Ice, Sea-level, and Global Climate
Ice, Sea-level, and Global Climate
The amount of water frozen in Earth's ice has changed throughout the planet's history. Earth's ice budget (total ice volume) grows when the planet's average temperature falls and shrinks when it rises. During colder periods called ice ages, ice caps (large dome-shaped glaciers) extend far from the North and South Poles, and mountain glaciers (large masses of moving ice) advance into lowlands. During warmer periods, ice retreats back toward the poles and up mountain valleys. Earth has even been completely ice-free several times during its long history.
Earth's water budget, the total amount of water on the planet, does not change over time. When more water freezes into ice on land, there is less water in the oceans, so global sea level falls during ice ages. Shorelines move seaward, and the edges of the continents are exposed above sea level. When polar ice caps and mountain glaciers melt, more water is stored in the oceans, and global sea level rises. Shorelines move landward and coastal regions are submerged below sea level. (Ice floating in the ocean does not affect global sea level when it melts or forms because no water is added or removed.)
Global warming and cooling
Energy from the Sun ultimately determines Earth's average temperature and thus, its ice budget. The planet warms and cools as its position changes relative to the Sun and it receives more and less sunlight over time. The shape of Earth's orbit (the path it revolves around other objects), the tilt of the planet as it spins in its orbits, and the intensity of its seasons all change in repeating patterns over thousands of years. Glacial advances and retreats in the past have matched the timing of these astronomical cycles with global warming and cooling. (They are called Milankovitch cycles after the Serbian mathematician who developed the theory relating Earth motions and long-term climate change.)
Global warming and cooling also occur in response to changes on Earth's surface and in its atmosphere (mass of air surrounding Earth). Gases like carbon dioxide and water vapor keep Earth warm by allowing incoming sunlight to pass and then trapping escaping heat. The layer of insulating gases in the atmosphere is called the greenhouse layer because it works like glass in a garden greenhouse. Without it, Earth would be a frigid, icy planet that could not support biological life. Processes like forest growth lead to green plants that take in the greenhouse gas carbon dioxide and release oxygen. The removal of greenhouse gases from the atmosphere leads to global cooling, advance of glaciers, and sea level fall. When more greenhouse gases escape into the atmosphere, Earth warms, ice melts, and sea level rises.
Ice ages and sea level lows
Massive ice sheets (flat layers of fresh water ice covering extensive regions of the world advanced and retreated across northern Europe, Asia, and North America many times during the ice ages of the Pleistocene Epoch (a division of geologic time that lasted from 2 million to 10,000 years ago). Canada and the northern portion of the United States are strewn with evidence of the last Pleistocene ice sheet in North America. The advancing ice polished, etched, and carved the solid rock layers of northern Canada. It tore rock from the ground and carried it south. When the ice melted, it stranded boulders and dropped piles of Canadian sediment (sand, grain, or silt) across the American Midwest and New England.
The last North American ice sheet was miles (kilometers) thick and reached as far south as Long Island, New York. Like an overloaded ship, the continent sank under the weight of the ice. The Great Lakes are meltwater-filled depressions created by the ice sheet, and the coastlines of New England and eastern Canada have been rising out of the sea since the ice melted away.
Collapse of the Larsen B Ice Shelf
A Rhode Island–sized piece of one of Antarctica's floating ice shelves broke into a fleet of thousands of icebergs over a few weeks in early 2002. Ice shelves are the floating edges of continental glaciers that form where a glacier flows out over the sea. The shelves that cover most of Antarctica's coastal inlets (narrow strip of water running into the land or between islands) and bays are the outlets of faster movements of ice called ice streams that drain ice from the interior of the ice sheets.
The breakup of the relatively small ice shelf, called Larsen B, in a bay along the eastern edge of the Antarctic Peninsula did not, in itself, cause global sea level rise or signal an impending crisis. Ice shelves are already floating in the water, so sea level does not rise when they break up. Ice melts and breaks off into the sea at the front of all glaciers, even advancing ones. Furthermore, a limited local climate change caused the demise of the Larsen B shelf, and there is no evidence that the event had anything to do with human-caused global warming.
