Facts for Kids

Scientists find signs of past ice ages in rocks, in tiny chemical clues, and in fossils. Moving ice scratches and smooths rocks, and it leaves piles of mixed stones and soil when it melts. Large rounded rocks called erratics got carried by ice far from their homes.

Other evidence comes from chemistry and preserved layers. Scientists study shells and mud for tiny chemical clues, and they drill into ice to get long tubes called ice cores. These cores hold layers and tiny air bubbles that tell us about old air and temperatures. Fossils show where animals and plants lived, so when warm-loving species suddenly disappear from a place, it can mean that area became much colder.

Ice age is a time when Earth becomes much colder than usual and large sheets of ice grow over continents and mountains. These sheets can cover whole countries, and smaller rivers of ice called glaciers move slowly like frozen rivers. Because so much water is locked up as ice, oceans can be lower and coastlines look different.

Earth’s long story has warm stretches and cold stretches. The cold stretches with lots of ice are called ice ages, while the warmer stretches have much less ice. These changes happen slowly over thousands or millions of years.

Earth has had at least five big cold times. One of the earliest is the Huronian ice age about 2.4–2.1 billion years ago, which happened after big changes in Earth’s air. The Cryogenian (about 720–630 million years ago) may have been especially cold — some scientists call that time a Snowball Earth because ice may have reached near the equator.

Later long cold times include the Andean-Saharan and the late Paleozoic ice ages. The most recent long cold time is the Quaternary Ice Age, which started a few million years ago and includes the small ice advances and retreats humans remember from long-ago times.

Last Glacial Maximum was the time about 26,000 to 13,300 years ago when ice covered much more land than today. In North America, ice sheets reached down toward the 45th parallel and were up to 3–4 kilometers thick in some places. Scientists name big glacial stages like the Illinoian, Eemian, and Wisconsin to talk about when ice grew and shrank; many older names are grouped together as Pre-Illinoian.

Huge ice sheets reshaped the land. They carved deep U-shaped valleys, left piles of rocks called moraines, and made many lakes. Because so much water was frozen, sea levels dropped and sometimes opened land bridges that let animals and people move to new places.

Inside an ice age, there are colder times called glacial periods and warmer times called interglacial periods. During glacials, ice sheets grow and spread; during interglacials, the ice pulls back and climates warm. For example, a warm interglacial not long before our modern time is called the Eemian.

These back-and-forth cycles happened in other ice ages too and changed where rivers flowed and where forests and animals could live. When ice grew, sea level fell and land patterns changed; when ice melted, seas rose again and new habitats opened up.

Milankovitch cycles are slow, natural changes in how Earth moves around the Sun and how much the planet tilts. Because of these changes, some years the Northern Hemisphere gets more warm summer sunlight and other years it gets less. For places around 65° north (where a lot of ice forms), July sunlight can change by about 22% — that is a big difference for melting snow.

When summers stay cooler because of less sunlight, winter snow can survive and pile up. Over many years that snow turns to ice and builds ice sheets. As ice grows, sea level falls (because water is stored in ice), and big ice sheets can also change ocean currents and weather far away.

Greenhouse gases are parts of the air, like carbon dioxide, that help keep Earth warm by trapping some heat. Scientists see in ice cores that when an ice age starts, these gas levels often fall, and when ice sheets shrink the gas levels rise. That pattern tells us greenhouse gases help make cold times colder and warm times warmer.

Most scientists think changes in sunlight from Earth’s orbit begin the move toward an ice age, and greenhouse gases then make the change larger. Other slow helpers, like rocks wearing down and volcanoes adding gas, can also change how much carbon dioxide is in the air and so change the climate over long times.