Ice age

An ice age is a time when Earth gets much colder. During an ice age, large areas of ice cover the land. Thick ice sheets and glaciers grow and change the shape of the land. These cold times happen when the planet's surface and air become much cooler than usual.

Earth's weather changes naturally between warmer and cooler times. For most of its history, Earth has been warm with little ice. But for the past 34 million years, we have been in what is called the Late Cenozoic Ice Age. This long ice age has had both very cold times, when ice grew, and warmer times, when ice melted.

In the last few million years, Earth has had many cycles of cold and warm weather. The most recent cold time was called the Last Glacial Period, which ended about 11,700 years ago. Before that, there was an even colder time known as the Last Glacial Maximum, when ice covered more of the land. Today, we are living in a warmer time called the Holocene, but Earth is still in an ice age because some ice remains.

History of research

See also: History of climate change science

In the 1700s and 1800s, people noticed strange rocks and marks in the land. They realized that glaciers must have moved long ago. For example, in 1742, an engineer in Geneva visited Chamonix in the Alps. He heard from locals that glaciers used to reach farther than they do today.

Later, scientists like Louis Agassiz and Jean de Charpentier studied these clues. They suggested the idea of "ice ages"—times when big glaciers covered parts of Earth that are warm now. It took many years for other scientists to agree, but by the late 1800s, most believed Earth had cold periods when glaciers grew large.

Evidence

There are three main types of evidence for ice ages: geological, chemical, and paleontological.

Geological evidence includes marks left on rocks by glaciers, such as scrapes and piles of dirt and rocks called glacial moraines. It also includes long hills shaped by glaciers called drumlins, and special mixtures of rocks and soil called till or tillites.

Chemical evidence looks at changes in the types of atoms, called isotopes, found in fossils and rocks. Scientists study ice cores taken from glaciers to learn about past temperatures.

Paleontological evidence studies how the locations of animal and plant fossils change, showing how species moved to colder or warmer areas during ice ages.

Major ice ages

There have been at least five major ice ages in Earth's history. The first well-known ice age, called the Huronian, happened around 2.4 to 2.1 billion years ago. It was caused by changes in the atmosphere during a big event called the Great Oxygenation Event.

Another major ice age, the Cryogenian, occurred from 720 to 630 million years ago. It may have been so cold that ice covered the whole planet. Later, the Andean-Saharan ice age took place from 460 to 420 million years ago.

The most recent major ice age, the Quaternary Ice Age, began about 2.58 million years ago. During this time, Earth has gone through cycles where ice sheets grew and shrank. We are currently in a warmer period called an interglacial, but large ice sheets still exist in places like Greenland and Antarctica.

Glacials and interglacials

See also: Glacial period and Interglacial

During ice ages, Earth has colder times called glacial periods and warmer times called interglacials. In glacial periods, it gets colder, and big sheets of ice cover land near the poles. Glaciers grow in mountains, and sea levels go down because much water is frozen in ice. These changes happen because of shifts in Earth’s orbit and tilt, known as Milankovitch cycles.

Right now, Earth is in an interglacial period called the Holocene. This has lasted about 11,700 years. Scientists think the next glacial period might not start for at least 50,000 years. This is because human activities, like adding greenhouse gases to the atmosphere, are changing the climate.

Feedback processes

During ice ages, some things can make the cold even colder, while others can help warm things up again. When ice and snow cover large areas, they reflect more of the sun's energy back into space. This makes Earth even colder. This is called a positive feedback.

When large ice sheets cover the land, they can wear away the soil and change the shape of the land. This can make it harder for new ice to form. Also, when it gets very cold, there is less moisture in the air. This can help stop glaciers from growing too much. Scientists have found that human activities, like burning fossil fuels, have added carbon dioxide to the air. This might delay the next ice age that would naturally have started soon.

Causes

The causes of ice ages are complex and not fully understood. Several factors work together to change Earth's climate. Important factors include changes in the amount of carbon dioxide and methane in the atmosphere, shifts in Earth's orbit around the Sun called Milankovitch cycles, the movement of tectonic plates, changes in ocean currents, and changes in the amount of energy from the Sun.

These factors can affect each other. For example, when greenhouse gases like carbon dioxide change, it can alter the climate. The positions of the continents also matter. When continents block warm water from reaching the poles, ice sheets can form. This creates a cycle where cooling leads to more ice, which leads to even more cooling.

Other factors, such as the rise of large mountain ranges like the Himalayas and changes in ocean circulation, also influence Earth's climate. Over long periods, these changes help explain why ice ages begin and end.

Recent glacial and interglacial phases

Main article: Timeline of glaciation

The current time period, called the Quaternary, began about 2.6 million years ago. During this time, Earth has had cold periods called glacials and warm periods called interglacials. Glacials last about 100,000 years, and interglacials last between 10,000 and 15,000 years. The last cold period, known as the Last Glacial Period, ended around 10,000 years ago. Today, Earth is in a warm phase called the Holocene.

In North America, large glaciers covered many areas during the last ice age. These glaciers changed the land, creating lakes such as the Great Lakes and the Finger Lakes. They also changed rivers and formed features like Niagara Falls. Some places, like the Driftless Area in Wisconsin, were not covered by these glaciers.

Effects of glaciation

The last ice age ended over 8,000 years ago, but we can still see its effects today. The moving ice changed the land in places like Canada, Greenland, northern Eurasia, and Antarctica. It left behind interesting shapes such as erratic boulders, till, drumlins, eskers, fjords, kettle lakes, moraines, cirques, and horns.

When the ice melted, the land slowly rose up again. This is still happening very slowly today. The weight of the ice had pushed down on Earth's surface, and now the land is bouncing back. This change also affected sea levels, causing coastlines to shift and creating new land shapes in places like Scandinavia and parts of North America.

Future ice ages

Main article: Next glacial period

Scientists used to think the next cold period might happen in about 11,000 years. But now, because people have added a lot of carbon dioxide to the air, this next cold period might not happen for a very long time.

Some studies from 2015 suggest that if the amount of carbon dioxide in the air stays above a certain level, we might not see another cold period for at least 50,000 years. This is because the extra carbon dioxide helps keep the Earth warmer.