About 20,000 years ago, ice sheets up to 3 kilometers thick covered much of North America, Northern Europe, and Asia. Sea levels were 120 meters lower than today, exposing land bridges between continents. Mammoths and giant ground sloths roamed alongside early humans. This was the last glacial maximum of the most recent ice age — a period when Earth's climate had dramatically cooled, transforming landscapes and ecosystems worldwide. Throughout Earth's history, multiple ice ages have shaped our planet, with profound consequences for life and geography.
The Short Answer
An ice age is a prolonged period when Earth's climate cools significantly, allowing large continental ice sheets and glaciers to form and persist for thousands of years. The term has both broad and narrow meanings. Broadly, an ice age refers to periods of millions of years when Earth has glaciers somewhere. Narrowly, it refers to the cooler "glacial" periods within a longer ice age, when ice sheets expand. We're currently in an ice age (the Quaternary Ice Age) but in an "interglacial" warmer period within it.
Two Definitions
The term "ice age" can be confusing because it has two meanings. In the broad sense, it refers to a long period (millions of years) when Earth has permanent ice somewhere, typically large polar ice caps. Earth has had several ice ages by this definition. In the narrow sense, it refers to a period of significant glaciation when ice sheets expand. Multiple of these "glacial periods" occur within a broader ice age. By the broad definition, we're currently in the Quaternary Ice Age (started 2.6 million years ago). By the narrow definition, we're in an "interglacial" — a warmer period between glacial cycles. The last glacial maximum was about 20,000 years ago.
Major Earth Ice Ages
Earth has experienced several major ice ages over geological history. The Huronian Ice Age (2.4-2.1 billion years ago) was one of the earliest, possibly Earth's most severe, and may have been "Snowball Earth" with global glaciation. The Cryogenian period (720-635 million years ago) had multiple "Snowball Earth" episodes; some scientists believe glaciers may have reached the equator. The Andean-Saharan ice age (450-420 million years ago) saw major glaciation during the Ordovician-Silurian periods. The Karoo Ice Age (360-260 million years ago) was a long glacial period during the Carboniferous-Permian. The Quaternary Ice Age (2.6 million years ago to present) includes the famous recent glaciations and our current interglacial.
Glacial and Interglacial Periods
Within an ice age, conditions fluctuate between cold "glacial" periods (when ice sheets expand) and warmer "interglacial" periods (when they retreat). The Quaternary Ice Age has had many glacial-interglacial cycles, typically lasting 100,000 years each. The last glacial maximum was about 20,000 years ago, when ice sheets were largest. We then entered an interglacial about 11,700 years ago. The current interglacial — the Holocene — is when modern civilization developed and now extends to today.
What Causes Ice Ages?
Multiple factors trigger ice ages. Milankovitch cycles are variations in Earth's orbital geometry over 100,000, 41,000, and 23,000-year cycles. These produce the cyclic glacial periods within ice ages. Solar radiation changes involve variations in solar output over long periods. Continental positions matter because plate tectonics moves continents to positions that favor or disfavor ice formation. Ocean circulation changes affect heat distribution globally. Greenhouse gases such as CO₂ and methane correlate with cooling at lower concentrations. Volcanic activity over long timescales affects atmospheric chemistry.
Milankovitch Cycles
Serbian scientist Milutin Milankovitch identified orbital cycles that influence Earth's climate. Three main cycles operate. Eccentricity (100,000 years) involves variations in the shape of Earth's orbit from more to less elliptical. Obliquity (41,000 years) is variation in Earth's axial tilt between 22.1° and 24.5°. Precession (23,000 years) involves Earth's wobble in its axis orientation. These combine to affect how much sunlight reaches different parts of Earth at different times of year. The 100,000-year cycle has been the strongest pacing of glacial-interglacial transitions over the past million years. Each cycle is precisely tracked by climate scientists to understand past and future climate change.
The Last Glacial Maximum
The most recent glacial maximum was about 20,000-22,000 years ago. During this period, ice sheets covered most of Canada, the northern United States, and much of Europe. The Laurentide Ice Sheet covered eastern Canada and parts of the US. The Cordilleran Ice Sheet covered western North America. The Fennoscandian Ice Sheet covered Northern Europe. The Antarctic Ice Sheet was significantly larger than today. Sea levels were about 120 meters lower than today. Global temperatures averaged about 6°C cooler than now. Land bridges connected continents.
The Bering Land Bridge
One of the most consequential effects of the last ice age was the Bering Land Bridge. With sea levels 120 meters lower, the Bering Strait between Asia and Alaska was dry land. This "Beringia" connected the continents and allowed humans to migrate from Asia to the Americas. The first peoples likely entered the Americas this way between 15,000 and 25,000 years ago. As the ice age ended and sea levels rose, Beringia was flooded, creating the modern Bering Strait. The land bridge was a key route for early human migration.
Mammoths and Ice Age Animals
The ice age supported distinctive megafauna. Woolly mammoths were massive elephants adapted to cold steppe. Saber-toothed cats were predators with enormous canine teeth. Giant ground sloths were some larger than modern bears. Cave bears were massive bears living in European caves. Glyptodonts were giant armadillo-like animals. Megaceros, the Irish elk, were deer with enormous antlers. Cave lions and dire wolves were larger predators. Many of these megafauna went extinct around the end of the last ice age, possibly due to climate change, human hunting, or both.
The Anthropocene and Climate
Some scientists propose we've entered a new geological epoch called the Anthropocene, marked by human-induced changes including climate change. The current interglacial would naturally last about 50,000 more years before potentially entering another glacial period. However, human-induced warming may delay or prevent the next glacial period. Some research suggests CO₂ levels from human activities have already disrupted natural climate cycles enough to skip the next glaciation entirely. The Anthropocene era will dramatically affect Earth's climate future.
