Greenland: Global Implications of Accelerated Melting

Introduction

Greenland, the world's largest island, bears the world's second-largest mass of ice, the Greenland ice cap or sheet. (Antarctica's is about 10 times larger.) In 2006, satellite observations confirmed that Greenland's ice is melting faster than ever before. Scientists disagree about whether the melting will continue to accelerate, but some specialists think further rapid melting is very likely.

Meltwater from ice that sits on land, such as mountain glaciers and ice sheets, raises sea levels. The addition of large amounts of freshwater to the North Atlantic also may slow the oceans' thermohaline (literally, “heat-and-salt”) overturning circulation, which moves water throughout the world's oceans and influences world climate. Melting in Greenland, therefore, has two major effects, rising sea level and slowed thermohaline circulation.

Historical Background and Scientific Foundations

About 80% of Greenland is covered with an ice sheet about 1 mi (1.6 km) deep at its deepest point. This sheet is the accumulation of hundreds of thousands of years of winter snows. Each year's snow has added a thin layer, which packs down to ice and is covered by the next year's snow. The oldest ice in the cap is about 120,000 years old, but the cap has been there for about a million years;

the older, deeper ice has long since flowed away to the sea. If all the ice in the Greenland sheet were to melt, it would raise the oceans by about 21 ft (6.5 m) or possibly more. Although most scientists do not presently think that complete melting of the ice cap is likely in the next century or so, greatly accelerated melting due to global warming has recently been measured by satellites and may force scientists to reconsider the possibility of more extreme melting in Greenland in the next century.

Impacts and Issues

Although it is not easy to predict how much of Greenland's ice will melt and how fast—past forecasts appear to have underestimated both—the consequences for rising sea levels of any given amount of melting are fairly easy to predict: given so many cubic miles of meltwater, the sea must rise by so many inches. Data from the last few years have shown that Greenland's contribution to global sea-level rise has risen from 0.006 in (0.15 mm) per year in the six years leading up to 2003 to 0.02 inches (0.5 mm) per year from 2004–2006 and probably since then as well. Two-thirds of the increased loss of ice mass is due to faster glacier flow, and one-third is due to increased surface melting and runoff.

Greenland's meltwater is also an important part of the global thermohaline circulation that transports water from the south polar regions to the north polar regions and back again, moving heat from the tropics to the polar zones. This circulation warms Europe and has other effects on global climate.

In the North Atlantic Ocean, the thermohaline circulation delivers a large surface current of warm water from the American tropics—the Gulf Stream—across the North Atlantic. This warm water swoops northward along the eastern coast of Europe. Westerly winds deliver heat from it to Europe, moderating its climate. As the water moves toward the North Pole, it cools and therefore becomes denser, until it finally sinks. This denser, colder water forms a returning current that follows the western coast of Greenland southward, eventually tracing the westward edge of North and South America all the way down to the South Atlantic Ocean.

Because freshwater is less dense than salty water, the addition of large amounts of freshwater to the sea from Greenland's melting ice tends to slow the sinking of cool Arctic waters. With slowed sinking, the return current must slow, and so must the northward surface current of warm water. (All the circulations must balance out because the ocean can't build up in one place.) The accelerated melting of Greenland's ice sheet, therefore, tends to slow down the entire Atlantic circulation system.

One possible consequence is a slow or sudden cooling of European climate. Data on prehistoric climate show that air temperatures in the North can drop by as much as 18°F (10°C) in only a few decades, and that such changes are linked to sudden changes in the ocean circulation. Scientists caution that the ocean circulation system is highly nonlinear, meaning that it does not always react in a smooth, steady way. Rather, when a certain (as-yet unknown) threshold is reached, it is possible that the circulation will switch suddenly to a new state that transports little or no heat to the North. Also, even when the flow of freshwater weakens again, as when the Arctic re-cooled or when most of the Greenland ice finished melting, the system might not quickly shift back to its earlier pattern.

In 2005, scientists reported that the conveyor has slowed by about 30% over the past 50 years. However, in 2007 the same group of scientists, after making more complete measurements of ocean flow over the Greenland-Scotland subsea ridge (which the conveyor circulation must cross to get to the Arctic), reported that they had found that the magnitude of the conveyor current fluctuates so much on a seasonal timescale that they cannot be sure that the conveyor has, in fact, slowed since the 1950s. If there has been a long-term slowing trend, recently accelerated Greenland warming cannot have been its cause but may accelerate it. A complete breakdown of the North Atlantic circulation due to human-caused climate change cannot be ruled out by current scientific knowledge, although such a breakdown could probably not occur for at least a century, if ever.

WORDS TO KNOW

GREENLAND-SCOTLAND RIDGE: Underwater ridge connecting Greenland to Scotland in the North Atlantic that separates the Nordic seas, where North Atlantic deep water formation occurs. Below a depth of 2,755 ft (840 m), the ridge forms a continuous barrier between the two basins; in some areas it rises to shallower depths or islands. Formation of the North Atlantic deep water is an essential part of the great conveyor belt or thermohaline circulation of the oceans, a key component of the global climate mechanism.

ICE SHEET: Glacial ice that covers at least 19,500 square mi(50,000 square km) of land and that flows in all directions, covering and obscuring the landscape below it.

THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.

The consequences of further, possibly sudden, slowing of the ocean circulation are hard to predict. The result might or might not be a net cooling of Europe. The tropics would probably warm, since the circulation removing heat to the polar regions would be less effective.

See Also Antarctica: Melting; Arctic Melting: Greenland Ice Cap; Arctic Melting: Polar Ice Cap; Great Conveyor Belt.

BIBLIOGRAPHY

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Larry Gilman