What is an ice age? Why Earth freezes and thaws

An ice age is a long stretch of Earth's history, lasting millions of years, when permanent ice sheets cover large areas of land. By that definition we are living in one now: ice sheets still bury Antarctica and Greenland. Within an ice age, climate swings between cold glacial periods, when ice spreads across continents, and warmer interglacials like today's. What paces those swings is slow, regular change in Earth's orbit, amplified by carbon dioxide and ice.

In short

• In the scientific sense, Earth has been in an ice age for about 2.6 million years, and we live in one of its warm interglacial periods.
• The last glacial maximum, about 20,000 years ago, was about 6°C colder than the 20th century, with sea level more than 120 meters lower.
• Glacial cycles are paced by wobbles in Earth's orbit over tens of thousands of years, far slower than today's warming.

What counts as an ice age?

Geologists use "ice age" for any long period when Earth has large ice sheets on land. In everyday speech, "the Ice Age" means the last glacial period, the time of woolly mammoths and ice over Chicago and Stockholm. The two are easy to confuse, so it helps to name the levels:

Term	Means	Example	Length
Ice age	Any period with lasting ice sheets on land	The Quaternary glaciation, from about 2.6 million years ago to now	Millions of years
Glacial period	A cold phase within an ice age, when ice sheets advance	The last glacial period, ending about 11,700 years ago	Tens of thousands of years
Interglacial	A warm phase between glacials	The Holocene, from about 11,700 years ago to now	Usually 10,000 to 30,000 years
Glacial maximum	The coldest point of a glacial, with the most ice	The Last Glacial Maximum, about 20,000 years ago	A few thousand years

Ice ages are the exception in Earth's 4.5-billion-year history, not the rule. For most of it there was little or no permanent ice. There have been at least five major ice ages, including one around 700 million years ago so severe that ice may have reached the equator, an idea known as "snowball Earth."

What was the last glacial maximum like?

About 20,000 years ago, ice sheets up to several kilometers thick covered Canada, the northern United States, Scandinavia and northern Britain. The U.S. Geological Survey (USGS) estimates that glaciers then covered about 25% of Earth's land, compared with about a tenth today. With so much water locked in ice, sea level was more than 400 feet (about 120 meters) lower than today, exposing land bridges such as the one between Siberia and Alaska.

How cold was it? A 2020 study led by Jessica Tierney of the University of Arizona, published in Nature, put the global average about 6°C colder than in the 20th century. That figure sounds small, but it is an average over the whole planet, ocean included. The Arctic was about 14°C colder.

Here is how that world compares with ours, using NOAA's 2024 temperature figures for the last two temperatures:

	Last Glacial Maximum, about 20,000 years ago	Before industry, about 1850	2024
Global temperature	About 6°C colder than the 20th century	About 0.2°C colder than the 20th century	About 1.3°C warmer than the 20th century
Carbon dioxide	About 180 ppm	About 280 ppm	About 423 ppm
Sea level	More than 120 meters lower	Near today's	Rising
Land under ice	About 25%	About 10%	About 10%

The step from glacial to interglacial was about 6°C and 100 parts per million of carbon dioxide. Since 1850 people have added more than 140 parts per million, a larger change in carbon dioxide than the whole swing out of the last glacial.

What causes glacial cycles?

The pacemaker is Earth's orbit. In the 1920s and 1930s the Serbian scientist Milutin Milanković worked out how three slow changes alter how much sunlight reaches the far north in summer. They are now called Milankovitch cycles, and NASA describes each:

1. Eccentricity. The shape of Earth's orbit around the Sun stretches from nearly circular to slightly oval and back over about 100,000 years.
2. Obliquity. The tilt of Earth's axis swings between 22.1 and 24.5 degrees in a cycle of about 41,000 years. It is now 23.4 degrees and slowly decreasing. Less tilt means cooler summers.
3. Precession. Earth's axis wobbles like a spinning top, which changes which season falls when Earth is closest to the Sun, on a cycle averaging about 23,000 years.

What matters most is summer in the northern continents. Ice sheets grow when summers are too cool to melt the winter's snow, and they shrink when summers warm. The orbit alone changes sunlight only a little. Feedbacks do the rest. Spreading ice and snow reflect more sunlight, which cools the planet further. Cooler oceans absorb more carbon dioxide, which weakens the greenhouse effect: ice cores show carbon dioxide swinging between about 180 parts per million in glacials and 280 in interglacials. Each push from the orbit is magnified by ice and carbon.

How is today's warming different?

Speed, cause and direction. Orbital cycles work over tens of thousands of years; today's warming has happened in about 150. NASA notes that the orbit alone would currently push Earth toward a slow cooling, a trend that began about 6,000 years ago, yet the planet has warmed by more than 1°C since 1850. Since 1750, the warming effect of greenhouse gases from burning fossil fuels has been more than 50 times the Sun's slight added warming.

The next glacial period has also been pushed back. A 2016 study in Nature by Andrey Ganopolski and colleagues found that, even without human emissions, the current interglacial would probably have lasted another 50,000 years, and that moderate cumulative emissions would postpone the next glacial by at least 100,000 years. In other words, the ice-age cycle is not about to rescue the planet from climate change, and climate change is not about to trigger a new glacial.

Did scientists predict an ice age in the 1970s?

No, not as a consensus. A few studies and magazine stories in the 1970s raised the idea of cooling, partly because global temperatures had dipped slightly since the 1940s, likely from air pollution that reflects sunlight. But a 2008 review of the scientific literature of the period by Thomas Peterson, William Connolley and John Fleck, published by the American Meteorological Society, found that greenhouse warming already dominated scientists' thinking as one of the most important forces on climate over human timescales. The "1970s ice age scare" is a popular myth, often used to cast doubt on climate science today.

Still argued

Why the rhythm of glacial cycles changed is an open question. Between about 1 and 3 million years ago, glacials came about every 41,000 years, matching the tilt cycle. Around 800,000 years ago they lengthened to about 100,000 years, matching the eccentricity cycle, though eccentricity changes sunlight the least of the three. NASA says there is not yet a clear explanation. Proposed answers include changes in the bedrock under the ice sheets, a slow fall in carbon dioxide, and ice sheets that grew big enough to survive weaker summers; researchers still debate which mattered most.

Check yourself

• Without looking back: in what sense are we living in an ice age right now?
• How can small changes in sunlight from Earth's orbit lead to a 6°C swing in global temperature?
• Name two ways today's warming differs from the warming at the end of the last glacial period.

Sources

• Milankovitch (orbital) cycles and their role in Earth's climate — NASA, 2024
• Why Milankovitch (orbital) cycles can't explain Earth's current warming — NASA, 2024
• How cold was the ice age? Researchers now know — University of Arizona, 2020
• How would sea level change if all glaciers melted? — U.S. Geological Survey, 2026
• Critical insolation–CO2 relation for diagnosing past and future glacial inception — Ganopolski, Winkelmann and Schellnhuber, Nature, 2016
• The myth of the 1970s global cooling scientific consensus — Peterson, Connolley and Fleck, Bulletin of the American Meteorological Society, 2008