Ice age

This article is about glacial periods in general. For specific recent glacial periods often called the "Ice Age", see Last Glacial Period, Pleistocene, and Quaternary glaciation. For the current ice age, see Late Cenozoic Ice Age. For other uses, see Ice age (disambiguation).

An ice age is a long period when Earth's surface and atmosphere become much colder. During these times, big sheets of ice spread across the land. Ice grows on high mountains as alpine glaciers, and it covers the far north and south as polar ice caps. Ice ages can be very long or shorter. An ice age can also have many smaller periods, when the world gets colder or warmer. The colder times are called glacials. The warmer times are called interglacials.

Earth's climate changes between icehouse and greenhouse periods. It depends on whether there is ice on the planet. For most of Earth's history, it has been a greenhouse period, with little or no permanent ice. Earth is now in an icehouse period called the Late Cenozoic Ice Age, which started 34 million years ago. Within this ice age, there have been colder and warmer times. The name is also used for the Quaternary glaciation, which started 2.58 million years ago.

During this period, the Last Interglacial ended 115,000 years ago. After it came the Last Glacial Period (LGP). Then the warm Holocene began 11,700 years ago, and we are still in it today. The coldest part of the LGP was the Last Glacial Maximum. It was at its peak between 26,000 and 20,000 years ago. The most recent glaciation was the Younger Dryas, which happened between 12,800 and 11,700 years ago.

History of research

See also: History of climate change science

Long ago, people in the Alps saw large rocks, called erratic boulders, far from where they first formed. They said glaciers had once reached much farther. In 1742, Pierre Martel, an engineer and geographer who lived in Geneva, visited the valley of Chamonix and wrote down this local explanation. Other people, including the chamois hunter Jean-Pierre Perraudin, also believed glaciers had carried the boulders. Later, scientists such as James Hutton, Göran Wahlenberg, and Jens Esmark suggested that ice was the cause. Esmark argued that there had been a series of ice ages across the world. Other scientists talked about his ideas, and Jean de Charpentier became convinced that the Alps had once been covered in ice.

In 1837, the German botanist Karl Friedrich Schimper created the term "ice age" ("Eiszeit"). Schimper, Charpentier, and others convinced Louis Agassiz that a time of glaciers had happened. Agassiz shared their ideas in 1837, but many scientists doubted them. At that time, most scientists believed that Earth had been slowly cooling since it was born, and the ice age idea did not fit with that view. Agassiz published Study on Glaciers in 1840. The theory was not fully accepted until the second half of the 1870s. By then, the work of James Croll had helped. His book Climate and Time, in Their Geological Relations (1875) gave a convincing explanation for what causes ice ages.

Evidence

Scientists use three main kinds of evidence to study ice ages: geological, chemical, and paleontological. Geological evidence includes rocks that have been scraped and scratched, glacial moraines, drumlins, valleys cut by ice, and layers of till or tillites and glacial erratics. Later ice ages can damage or erase the signs of earlier ones, which makes them hard to read. Early ideas assumed the cold periods were short. But sediment and ice cores showed that cold periods are long and warm periods are short. Chemical evidence mainly comes from changes in the ratios of isotopes in fossils found in sedimentary rocks and ocean sediment cores. For the most recent cold periods, ice cores give clues about climate, both from the ice and from air bubbles trapped inside. Water with lighter isotopes has a lower heat of evaporation, so its share goes down in warmer conditions. This allows scientists to build a record of temperature. Other factors can also affect these ratios, which can make the evidence confusing.

Paleontological evidence looks at where fossils are found. During a cold period, cold-adapted living things spread into lower latitudes. Living things that prefer warmer conditions either die out or move toward lower latitudes. This evidence is hard to read. It needs long sets of sediments across many latitudes, ancient living things that stayed unchanged for millions of years, and the right fossils being found. Even so, ice cores and ocean sediment cores have provided a believable record of cold and warm periods over the past few million years. They also support the link between ice ages and land features such as glacial moraines, drumlins, and glacial erratics. Because of this, these features are accepted as good evidence of earlier ice ages when they are found in much older layers.

