedule.