Ice Age

An ice age is a time when large ice sheets exist; intervals without such sheets are called interglacial periods. The most recent glacial period occurred between about 120,000 and 11,500 years ago, and since then Earth has been in an interglacial period called the Holocene.

These glacial–interglacial cycles have waxed and waned throughout the Quaternary Period, the past 2.6 million years. Since the middle Quaternary they have had a frequency of about 100,000 years.

The Pleistocene Epoch spans the interval from 2.6 million to 11,700 years ago, a period during which a succession of glacial and interglacial climatic cycles occurred. It was an epoch defined by that alternation of glacial and interglacial phases rather than by a single unbroken cold.

Beginning roughly 2.6 million years ago, global climate entered a cooling phase, and as snow in northern latitudes compacted into ice several kilometers thick, it flowed as glaciers southward across the North American continent.

Pleistocene biotas were extremely close to modern ones, with many genera and even species of Pleistocene conifers, mosses, flowering plants, insects, mollusks, birds and mammals surviving to this day.

The Pleistocene also saw the evolution and expansion of our own species, Homo sapiens, and by the close of the Pleistocene humans had spread through most of the world.

During the Pleistocene, mountain glaciers formed on all continents, while the icecaps of Antarctica and Greenland stood more extensive and thicker than they are today, and vast glaciers spread across northern North America and Eurasia. The glaciers of the Great Ice Age were so extensive that almost a third of the present land surface of the Earth was intermittently covered by ice. The last major ice sheet to spread across the north-central United States reached its maximum extent about 20,000 years ago and lingered in Canada until about 6,000 years ago.

The Last Glacial Maximum occurred about 20,000 years ago, during the last phase of the Pleistocene.

At that time, global sea level stood more than 400 feet lower than it does today. Glaciers covered approximately 8% of Earth's surface and 25% of its land area, and 33% of Alaska.

Ice sheets up to two miles thick covered large parts of North America, Europe and Asia, locking up much of the earth's water.

The glaciers began to retreat about 15,000 years ago. Sea level rose to its current height about 8,000 years ago and has fluctuated since.

Milutin Milankovitch proposed that the collective effects of changes in Earth's position relative to the Sun drive long-term climate and set in motion the beginning and end of glaciation periods. His career in science followed an early period as a construction company's chief engineer, after which he accepted a faculty position in applied mathematics at the University of Belgrade in 1909 and held the post for the rest of his life.

The hypothesis rests on three orbital movements. Eccentricity varies on a cycle of about 100,000 years; obliquity, or axial tilt, varies between 22.1 and 24.5 degrees; and axial precession spans about 25,771.5 years.

The mechanism works through the seasons. As obliquity decreases, milder seasons with warmer winters and cooler summers allow snow and ice at high latitudes to build into large ice sheets, and the increasing ice cover reflects more solar energy, promoting further cooling.

Milankovitch calculated that ice ages occur approximately every 41,000 years, a figure confirmed for the interval between one and three million years ago. About 800,000 years ago the cycle lengthened to 100,000 years, matching Earth's eccentricity cycle.

Fluctuations in sunlight alone are not enough to bring about full-blown ice ages and interglacials; they trigger feedback loops that amplify the original warming or cooling.

Milutin Milankovitch was born in 1879 in the rural village of Dalj, then part of the Austro-Hungarian Empire and today in Croatia, and graduated from the Vienna Institute of Technology in 1904 with a doctorate in technical sciences.

He built a comprehensive mathematical model that calculated latitudinal differences in insolation and the corresponding surface temperature for 600,000 years prior to the year 1800, and he then attempted to correlate those changes with the growth and retreat of ice ages.

For about 50 years Milankovitch's theory was largely ignored, until a 1976 study published in the journal Science examined deep-sea sediment cores and found that the theory did in fact correspond to periods of climate change.

Beringia is the name given to the unglaciated region that extended from the Yukon Territory in Canada west across to eastern Siberia. Because so much of the planet's water was locked up in glaciers, sea level was much lower, and Alaska and Siberia were connected by the Bering Land Bridge.

During the cold, dry glacial periods, the emergence of the land bridge allowed plant and animal ranges to shift and mobile species to migrate between the continents.

As the climate began warming about 18,000 years ago, the Beringia region became more moist and the sea level rose, submerging the land bridge and causing shrub tundra vegetation to expand. The first flooding of the land bridge happened around 10,000 years ago.

At the peak of the last ice age, 20,000 years ago, when most of northern North America was covered by massive glaciers, much of Alaska was ice-free and home to a diverse assortment of large mammals.

The Paleoarctic were the very first humans to set foot in North America, moving from eastern Siberia about 1,000 miles across the Bering Land Bridge during the final millennia of the Earth's last Ice Age.

Most of interior Alaska, south of the Brooks Range and north of the Alaska Range, was a non-glaciated grassland refuge habitat for a number of plant and animal species during the maximum Pleistocene glaciation. This ice-free corridor also provided one route for humans to move into North America.

Distinctive large mammals characterized Pleistocene habitats in North America, Asia, and Europe, among them mammoths and their cousins the mastodons, longhorned bison, saber-toothed cats, and giant ground sloths. Around the end of the Pleistocene, all these creatures went extinct.

The loss was not confined to a handful of species. Hundreds of large mammal species disappeared during the transition from the last glaciation to the present interglacial period, a span running from around 50,000 to 5,000 years ago.

The woolly mammoth illustrates the scale of that history. Woolly mammoths roamed parts of Earth's northern hemisphere for at least half a million years, and were still in their heyday 20,000 years ago. By 4,000 years ago they were gone.

Two great ice masses, the Antarctic and Greenland ice sheets, stand out in the world today and may be similar in many respects to the large Pleistocene ice sheets.

Parts of the Antarctic continent have had continuous glacier cover for perhaps as long as 20 million years, while valley glaciers of the Antarctic Peninsula and glaciers of the Transantarctic Mountains may date from the early Pleistocene.

For Greenland, ice cores and related data suggest that all of southern Greenland and most of northern Greenland were ice-free during the last interglacial period, approximately 125,000 years ago.

Where ice once gathered in mountain ranges, the land still carries its signature. Falls drop at the outlets of cirques, the bowl-shaped depressions in the headwaters of drainage areas formed by the accumulation of ice and its erosive action on the underlying bedrock.

Ice age glaciation shaped mountaintops into amphitheatre-like cirques, arête ridges, and majestic peaks such as the Matterhorn and Grossglockner.

Not all ice in the landscape is ancient. Temperate glaciers in Alaska are generally much younger than the great Pleistocene ice, and many formed as recently as the start of the Little Ice Age, approximately 700 years ago.

The concept of an ice age was enlarged and popularized by the Swiss geologist Louis Agassiz, whose arrival in the United States in 1846 marks the beginning of the study of this period of Earth history in North America.

A chronology developed from European river terraces suggested that the Pleistocene consisted of four or five major glacial stages separated by interglacial stages with climates generally similar to those of today.

The World Data Service for Paleoclimatology maintains archives of ice core data from polar and low-latitude mountain glaciers and ice caps throughout the world.