For a chronological guide, see Timeline of glaciation.

Earth has had at least five major ice ages. They are the Huronian, Cryogenian, Andean-Saharan, late Paleozoic, and the latest Quaternary Ice Age. Between these ice ages, Earth was once thought to be ice-free, even in the far north and south. These warm times are called greenhouse periods. Other studies disagree. They find signs of some ice at high latitudes even during these warm times.

The earliest well-known ice age, the Huronian, happened about 2.4 to 2.1 billion years ago, in the early Proterozoic Eon. Its rocks are found in the Huronian Supergroup, north of Lake Huron, from near Sault Ste. Marie to Sudbury. They include dropstones, varves, and outwash. Similar rocks are found near Marquette, Michigan. The Huronian ice age was caused when atmospheric methane, a greenhouse gas, was removed during the Great Oxygenation Event.

The next well-known ice age was probably the most severe of the last billion years. It happened from 720 to 630 million years ago, during the Cryogenian period. It may have created a Snowball Earth, where ice sheets reached the equator. It may have ended when greenhouse gases such as CO₂, released by volcanoes, built up. Ice on land and sea would slow silicate weathering and photosynthesis, which are two major ways CO₂ is removed. Some scientists have suggested that its end helped lead to the Ediacaran and Cambrian explosion. This idea is new and still debated.

The Andean-Saharan ice age happened from 460 to 420 million years ago, during the Late Ordovician and the Silurian period.

Land plants first spread during the Devonian period. This raised oxygen levels and lowered CO₂ levels, which led to the late Paleozoic icehouse. It was once called the Karoo glaciation, after the glacial deposits in the Karoo region of South Africa. Large polar ice caps existed from 360 to 260 million years ago in South Africa, during the Carboniferous and early Permian periods. Similar deposits are found in Argentina, also in the center of the ancient supercontinent Gondwanaland.

The Mesozoic Era was warm overall. Newer studies suggest brief ice ages happened in both hemispheres during the Early Cretaceous, in the Valanginian, Hauterivian, and Aptian stages. Ice-rafted dropstones suggest ice sheets may have reached as far south as the Iberian Peninsula in the Northern Hemisphere. Ice later disappeared for the rest of the period. Possible ice reports from the Turonian are disputed. Ice sheets briefly returned to Antarctica near the end of the Maastrichtian, just before the Cretaceous-Paleogene extinction event.

The Quaternary Glaciation / Quaternary Ice Age began about 2.58 million years ago, at the start of the Quaternary Period, when ice sheets began spreading in the Northern Hemisphere. Since then, ice sheets have advanced and retreated in cycles of 40,000 and 100,000 years. The cold times are called glacial periods, and the warmer times are called interglacial periods. Earth is now in an interglacial. The last glacial period ended about 11,700 years ago. Today, the main ice left from the continental ice sheets is the Greenland and Antarctic ice sheets, along with smaller glaciers such as those on Baffin Island. The start of the Quaternary is based on when the Arctic ice cap formed. The Antarctic ice sheet began earlier, about 34 million years ago, in the mid-Cenozoic (Eocene-Oligocene Boundary). The term Late Cenozoic Ice Age includes this earlier phase.

Ice ages can also be divided by place and time. For example, Riss (180,000–130,000 years bp) and Würm (70,000–10,000 years bp) refer to ice in the Alpine region. The largest extent of the ice did not last the whole time. Each ice age tends to wipe out most of the evidence of earlier ice sheets, except in places the later sheet did not fully cover.

Glacials and interglacials

See also: Glacial period and Interglacial

Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the northern hemisphere

Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the southern hemisphere

Even within the current ice age, the weather has changed. Colder times are called glacial periods. Warmer times are called interglacials, such as the Eemian Stage. Earlier ice ages seem to have had similar cycles of cold and warm. The changes in the late Paleozoic ice house are likely part of how cyclothems formed.

During glacial periods, most of Earth is colder and drier. Large areas of ice spread out from the poles. Mountain glaciers reach lower down because the snow line is lower. Sea levels drop because a lot of water is frozen in the ice caps. Changes in orbital forcing of the climate, caused by Milankovitch cycles, match these cold and warm periods. Milankovitch cycles are regular changes in Earth's orbit and the tilt of its axis. Earth is now in an interglacial called the Holocene. It has lasted about 11,700 years. Greenhouse gases added by people may affect the climate more than these orbital changes over hundreds of thousands of years. greenhouse gases

Feedback processes

Each ice age has positive feedback that makes it stronger, and negative feedback that softens the overall cooling. During the Quaternary ice ages, Earth's positive feedback and negative feedback worked together. Ice sheets and dust reflected a lot of sunlight, and the air held little carbon dioxide (CO₂). Both helped make the glacial climates cold. Albedo is the amount of the sun's energy that is reflected instead of absorbed. Albedo is high where there is ice and snow, and it is low in forests. As the air gets colder, ice and snow spread and forests shrink. This continues until a negative feedback balances the system.

Ice sheets wear down the land under them. This lowers the land above sea level and limits where ice can form. Rising sea levels also lower albedo, which slows the cooling. Dry conditions during the coldest parts of a glacial period are another negative feedback, because there is less snow to keep glaciers growing. A study in Nature Geoscience suggests that human release of carbon dioxide (CO₂) will delay the next glacial period.

See also: Abrupt climate change and Atlantic meridional overturning circulation

Scientists do not yet fully understand what causes ice ages or the shorter cold and warm periods within them. Most agree that several factors matter. These include the amounts of carbon dioxide and methane in the air. They also include slow changes in Earth's path around the Sun, called Milankovitch cycles. The slow movement of tectonic plates is another factor, because it changes where land and ocean are and affects ocean currents. Changes in solar output, large meteorites, and supervolcanoes can also play a part. These factors can affect each other. For example, greenhouse gases can change the climate, and a changing climate can change the gases in the air. One way this happens is by changing how fast weathering removes CO₂. Some researchers, including Maureen Raymo and William Ruddiman, think the Colorado Plateaus and the Tibetan Plateau may have removed enough CO₂ from the air to be a major cause of the long Cenozoic Cooling trend.

Greenhouse gas levels seem to fall at the start of ice ages and rise as the ice sheets melt back. It is hard to know which one causes the other. The Snowball Earth idea says that the severe freezing of the late Proterozoic ended when CO₂ levels rose, mainly from volcanoes, and warmed the planet. The position of the continents also matters. When land blocks warm water from reaching the poles, ice sheets can form. Ice reflects more sunlight, which cools the air and lets the ice grow even more. Changes in ocean currents, the rising Himalayas, and small shifts in Earth's orbit have also been linked to ice ages. During the present ice age, the timing of cold and warm periods matches the Milankovitch cycles closely. Scientists still debate why some cycles are stronger than others.

Recent glacial and interglacial phases

For a chronological guide, see Timeline of glaciation.

The current geological period, the Quaternary, began about 2.6 million years ago and continues today. It has had warm and cold times. The cold times, called glacials (Quaternary ice age), last about 100,000 years. The warm times, called interglacials, last 10,000 to 15,000 years. The last cold time, the Last Glacial Period, ended about 10,000 years ago. Earth is now in a warm time of the Quaternary, called the Holocene.

See also: Glacial history of Minnesota

The main cold stages of the current ice age in North America are the Illinoian, Eemian, and Wisconsin glaciation. Scientists no longer use the Nebraskan, Afton, Kansan, and Yarmouthian stages to divide this ice age. In the 1980s, these stages were combined into the Pre-Illinoian.

