Quaternary glaciation

The Quaternary glaciation is a massive, ongoing period of Earth's history. It began approximately 2.58 million years ago (Ma). Geologists define an ice age by the presence of large land-based ice. Because the Antarctic ice sheet has existed continuously, the Quaternary is considered an ice age. We are currently living in an interglacial period called the Holocene. This is a warmer interval between the colder glacial periods.

This period works through a cycle of expanding and contracting ice. During glacial periods, massive ice sheets cover huge areas. These sheets once reached across much of Europe, North America, and Siberia. When the climate warms, we enter an interglacial period. During these times, the continental glaciers retreat and melt. Today, glaciers occupy only about 10% of Earth's land surface. They are mostly found in Greenland, Antarctica, and some mountain regions.

These cycles change the physical world in profound ways. As ice sheets move, they cause continental erosion of the land. They also deposit vast amounts of material in new places. The movement of ice can modify entire river systems. It also creates millions of lakes, including pluvial lakes far from the ice. The heavy weight of the ice can even cause isostatic adjustment. This is a process where the Earth's crust sinks under the weight.

Scientists have spent centuries uncovering the history of these cycles. During the 18th and 19th centuries, researchers first began to understand this process. Later, hundreds of geologists performed extensive fieldwork. They mapped specific glacial features to see where ice once flowed. They looked for drumlins, eskers, and moraines. They also studied striations, which are scratches in the rock. These features helped them trace the direction of ancient ice flow.

One major cause of these cycles involves Earth's movement in space. These are known as Milankovitch cycles. They are caused by changes in Earth's orbital properties. First, orbital eccentricity changes the shape of Earth's orbit every 100,000 years. Second, the axial tilt varies between 22° and 24.5° every 41,000 years. This tilt changes the contrast between summer and winter temperatures. Third, precession is a wobble in Earth's rotation that lasts 26,000 years. These cycles change how much seasonal melting occurs.

Atmospheric composition also drives these temperature changes. Carbon dioxide is a greenhouse gas that helps trap heat. Geological evidence shows that carbon dioxide levels have dropped by over 90% since the Mesozoic Era. Lower levels of carbon dioxide can lead to long-term cooling. This cooling helps continental ice sheets form. During the Quaternary, high carbon dioxide levels correspond to warm interglacials. Low levels correspond to cold glacials. Some scientists believe carbon dioxide acts as a feedback to these changes.

Finally, the movement of continents and ocean currents plays a vital role. As tectonic plates move, they change how heat travels through the oceans. For example, the Drake Passage opened 33.9 million years ago. This separated Antarctica from South America. It allowed the Antarctic Circumpolar Current to flow freely. This current isolated Antarctica from warm water and helped ice sheets grow. Later, the Isthmus of Panama formed about 2.6 million years ago. This change in land also altered how the oceans circulated heat.