Snowball Earth
Some ancient ice ages may have been so severe that ice covered most or all of Earth's surface — including the equator. This "Snowball Earth" hypothesis is supported by evidence of glacial deposits in low-latitude regions during the Cryogenian period (635-720 million years ago). Some scientists believe ice may have completely covered the planet during these episodes. The conditions would have been incredibly harsh, with global temperatures near -40°C. Life would have been mostly limited to deep ocean hydrothermal vents, ice-covered seas, and other refugia. The cause and extent of Snowball Earth events remain debated.
Ice Sheets and Sea Level
During ice ages, massive amounts of water are locked in ice sheets, dramatically lowering sea levels. At the last glacial maximum, sea level was about 120 meters lower than today. Many coastlines were dramatically different. The English Channel was a land area. The Persian Gulf was largely dry. The Indonesian islands were connected to mainland Asia. Australia was connected to Tasmania and New Guinea. As ice sheets melted, sea levels rose rapidly, drowning coastal lands and disrupting human settlements.
Evidence of Past Ice Ages
Multiple lines of evidence reveal past ice ages. Glacial deposits include tills, moraines, and erratic boulders. Ice cores from Greenland and Antarctic ice contain ancient atmospheric samples in trapped air bubbles. Ocean sediments show changes in temperature and species composition. Pollen records reveal vegetation changes. Cave deposits including stalagmites and stalactites record past climates. Isotope analysis of oxygen isotopes in marine fossils indicates past temperatures. Glacial landforms include U-shaped valleys, drumlins, eskers.
Ice Cores and Climate History
Ice cores from Greenland and Antarctica are particularly valuable. They contain trapped air bubbles, providing direct samples of ancient atmospheres. By analyzing these bubbles, scientists can reconstruct past CO₂ levels, methane levels, and other atmospheric gases. Ice cores also record temperature variations through oxygen isotope analysis. Some cores cover the past 800,000 years, with annual resolution at the top. They reveal cyclical patterns matching Milankovitch predictions, showing how climate has varied through multiple glacial-interglacial cycles. Ice core data is fundamental to climate science.
The Little Ice Age
The "Little Ice Age" (approximately 1300-1850) was a period of slightly cooler temperatures, not a true ice age but significant cooling that affected agriculture and society. Glaciers expanded in Europe. Rivers like the Thames froze over more frequently. Crop failures contributed to famines. The cause was likely a combination of reduced solar activity (the Maunder Minimum), increased volcanic activity, and other factors. The Little Ice Age ended in the 19th century as Earth's climate gradually warmed. It's an example of how relatively small climate changes can have significant social impacts.
Glaciers Today
Although we're in an interglacial period, glaciers still exist worldwide. Antarctica has the largest ice sheet, containing 90% of Earth's ice. Greenland has the second largest ice sheet. High mountain ranges including the Alps, Andes, Himalayas, Rockies, and Alaska have many glaciers. Iceland has significant glaciers. Patagonia has large icefields. Subarctic regions have various ice caps and glaciers. Most glaciers are retreating due to current warming, which affects sea levels and water resources globally.
Future Ice Ages
Natural climate cycles would suggest another glacial period within the next 50,000-100,000 years. However, human-induced CO₂ emissions may disrupt this. Some research suggests current atmospheric CO₂ levels could prevent the next glaciation entirely, or significantly delay it. The interaction between natural cycles and anthropogenic forcing is complex. Earth's climate may enter a new state different from past patterns. Scientists actively study these dynamics to project future climate trajectories.
Climate Change and Ice Ages
Current human-induced climate change differs from natural ice age cycles in several ways. The rate of current warming is happening 10-100 times faster than natural rates. The direction is warming during an interglacial, not transitioning into one. The magnitude of projected warming may exceed natural variations. The cause is anthropogenic CO₂, not natural orbital cycles. Sea level may rise faster than during natural interglacials. Understanding past ice ages helps put current change in context and predict future possibilities.
Key Facts
An ice age is a long period when Earth's climate cools enough for large ice sheets. We're currently in the Quaternary Ice Age but in a warmer interglacial period. The last glacial maximum was about 20,000 years ago. Multiple Snowball Earth episodes may have occurred 635-720 million years ago. Milankovitch orbital cycles drive most natural climate variations. Ice cores provide direct evidence of past climate.
Fun Facts
Mammoths still existed when the Egyptian pyramids were being built — they didn't fully go extinct until about 4,000 years ago on Wrangel Island. During the last glacial maximum, you could walk from Russia to Alaska across dry land. Some scientists believe ice may have once covered the entire Earth during ancient ice ages. The current interglacial period (Holocene) is when all of human civilization developed. Earth's most recent ice age may have been triggered partly by the closure of the Isthmus of Panama, changing ocean circulation. The largest glaciers are in Antarctica, which holds 90% of all Earth's ice.
The Bottom Line
An ice age is a prolonged period when Earth's climate cools enough for continental ice sheets and glaciers to form and persist. Multiple ice ages have shaped Earth's history, with the most recent being the ongoing Quaternary Ice Age — currently in a warmer interglacial period. Natural cycles driven by Earth's orbital geometry trigger glacial-interglacial transitions. The last glacial maximum about 20,000 years ago dramatically transformed landscapes, ecosystems, and human migration. Today's anthropogenic climate change is occurring during an interglacial, with implications that may affect Earth's climate for millions of years.