During the most recent glaciation in North America, ice sheets reached about the 45th parallel north. These sheets were 3 to 4 kilometres (1.9 to 2.5 mi) thick. This happened in the later part of the Last Glacial Maximum, between 26,000 and 13,300 years ago.

The Wisconsin glaciation changed the North American landscape in many ways. The Great Lakes and the Finger Lakes were carved by ice that deepened old valleys. Most lakes in Minnesota and Wisconsin were dug out by glaciers and later filled with melted glacial water. The old Teays River system was greatly changed and mostly reshaped into the Ohio River system. Other rivers were blocked and moved to new paths. One example is Niagara Falls, which formed a dramatic waterfall and gorge when the water reached a limestone cliff. Another waterfall once formed at the present Clark Reservation State Park near Syracuse, New York, but it is now dry.

The area from Long Island to Nantucket, Massachusetts was formed from glacial till. Many lakes on the Canadian Shield in northern Canada were also formed by the ice. As the ice melted and the rock dust dried, winds carried the dust far away, forming thick layers of loess in the Missouri Valley. Post-glacial rebound still continues to reshape the Great Lakes and other areas that were once under the weight of the ice sheets. The Driftless Area, a part of western and southwestern Wisconsin along with parts of nearby Minnesota, Iowa, and Illinois, was not covered by glaciers.

Effects of glaciation

See also: Glacial landform

The last glacial period ended more than 8,000 years ago, but its effects can still be seen today. Moving ice carved the landscape in Canadian Arctic Archipelago, Greenland, northern Eurasia and Antarctica. Glaciers left behind features such as erratic boulders, till, drumlins, eskers, fjords, kettle lakes, moraines, cirques and horns. The great weight of the ice pressed down on Earth's crust and mantle. After the ice melted, the land rebounded, slowly rising back up. This happens slowly because of the high viscosity of Earth's mantle. Today, the land near the center of the rebound area rises about 1 cm per year.

As ice formed far to the north and south, water was taken from the oceans, and global sea level dropped by about 110 meters. When the ice melted during deglaciation, the water returned to the oceans and sea level rose. This could suddenly change coastlines, break ice dams so that lakes became salination lakes, and create new ice dams that formed large areas of freshwater. It could even cause temporary reglaciation. Researchers have suggested this kind of changing pattern may explain the Scandinavian region, the Baltic region and much of central North America at the end of the last glacial maximum. The moved ice-water also changed the gravitational field and the moment of inertia of Earth, which affected its angular velocity, axis and wobble. The weight of the ice caused the lithosphere to flex and created stress inside Earth. While glaciers were present, they held back movement along faults below. During deglaciation, the faults slipped more quickly and triggered earthquakes. These earthquakes may have helped cause ice calving and the Heinrich events. More intraplate earthquakes may explain how ice sheets collapsed so quickly. In Europe, glacial erosion and isostatic sinking created the Baltic Sea. This area had been land drained by the Eridanos River before the Ice Age.

Future ice ages

Main article: Next glacial period

Past estimates suggested that warm periods last about 10,000 years. Because of this, some scientists were worried in the 1970s that the next ice age would come soon. Today, people think human activity may be making an already unusually long warm period even longer. Ice ages go through cycles of about 100,000 years, but human carbon dioxide emissions may stop the next one from happening. Stephen Barker of Cardiff University says that without human effects, the next ice age would "occur within the next 11,000 years, and it would end in 66,000 years' time."

A 2015 report by the Past Global Changes Project used computer simulations. It found that a new ice age is unlikely in about the next 50,000 years. This is true "if either atmospheric CO₂ concentration remains above 300 ppm or cumulative carbon emissions exceed 1000 Pg C" (that is, 1,000 gigatonnes of carbon). Because humans keep adding carbon to the air, the report says an ice age is very unlikely to start in the next 50,000 years. (A precessional cycle lasts around 21,000 years. This is the time it takes for the perihelion to move all the way around the tropical year.)