An ice age is a long stretch of time when Earth’s temperatures drop enough for massive ice sheets to spread across continents. The planet has gone through several ice ages over billions of years, but the one most people think of, the era of woolly mammoths and mile-thick glaciers, is part of a cold period that actually began about 2.6 million years ago and, technically, hasn’t ended yet.
Glacial and Interglacial Periods
The term “ice age” can be confusing because it refers to two different things. In the broadest sense, an ice age is any era when permanent ice sheets exist on Earth. By that definition, we’re still in one: the Quaternary Ice Age, which started 2.6 million years ago. Large ice sheets still sit on Greenland and Antarctica.
Within that larger ice age, the climate swings between colder phases called glacials and warmer phases called interglacials. During a glacial period, ice sheets expand far beyond the poles, covering huge portions of North America, Europe, and Asia. During an interglacial, the ice pulls back and temperatures rise. We’re currently living in an interglacial period called the Holocene, which began about 11,700 years ago. So when people ask “are we in an ice age,” the answer depends on which definition you’re using. We’re in the warm phase of one.
What Caused the Ice Ages
The primary driver of glacial cycles is a set of slow, predictable changes in how Earth orbits the Sun and how it’s tilted on its axis. These are known as Milankovitch cycles, named after the Serbian scientist who calculated them in the early 1900s. Three orbital variations work together to shift how much sunlight different parts of the planet receive over thousands of years.
The first is the shape of Earth’s orbit. Our path around the Sun isn’t a perfect circle; it stretches slightly into an oval and back again over a cycle of roughly 100,000 years. The second is the tilt of Earth’s axis, which wobbles between about 22 and 24.5 degrees over a 41,000-year cycle. (It’s currently at 23.4 degrees and slowly decreasing.) The third is precession, a slow wobble in the direction Earth’s axis points, completing a full rotation roughly every 23,000 years. When these cycles align in a way that reduces summer sunlight in the Northern Hemisphere, snow and ice from winter don’t fully melt. Over millennia, that leftover ice accumulates into continental-scale sheets.
Orbital cycles set the rhythm, but they don’t work alone. Carbon dioxide levels in the atmosphere amplify the effect. During the peak of the last glacial period, about 20,000 years ago, CO₂ concentrations were around 180 to 190 parts per million, less than half of today’s levels. Lower CO₂ meant the atmosphere trapped less heat, reinforcing the cooling. As ice retreated, CO₂ rose, and warming accelerated. This feedback loop helps explain why relatively small changes in sunlight can produce such dramatic shifts in climate.
How Big the Ice Sheets Got
The scale of glacial ice is hard to overstate. The Laurentide Ice Sheet, which covered most of Canada and a large portion of the northern United States, reached as far south as roughly the 37th parallel, the latitude of southern Missouri and Virginia. It covered more than 13 million square kilometers (5 million square miles) and in some places was 2,400 to 3,000 meters thick. That’s nearly two miles of ice stacked on top of the land.
All that water had to come from somewhere. Global sea levels dropped by an estimated 130 to 140 meters (roughly 430 to 460 feet) at the peak of the last glacial period. Coastlines extended far beyond where they sit today. Land that is now submerged was exposed, including a wide strip of territory between Siberia and Alaska known as the Bering Land Bridge. Recent research from the National Science Foundation found this land bridge only became passable around 35,700 years ago, much later than scientists previously assumed, shortening the window during which humans could have walked from Asia into the Americas.
Life During the Ice Age
The Pleistocene epoch, spanning from 2.6 million to 11,700 years ago, was home to an extraordinary collection of oversized animals. Woolly mammoths, giant ground sloths, saber-toothed cats, and massive flightless birds roamed landscapes that looked nothing like what we see today. These creatures are collectively called megafauna, animals with adult body weights over roughly 100 pounds.
Most of these species went extinct toward the end of the Pleistocene, and the cause remains one of the longest-running debates in science. The leading idea, often called the “overkill hypothesis,” argues that human hunters drove megafauna to extinction within a few thousand years of arriving on each continent. A competing explanation points to rapid climate change at the end of the glacial period, which reshaped habitats faster than large animals could adapt. Other researchers have pointed to habitat destruction through landscape burning, disease, and the collapse of food webs. The reality was likely some combination of these pressures.
One telling detail: Africa is the only continent that still has a diverse community of large animals, including elephants, hippos, and rhinoceroses. Because modern humans evolved in Africa alongside these species, the animals there had millions of years to develop wariness of human hunters. On other continents, where humans arrived suddenly, the megafauna had no such evolutionary preparation. Some surviving species adapted by changing their behavior, shifting from daytime to nighttime grazing, for example, but many couldn’t adjust fast enough.
Evidence Left Behind
You don’t need to travel far to see proof of past glaciers. Ice sheets reshaped entire landscapes, and the evidence is written into the terrain across North America, Europe, and beyond. Glaciers carved U-shaped valleys, fjords, and bowl-shaped depressions called cirques into mountainsides. They deposited long ridges of rock and debris known as moraines, which mark the edges and farthest reach of ancient ice. Boulders transported hundreds of miles from their origin, called glacial erratics, sit in fields and forests where the ice eventually dropped them.
At a smaller scale, glaciers left scratches in bedrock called striations, created by rocks frozen into the ice grinding across the surface like sandpaper. These scratches reveal the direction the glacier was flowing. Rolling hills of sediment called drumlins, pockmarked plains with round depressions called kettles, and chains of small lakes called paternoster lakes all tell the story of ice that once covered the land. Many of the Great Lakes, the Finger Lakes in New York, and the fjords of Norway and Alaska owe their existence to glacial carving.
The Younger Dryas: A Dramatic Interruption
The transition from the last glacial period to our current warm era wasn’t smooth. About 12,870 years ago, just as the planet was warming and ice was retreating, temperatures in the North Atlantic region plunged back to near-glacial conditions in as little as 20 years. This cold snap, called the Younger Dryas, lasted roughly 1,300 years before ending almost as abruptly as it began, with the final warming happening over about 90 years.
The Younger Dryas is a striking example of how quickly climate can shift. The leading explanation involves a massive influx of freshwater from melting glaciers into the North Atlantic, which disrupted ocean circulation patterns that carry warm water northward. For early human populations in Europe and the Near East who were beginning to settle into farming and more permanent communities, this return to harsh cold would have been a serious setback. The event ended around 11,600 years ago, ushering in the stable, warm Holocene climate that allowed agriculture and civilization to develop.
Where We Stand Now
Based on the natural rhythm of Milankovitch cycles, Earth would ordinarily be heading slowly toward another glacial period over the next several tens of thousands of years. The current interglacial has already lasted about 11,700 years, which is typical for warm phases in the Quaternary. But the massive increase in atmospheric CO₂ from burning fossil fuels has fundamentally altered the equation. Pre-industrial CO₂ levels were around 280 parts per million. During the last glacial maximum, they were 180 to 190 ppm. Today, they exceed 420 ppm. That concentration of greenhouse gases is expected to delay or prevent the next glacial period entirely, pushing the climate system into territory that has no precedent in the Quaternary record.