Glaciation

Introduction

Glaciation is the ice and snow cover that results when more snow falls in winter than melts in summer. When the snow accumulates over the years, the weight of it turns the mass into ice that flows outward and downward to cover an ever-larger land mass. If the advancing glaciation covers more than 12 million acres (50,000 sq km), it is called an ice sheet. Antarctica and Greenland are each largely covered by an ice sheet.

When the climate changes and less snow falls in winter than melts in summer, glaciation retreats. Left behind from glaciation are tell-tale signs that paleoclimatologists use to trace the history of the fairly regular advances and retreats of glaciation over very large portions of Earth's land mass, especially over a period of 1.8 million years ago to 10,000 years ago called the Pleistocene, an epoch in geologic history that is often referred to as the Ice Ages.

Historical Background and Scientific Foundations

The history of repeating global glaciation is determined from deposits left behind on land, sea floor sediments, and actual ice core samples that are collected by drilling deep into an ice sheet. In 2003, a team of European scientists

completed drilling of an ice core 9,800 ft (3,000 m) into the North Greenland ice sheet. The effort took seven years. When the core is completely analyzed it will trace the history of the ice sheet back about 120,000 years.

Swiss born geologist Louis Agassiz (1807–1873) first suggested the concept of glaciation over large areas of land when he came to North America and saw evidence of glacial changes in the landscapes of vast areas. He saw boulders of rock types from distant locations that were scarred from being pushed by advancing glacial ice that reminded him of the glacier scoured rocks in valleys in Switzerland but on a much larger scale.

Repeated advances and retreats of ice are also visible in channels carved in the landscape that are now lakes and in glacial deposits of collected sand and rock that form long regular hills called eskers. In some elevated locations, ancient fossil shorelines are evidence of periods when glaciation had retreated and sea levels were much higher than they are now.

Serbian engineer Milutin Milankovitch (1879–1958) developed a mathematical explanation for the evidence of past periods of glaciation based on cyclical variations of Earth's orbit that would change the amount of sunlight reaching Earth and therefore change the climate. At least for the Pleistocene period Milankovitch's theory is in close agreement with glaciation cycles of 20,000, 40,000, and 100,000 years. The variations Milankovitch considered are variations in the shape of Earth's orbit around the sun; changes in the tilt of Earth's axis to the orbit; and the precession effect that changes the direction of Earth's axis of rotation much as a spinning top wobbles.

Periods of glaciation that are more ancient than the Pleistocene Epoch are attributed by scientists to shifting land masses that were associated with Earth's plate movements. The plate movements created polar wandering, mountain ranges where plates converged, and opened and closed ocean basins. Pre-Pleistocene plate movements resulted in changes in the atmosphere and ocean circulation, driving changes in climate.

Impacts and Issues

The most recent period of widespread global glaciation began about 70,000 years ago. It reached its peak about 20,000 years ago and ended about 10,000 years ago. During that period about 32% of the total land area was covered in ice. Presently about 10% is covered. The glaciation that remains stores about 70% of Earth's freshwater.

Historically the warm periods have lasted about 10,000 years. Climate is affected by small changes. Some scientists believe in a Chaos Theory that refers to small indeterminate events as the trigger of severe climate changes. There is speculation that adding pollution to the atmosphere is one of those events. The rise of two key greenhouse gases, carbon dioxide and methane, is changing the climate. The combination of adding atmospheric changes to changing oceanic dynamics as glacier ice melts and adds freshwater to dilute salty ocean water at a record pace, particularly around Greenland, could trigger severe climate changes.

WORDS TO KNOW

ICE AGE: Period of glacial advance.

ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

MILANKOVITCH CYCLES: Regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun. Named after Serbian scientist Milutin Milankovitch (1879–1958), though he was not the first to propose such cycles.

PALEOCLIMATOLOGY: The study of past climates, throughout geological history, and the causes of the variations among.

PLEISTOCENE EPOCH: Geologic period characterized by ice ages in the Northern Hemisphere, from 1.8 million to 10,000 years ago.

PRECESSION: The comparatively slow torquing of the orbital planes of all satellites with respect to Earth's axis, due to the bulge of Earth at the equator, which distorts Earth's gravitational field. Precession is manifest by the slow rotation of the line of nodes of the orbit (westward for inclinations less than 90 degrees and eastward for inclinations greater than 90 degrees).

As ice ages in the past have ended, the rising temperatures stimulated changes that increased carbon dioxide and methane levels in the atmosphere. These changes sent temperatures even higher. Current levels of carbon dioxide and methane in the atmosphere are higher than the highest levels ever detected through studies of ice cores from Antarctica. Present CO₂ and methane concentrations are higher than any time in at least the last 650,000 years, and growing. Questions remain among climatologists on whether Earth is approaching another ice age as past cycles suggest or whether the human factor will change the cycles of glaciation.

See Also Glacier; Glacier Retreat; Ice Ages.

BIBLIOGRAPHY

Books

Lurie, Edward. Louis Agassiz:A Life in Science. John Hopkins University Press, Baltimore, MD: 1996.

Weart, Spencer R. The Discovery of Global Warming. Harvard University Press, Cambridge, MA: 2004.

Web Sites

“All About Glaciers.” National Snow and Ice Data Center, 2007. < http://www.nsidc.org/glaciers> (accessed August 16, 2007).

“Atmospheric Changes.” U.S. Environmental Protection Agency, 2007. < http://www.epa.gov/climatechange/science/recentac.html> (accessed August 16, 2007).

“Past Climate Cycles: Ice Age Speculations.” American Institute of Physics, 2007. < http://www.aip.org/climate/cycles.htm> (accessed August 16, 2007).