Quaternary glaciation

The Quaternary glaciation, also known as the Pleistocene glaciation, is a long series of times when huge sheets of ice covered much of the Earth. It began about 2.58 million years ago and is still happening today, though we are currently in a warmer time between icy periods, called an interglacial.

During these icy times, massive ice sheets grew and covered large parts of the land, and during warmer times, they melted away.

Only two big ice sheets remain today: one over Antarctica and another over Greenland. Other large ice sheets, like the Laurentide Ice Sheet that once covered much of North America, have completely melted since the last icy time ended.

The Quaternary glaciation changed the Earth in many ways. It scraped and shaped the land, created millions of lakes, changed how rivers flow, and even moved the Earth's crust. These changes affected where plants and animals could live and helped shape the world we see today. The ice sheets also reflected sunlight back into space, which helped keep the Earth cooler during these icy times.

Discovery

Main article: Ice age § Origin of ice age theory

In the 18th and 19th centuries, people began to understand that Earth went through big cold times called ice ages. They looked at places in Europe, North America, and Siberia and saw signs that huge glaciers had covered the land. By studying marks left in the ground like long hills called drumlins, ridges called eskers, and piles of rocks called moraines, scientists learned how the ice moved and melted away. This showed them that the ice moved forward and backward many times over a long time.

Description

Further information: Marine isotope stages

An ice age is when lots of ice covers parts of the land. Today, scientists call the time from about 2.58 million years ago until now an ice age because there has always been a big sheet of ice over Antarctica.

During this long ice age, Earth has had cold times and warm times. In the cold times, huge sheets of ice covered parts of Europe, North America, and Siberia. In the warm times, these ice sheets melted. These warm times are called interglacials.

The pattern of cold and warm times changed over millions of years. In the past, the cycle between cold and warm was about 41,000 years. But about a million years ago, this cycle slowed to about 100,000 years. Today, Earth is in a warm time called the Holocene, which began 11,700 years ago. The big ice sheets from the last cold time are still melting.

Causes

Further information: Ice age

The Earth has had big cold periods called glaciations. These come from changes inside the climate system, like ocean currents and the carbon cycle. They also come from changes outside, like Earth's orbit, volcanic activity, and the Sun's energy.

Astronomical cycles

Main articles: Milankovitch cycles and orbital forcing

See also: 100,000-year problem

Changes in Earth's orbit were studied by James Croll a long time ago. A scientist named Milutin Milanković found these changes could cause climate cycles called Milankovitch cycles. These cycles come from three main changes:

First, Earth's orbit shape changes every 100,000 years. Second, Earth's tilt changes between 22° and 24.5° every 41,000 years. This tilt helps create seasons, and a bigger tilt means bigger differences between summer and winter. Third, Earth's axis wobbles every 26,000 years. These changes make Earth cooler at certain times, about every 40,000 years. The biggest effect is on how different the seasons are, not on how much heat Earth gets. This means less ice melts and glaciers grow.

Atmospheric composition

One idea is that lower levels of a gas called CO₂ started a cooling trend that led to big ice sheets in the Arctic. Evidence shows CO₂ levels dropped a lot since the middle of the Mesozoic Era. Studies of tiny sea creatures show CO₂ levels fell before and during the growth of ice in Antarctica. Lower CO₂ levels later may have helped start big ice sheets in the Northern Hemisphere. CO₂ levels change between warm and cold times, but it may not be the main cause.

Plate tectonics and ocean currents

Further information: Plate tectonics and ocean current

Where continents are placed matters for ice ages. For most of Earth's history, the North Pole was in open water, which kept things warm. But during the Cenozoic Era, big land masses moved. North and South America moved west, creating the Atlantic Ocean and leaving the North Pole in a smaller, almost trapped Arctic Ocean. The opening of the Drake Passage let cold water circle Antarctica, helping it freeze. Changes in ocean currents around 3.65 million years ago cooled the Arctic, helping sea ice form. The Isthmus of Panama formed about 2.6 million years ago, changing ocean currents and helping start ice ages in the Northern Hemisphere.

Collapse of permanent El Niño

A strong El Niño effect existed in the early to middle Pliocene. Warmer water in the eastern Pacific made the planet trap more heat and reflect less sunlight, which kept polar regions warmer and delayed ice ages in the Northern Hemisphere. When cold water appeared in the eastern Pacific about 3 million years ago, it may have helped cool the planet.

Rise of mountains

The formation of mountains is thought to have helped cause the Quaternary glaciation. As Earth's land moved away from the tropics and more mountains formed, there was more land at high altitudes and latitudes, which helps glaciers form. For example, the Greenland ice sheet formed as mountains in Greenland rose. Uplift of the Rocky Mountains and Greenland's west coast may have cooled the climate and increased snowfall. In the Andes, mountains rose high enough about 1 million years ago to allow valley glaciers to form.

Effects

The huge amount of ice during the Quaternary glaciation changed Earth in many ways. It created new landscapes, forming beautiful mountains and wide areas of land in a short time. These changes affected the whole planet, not just where glaciers were.

Glaciers created many lakes by scraping the land and leaving behind dips that filled with water. Very large lakes formed along the edges of glaciers, such as the Baltic Sea and the Great Lakes in North America. Other lakes, known as pluvial lakes, grew in dry regions because of more rain during icy times.

The weight of the ice pushed the land down. After the ice melted, the land rose again in a process called isostatic adjustment. This rising sometimes caused earthquakes, especially in Scandinavia. Today, the land in some places still rises by about 1 centimeter each year.

Glaciers also changed wind patterns, spreading fine sediments called loess. These sediments covered large areas, including parts of the central United States, Europe, and China. Big glaciers influenced ocean currents by blocking sea passages and affecting global heat.

Finally, materials left behind by glaciers, called moraines, sometimes gathered valuable minerals like gold in certain areas, such as southernmost Chile.

Main article: Glacial lake

Main article: Post-glacial rebound

Main article: Pluvial lake

Records of prior glaciation

Main article: Timeline of glaciation

See also: Ice age and paleoclimatology

Glaciers have not always covered Earth, but there are signs of big ice ages long ago. During the late Paleozoic Era (about 300 to 200 million years ago) and the late Precambrian (800 to 600 million years ago), large parts of Earth were covered in ice.

Before the current ice age, which started about 2 to 3 million years ago, Earth’s weather was usually gentle and stayed the same for many years. We know this from old plants and animals that have been preserved, as well as from layers of rock. But there are also signs of older times when glaciers covered many places. One well-known ancient ice age, called the Karoo Ice Age, left traces in rocks in South Africa, India, South America, Antarctica, and Australia. Even older glacial deposits can be found on every continent except South America. These show that Earth went through two other big ice ages during the late Precambrian, possibly turning into a “Snowball Earth” during the Cryogenian period.

Next glacial period

Further information: Milankovitch cycles, Past sea level, Climate change, and Human impact on the environment

After a big cold time ended about 20,000 years ago, the Earth has been getting warmer. This warming made sea levels rise by about 121 meters. Since about 6,000 years ago, sea levels have stayed about the same. This warm, steady time may have helped humans start farming and build early communities.

Scientists look at how Earth's path around the Sun changes. These changes can affect our weather. Some models think Earth might stay warm for another 50,000 years, even without people. But other studies show that extra gases we release might stop glaciers from coming back for much longer — maybe forever. The amount of a gas called CO₂ in the air is very important for these predictions.

Changes in ocean currents, especially a big current in the Atlantic, could also change future weather patterns.