An ice age is a long stretch of Earth’s history, lasting millions of years, during which large ice sheets persist on the planet’s surface. At least five major ice ages have occurred over the past 2.4 billion years, and we are technically still in one. The current ice age began roughly 2.5 million years ago, but we live in a warm phase within it called an interglacial period, which is why most of the planet isn’t buried under glaciers right now.
Glacial vs. Interglacial Periods
The distinction that confuses most people is the difference between an ice age and a glacial period. An ice age is the entire multi-million-year era of recurring cold. Within that era, the climate swings back and forth between colder glacial periods, when ice sheets advance across continents, and warmer interglacial periods, when they retreat. Over the past 2 million years, Earth has cycled between these two states repeatedly.
We currently live in the Holocene Epoch, an interglacial period that began about 11,700 years ago when the last major ice sheets collapsed. Because the Holocene has lasted a shorter time than some previous interglacial phases, some scientists consider it possible that large-scale glaciation could return at some point in the distant future, making the Holocene just another warm pause rather than a permanent shift.
What Causes Ice Ages
Ice ages are driven by a combination of factors, but the most important pacemaker is the way Earth moves through space. Three types of orbital variation, known as Milankovitch cycles, change how much sunlight different parts of the planet receive over thousands of years:
- Eccentricity: The shape of Earth’s orbit around the Sun shifts between more circular and more elliptical over a cycle of about 100,000 years. A more elliptical orbit means greater variation in how close Earth gets to the Sun throughout the year.
- Obliquity: Earth’s axis is currently tilted at 23.4 degrees, but that angle wobbles between about 22 and 24.5 degrees over a 41,000-year cycle. A greater tilt means more extreme seasons, which affects how much summer warmth reaches polar ice.
- Precession: Earth’s axis slowly traces a circle in space, like a wobbling top, completing a full rotation roughly every 26,000 years. This changes which hemisphere is tilted toward the Sun during the closest point in Earth’s orbit, shifting the intensity of seasons between the Northern and Southern Hemispheres.
These orbital cycles don’t work alone. Carbon dioxide in the atmosphere plays a critical amplifying role. As CO₂ levels dropped over tens of millions of years, they eventually fell to around 270 to 280 parts per million by about 2.5 million years ago, low enough to kick off the series of glacial cycles we’ve experienced since. When orbital shifts cool the planet slightly, ice reflects more sunlight, oceans absorb more CO₂, and the cooling deepens. The reverse happens during warming. It’s a feedback loop where small orbital nudges get magnified by changes in greenhouse gases and ice cover.
Earth’s Five Major Ice Ages
The earliest known ice age, the Huronian glaciation, lasted from roughly 2.4 billion to 2.1 billion years ago. Scientists have found glacial deposits near Lake Huron in Ontario and other locations dating to this period.
The most dramatic glaciation happened during the Cryogenian Period, sometimes called “Snowball Earth.” Between about 700 and 650 million years ago, two glacial episodes each lasted around 20 million years. Scientists hypothesize that ice covered the entire planet during these events, including the equatorial oceans, with sea ice potentially reaching up to a kilometer thick. The causes aren’t fully understood, but the result was the most intense cold Earth has ever experienced.
Three more ice ages followed: one in the late Ordovician period (around 450 million years ago), one in the late Carboniferous through early Permian (around 360 to 260 million years ago), and the current Quaternary ice age, which started roughly 2.5 million years ago. Each left its mark in the geological record through distinctive features like moraines, which are ridges and mounds of rock, clay, and sand bulldozed into place by advancing glaciers, then left behind when the ice melted.
The Last Glacial Maximum
The most recent peak of glaciation, called the Last Glacial Maximum, occurred about 20,000 years ago. Ice sheets covered roughly 8% of Earth’s total surface and 25% of its land area. In North America, a massive ice sheet extended from the Arctic deep into what is now the northern United States. In Europe, ice blanketed Scandinavia and reached into the British Isles and northern Germany.
All that water locked up in ice had to come from somewhere. Global sea levels dropped more than 400 feet (about 122 meters) below where they are today, exposing vast stretches of continental shelf. Land bridges appeared where ocean exists now, including the Bering land bridge connecting Asia to North America, which enabled human migration into the Americas. Coastlines extended miles beyond their current positions. The world map looked fundamentally different.
Are We Still in an Ice Age?
Technically, yes. Geologists define an ice age by the presence of permanent ice sheets on the planet, and both Greenland and Antarctica still hold massive ones. The Holocene, our current warm phase, sits within the broader Quaternary ice age. By contrast, for most of Earth’s 4.5-billion-year history, the planet had no permanent ice at all. Polar ice caps are the exception, not the rule.
Left to its natural rhythm, Earth would likely begin sliding toward another glacial period in roughly 10,000 years, based on the orbital cycles that have paced these transitions for millions of years. But human greenhouse gas emissions have almost certainly disrupted that timeline. CO₂ concentrations have risen far above the 270 to 280 ppm range that characterized the pre-industrial interglacial, and researchers at the Alfred Wegener Institute and University College London have concluded that a natural transition back to glacial conditions in 10,000 years is now very unlikely. Human emissions have diverted the climate from its natural course, with impacts that will persist far into the future.
So while we live in an ice age in the geological sense, the more pressing concern isn’t the return of glaciers. It’s the rapid warming that’s pushing us in the opposite direction, faster than at almost any point in the planet’s history.