Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

How does an ice age happen?

ScienceLesson on requestFree to read

Quick answer

Ice ages begin when slow changes in Earth's orbit and tilt reduce summer sunlight in the north, letting snow survive all year. The growing white ice reflects more sunlight, cooling the planet further. Lower greenhouse gases, especially carbon dioxide, then lock in the cold for tens of thousands of years.

An ice age happens when global temperatures fall enough for ice sheets to expand across continents, driven mainly by slow changes in Earth's orbit and axis that reduce summer sunlight in the northern hemisphere. These orbital cycles, combined with feedbacks like higher reflectivity from snow and lower greenhouse gases, can sustain glacial conditions for tens of thousands of years.

Step by step

  1. Slow variations in Earth's orbit, axial tilt and wobble change how sunlight is distributed across the seasons.
  2. When northern hemisphere summers become cooler, winter snow does not fully melt and accumulates year after year.
  3. The expanding white snow and ice reflect more sunlight back to space, which cools the region further and spreads the ice.
  4. Colder oceans absorb more carbon dioxide from the atmosphere, weakening the greenhouse effect and lowering global temperatures.
  5. These cooling feedbacks reinforce each other until ice sheets cover large parts of continents, creating a glacial period.
  6. Later, orbital changes increase summer sunlight, ice melts, oceans release carbon dioxide, and the climate warms into an interglacial.

Key things to understand

  1. Ice ages are long intervals when Earth has permanent ice sheets at one or both poles, lasting millions of years.
  2. Within an ice age, colder glacial periods alternate with warmer interglacials like the one we live in now.
  3. The main trigger is changes in Earth's orbit and axial tilt, known as Milankovitch cycles, which alter sunlight distribution.
  4. When northern summers cool, winter snow survives year-round, and its white surface reflects more sunlight, cooling the planet further.
  5. Lower atmospheric carbon dioxide during glacial periods amplifies cooling, while higher CO2 during interglacials helps warm the planet.

Labelled diagram

  1. Orbital change

    Earth's orbit and tilt shift, reducing summer sunlight in the northern hemisphere.

  2. Snow survives

    Cooler summers let winter snow remain, accumulating into deeper layers.

  3. Ice reflects

    White ice bounces sunlight back to space, cooling the surface further.

  4. CO2 drops

    Colder oceans absorb carbon dioxide, weakening the greenhouse effect.

  5. Glacial period

    Ice sheets expand across continents, and the planet stays cold for millennia.

How orbital cycles build an ice age

Real-world examples

  • The Laurentide Ice Sheet covered most of Canada and the northern United States during the last glacial period, reaching thicknesses of several kilometres.
  • During the last glacial maximum about 20,000 years ago, sea level was roughly 120 metres lower, exposing a land bridge between Siberia and Alaska.
  • The Little Ice Age, a cooler period from about 1300 to 1850, froze the Thames River in London and caused crop failures in Europe.
  • Antarctica has been covered by ice for around 34 million years, making it the longest-lasting ice sheet on Earth today.

Common mistake

✗ Myth: Ice ages are caused by the Sun getting dimmer or by a sudden global freeze.

✓ Fact: Ice ages are triggered by slow changes in Earth's orbit and axial tilt that redistribute sunlight, not by a change in the Sun's output. Feedback from ice reflectivity and greenhouse gases then amplifies the cooling over thousands of years.

People also ask

What causes an ice age?
Ice ages are caused by long-term changes in Earth's orbit and axial tilt, called Milankovitch cycles, which reduce summer sunlight in the northern hemisphere. When summers are too cool to melt winter snow, ice sheets grow, reflect sunlight, and cool the planet further, reinforced by falling carbon dioxide levels.
How often do ice ages occur?
Major ice ages have occurred only a handful of times in Earth's history, separated by long warm intervals, but within the current ice age, glacial periods have cycled about every 100,000 years for the past million years. These cycles are paced by changes in Earth's orbit.
What was Earth like during the last ice age?
During the last glacial maximum about 20,000 years ago, ice sheets up to several kilometres thick covered northern continents, sea levels were about 120 metres lower, and the tropics were drier. Mammoths, sabre-toothed cats and early humans lived in these conditions.
Are we currently in an ice age?
Yes. Earth is in an ice age that began about 2.6 million years ago, because permanent ice sheets exist on Greenland and Antarctica. We are in a warmer interglacial phase called the Holocene, which started roughly 11,700 years ago.
What ends an ice age?
Ice ages end when orbital cycles increase summer sunlight in the northern hemisphere, melting ice sheets. As ice retreats, the surface reflects less sunlight and oceans release carbon dioxide, creating a warming feedback that pushes the climate into an interglacial period.
How long do ice ages last?
Major ice ages typically last tens of millions of years, with glacial periods within them lasting about 100,000 years and interglacials around 10,000 to 20,000 years. The current ice age has lasted about 2.6 million years so far.
Do ice ages affect the whole planet?
Ice ages cool the whole planet, but ice sheets mainly grow on continents near the poles. The tropics stay warm but become drier, and sea levels drop worldwide as water is locked up in ice, exposing land bridges between continents.

Part of the guide: Water & Weather Phenomena →