Ice age

An ice age is a time when Earth's temperatures drop, allowing large areas of ice to cover the land. During an ice age, thick ice sheets and glaciers grow, changing the shape of continents and affecting the climate. These periods happen when the planet's surface and atmosphere become much colder than usual.

Earth's climate naturally changes 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 includes both colder times, when ice grew, and warmer times, when ice melted.

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

History of research

See also: History of climate change science

In the 1700s and 1800s, people began to notice strange rocks and marks in the land that could only be explained by glaciers moving long ago. For example, in 1742, an engineer in Geneva visited Chamonix in the Alps and heard locals say glaciers once reached farther than they do today.

Later, scientists like Louis Agassiz and Jean de Charpentier studied these clues and proposed the idea of "ice ages"—times when huge glaciers covered parts of Earth that are now warm. It took many years for other scientists to accept this idea, but by the late 1800s, most agreed that Earth had gone through periods of cold when glaciers grew and covered large areas.

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, 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. For example, 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, possibly ending when volcanoes added gases to the air. 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 goes through colder periods called glacial periods and warmer periods called interglacials. In glacial periods, temperatures drop, and large ice sheets cover much of the land near the poles. Glaciers grow even in mountain areas, and sea levels fall because so much water is locked up 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, which has lasted about 11,700 years. Scientists think the next glacial period might not start for at least 50,000 years, especially because human activities, like adding greenhouse gases to the atmosphere, are changing the climate.

Feedback processes

During ice ages, certain processes 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, which makes Earth even colder. This is called a positive feedback.

On the other hand, 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, which helps to balance out the cold. Also, when it gets very cold, there is less moisture in the air, which can also help stop glaciers from growing too much. Scientists have found that human activities, like burning fossil fuels, have added so much carbon dioxide to the air that they might delay the next ice age that would naturally have started soon.

Causes

The causes of ice ages are complex and not fully understood. We know that several factors work together to influence 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 known as Milankovitch cycles, the movement of tectonic plates, changes in ocean currents, and variations in the amount of energy the Sun gives off.

These factors can affect each other. For example, when greenhouse gases like carbon dioxide change, it can alter the climate, which in turn can change how much carbon dioxide is removed from the atmosphere by natural processes. The positions of the continents also play a role. When continents block warm water from reaching the poles, ice sheets can form, reflecting more sunlight and cooling the planet further. 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 patterns, 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, started about 2.6 million years ago. During this time, Earth has gone through cold periods called glacials and warm periods called interglacials. Glacials last about 100,000 years, while 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, big glaciers covered large areas during the last ice age. These glaciers shaped many landscapes, creating lakes such as the Great Lakes and the Finger Lakes. They also changed rivers and formed features like Niagara Falls. Some areas, like the Driftless Area in Wisconsin, were not covered by these glaciers.

Effects of glaciation

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

When the ice melted, the land slowly rose up again, a process that 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 that the next cold period, or glacial period, might happen very soon—within 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 glacial period for at least 50,000 years. This is because the extra carbon dioxide helps keep the Earth warmer.