A glacial period is a stretch of time, typically lasting tens of thousands of years, when Earth’s temperatures drop significantly and massive ice sheets spread across continents. During the most recent one, which peaked roughly 26,000 to 20,000 years ago, global average temperatures were about 5 to 6°C (9 to 11°F) cooler than preindustrial levels, and ice covered much of North America and northern Europe.
The term often gets mixed up with “ice age,” but they aren’t the same thing. An ice age is the broader era, spanning millions of years, during which Earth’s climate swings back and forth between colder glacial periods and warmer stretches called interglacials. We’re currently living in an interglacial period called the Holocene Epoch, which began about 10,000 years ago. The larger ice age itself hasn’t ended.
What Triggers a Glacial Period
Glacial periods are driven primarily by slow, predictable shifts in how Earth moves through space. These are known as Milankovitch cycles, and they come in three flavors. First, the shape of Earth’s orbit around the Sun stretches from nearly circular to slightly more oval and back again over a roughly 100,000-year cycle. Second, the tilt of Earth’s axis wobbles between about 22° and 24.5° over a 41,000-year cycle. Third, Earth’s axis slowly traces a circle like a spinning top, completing one full wobble every 23,000 years or so.
None of these changes alone is dramatic enough to freeze a continent. But when they align in ways that reduce the amount of summer sunlight hitting the Northern Hemisphere, snow from winter survives through summer, piles up year after year, and eventually compresses into continent-spanning ice sheets. Once ice covers enough land, it reflects sunlight back into space, cooling the planet further in a self-reinforcing loop. Falling temperatures also pull carbon dioxide out of the atmosphere, amplifying the chill even more.
Before about a million years ago, glacial cycles followed the 41,000-year rhythm of Earth’s axial tilt. Then something shifted. Glacial periods became longer and more intense, stretching to roughly 100,000-year cycles and producing ice sheets large enough to reshape entire landscapes. Scientists are still working out exactly why the longer cycle took over, but the pattern has held through all of recent geologic time.
How Cold It Actually Gets
The most recent glacial period reached its coldest point during what geologists call the Last Glacial Maximum, roughly 26,000 to 20,000 years ago. Constrained by ocean temperature records from that era, the best estimates put global cooling at about 5.8°C below preindustrial temperatures, with a plausible range of 4.4 to 7.2°C. That might sound modest, but a drop of even a few degrees globally translates to radically different conditions on the ground: permafrost spreading across Europe, forests retreating to narrow refuges near the equator, and deserts expanding as the air dried out.
The cooling wasn’t evenly distributed. Polar regions and continental interiors cooled far more than the tropics. Parts of central North America sat under kilometers of ice, while tropical sea surface temperatures dropped by a comparatively mild 2 to 3°C. Atmospheric dust increased sharply as vegetation died back and exposed bare soil to wind, and that dust further reduced the amount of sunlight reaching the surface.
Ice Sheets and Sea Levels
The most visible feature of a glacial period is the ice itself. The Laurentide Ice Sheet, which covered most of Canada and reached into the northern United States, was the largest ice mass in the Northern Hemisphere. At its peak, it spread across roughly 12 to 16 million square kilometers and reached a maximum thickness of about 2.7 to 2.9 kilometers. For perspective, that’s ice stacked nearly two miles high over what is now Hudson Bay. The sheet had two main domes: one centered over Hudson Bay and a second farther west, near present-day Alberta.
Scandinavia and northern Russia carried their own massive ice sheet, and smaller glaciers expanded in the Alps, Andes, Himalayas, and mountain ranges across the Southern Hemisphere. All that water locked up on land had to come from somewhere, and it came from the ocean. Global sea levels during the Last Glacial Maximum dropped about 130 meters (425 feet) below where they sit today. That exposed enormous stretches of continental shelf, connecting Asia to North America via a land bridge across what is now the Bering Strait and linking the British Isles to mainland Europe.
How Life Adapted
Glacial periods didn’t simply freeze everything in place. They created a patchwork of rapidly shifting ecosystems, and many large animals were well suited to that instability. Megafauna like woolly mammoths, giant ground sloths, and steppe bison thrived in part because of traits that helped them exploit unpredictable landscapes: they could travel long distances at relatively low energy cost, they ate a wide variety of plants, and their large bodies were metabolically efficient, losing heat more slowly than smaller animals. These weren’t just cold-weather specialists. They were generalists built for a world in constant flux.
Plants responded differently. Forests contracted toward lower latitudes and sheltered valleys, replaced at higher latitudes by vast grasslands and tundra. Many tree species survived glacial periods in small pockets called refugia, then slowly recolonized northward as the ice retreated. The genetic fingerprints of those bottlenecks are still visible in modern forest populations across Europe and North America.
Where We Are Now
The current interglacial, the Holocene, began roughly 10,000 years ago as the great ice sheets melted and sea levels rose to near their present position. Based on the rhythm of past cycles, the next glacial period would ordinarily be expected tens of thousands of years from now. However, the orbital conditions that trigger glaciation require a specific reduction in Northern Hemisphere summer sunlight, and Earth’s orbit is currently near its most circular, which dampens the influence of orbital cycles. Some analyses suggest the next natural glacial onset could be delayed by 50,000 years or more even without human influence.
Human-caused increases in atmospheric carbon dioxide add another layer. The greenhouse warming already underway is likely sufficient to override the orbital cooling that would otherwise nudge Earth toward glaciation, effectively postponing the next glacial period well beyond any natural timetable. The ice age cycle hasn’t stopped, but its next cold chapter has been pushed further into the future than at any point in the past several million years.