The breakup of the Larsen B shelf did, however, cause concern among scientists for a number of reasons. First, the shelf has been losing ice more rapidly than new ice has been accumulating on the glaciers that feed it. This is true of many of Antarctica's ice shelves, including the very large ones that float in the Weddell and Ross Seas at the outlets of ice streams in the West Antarctic ice sheet. It indicates that the Antarctic ice sheets are indeed melting into the ocean. Second, the ice streams that feed the Larsen ice shelf all sped up immediately after the breakup. Apparently, the mass of floating ice acts as a buttress that slows the ice streams. Once a shelf collapses, ice flows rapidly into the ocean. If a very large shelf like the Ross Ice Shelf in west Antarctica were to collapse, it could potentially trigger a rapid rise in global sea level. Third, the Larsen B's breakup and dramatic increase in rate of ice loss followed a very minor local temperature increase (about 4.5°F (2.5°C) in its last 65 years). So, small climate changes can have large consequences.
When the last Pleistocene glaciers reached their maximum extent about 20,000 years ago, ice covered about 30% of the Earth surface. Global sea level was about 350 feet (107 meters) lower than its present-day level. Rivers flowed across exposed continental shelves (gently sloping shallow seabed extending into the ocean from a continent) to shorelines far seaward of their present locations. Land animals lived on regions that are submerged below sea level today, and migrated between islands and continents on exposed land bridges. Land bridges are strips of dry land that connects islands or continents. Some of North America's first human inhabitants arrived via a land bridge between Siberia and Alaska.
Global warming and sea level highs
Earth was almost (if not completely) ice-free about 100 million years ago during the Cretaceous period of geologic time (this period lasted from 144 to 65 million years ago). High mountains and continental areas that were over the poles bear no trace of deposits from glaciers. Geologists (Earth scientists) have even found fossils of tropical plants that resemble palm trees in rocks that formed near the poles. Sea level was much higher in the Cretaceous period than it is today. Only central highlands of the continents remained exposed. A shallow seaway covered the interior of North America.
Humans and global climate change
Today, Earth is in a period that is moving away from the "ice-house" conditions of the Pleistocene. Global temperature and sea level have been rising and ice has been melting since the last glacial maximum. Ice now covers only about 10% of Earth's surface. Most continental ice is bound up in the massive ice sheets covering the continent of Antarctica at the South Pole. Small mountain glaciers and the ice sheet covering Greenland are all that remain of the massive Pleistocene ice sheets of the northern hemisphere. Much of Earth's continental ice cover is presently retreating, including the Greenland ice sheet, the West Antarctic ice sheet, and many mountain glaciers. The melting is probably adding to a rise in sea level of a few inches per century.
Scientists and environmentalists are concerned that humans are contributing to naturally-occurring global warming by releasing greenhouse gases into the atmosphere. Burning fuels like wood, coal, oil, and natural gas releases carbon dioxide, and scientists agree that booming human population growth and industrialization over the last century have significantly raised the level of carbon dioxide in the atmosphere. Most researchers also accept the conclusion that the additional carbon dioxide will have some effect on Earth's climate.
Scientists do not, however, agree on what the effects of adding carbon dioxide and other gases to the atmosphere will be in the future. Some argue that enhanced greenhouse warming could lead to rapid sea level rise and dramatic global climate change that would threaten human property, food supplies, and even lives. Others predict that the additional greenhouse gases may have little effect, and that slow, steady warming will continue at a rate that allows humans to gradually adjust. There are also climate scientists who theorize that upsetting the balance of greenhouse gases, ice cover, and sea levels could trigger the start of a new ice age.
The question of human-caused greenhouse warming is one of the most pressing and controversial environmental issues of our time. Scientists are working to understand the complex nature of Earth's past and present climate in hopes of predicting changes in the future. Policy makers, environmentalists, and business leaders hope to use scientists' predictions and recommendations to design workable solutions.
Laurie Duncan, Ph.D.
For More Information
Alley, Richard. The Two-Mile Time Machine. Princeton, NJ: Princeton University Press, 2002.
"Antarctic Ice Shelves and Icebergs in the News." National Snow and Ice Data Center.http://nsidc.org/iceshelves/ (accessed on August 17, 2004).
Rott, Helmut, and U. Innsbruk. "Antarctic Ice Shelf Vista: Astronomy Picture of the Day." National Aeronautical and Space Administration.http://antwrp.gsfc.nasa.gov/apod/ap020527.html (accessed on August 17, 2004).