Source: Copyright 2004, USA Today
Date: November 21, 2004
Byline:  Jack Williams, USATODAY.com

Q: How much would sea level rise if the ice at both poles melted?

A science camp on the Beardmore Glacier, Antarctica. Miles-deep ice covers most of Antarctica.
By Kristan Hutchison, National Science Foundation

A: Scientists at the U.S. Geological Survey estimate that If all of the ice sitting on land in Greenland and Antarctica melted it would cause global sea levels to rise by about 215 feet, or about 65 meters.

Fortunately, even the most dire scientific, global warming scenarios do not have all of the ice melting, especially in Antarctica.

To understand what could happen as the polar regions warm up, you need to have a clear picture of how glaciers, ice sheets, and sea ice work.

A glacier, or a large ice sheet, forms when more snow falls than melts each year. As the snow falls, it compresses the snow under it, turning it into dense "glacier ice." For more on how a glacier or an ice sheet grows, see my answer to a question about the age of Greenland's ice.

As a glacier, or ice sheet builds up, the weight pushes the ice outward. A glacier in a valley will spread down the valley. An ice sheet will spread out in all directions.

The edge of a glacier can be on the water, or on land. If its on the water, pieces break off as icebergs. On land, the edges melt and the water flows to the sea in streams and rivers.

The answer to the question: Is polar ice melting? is: It's been melting since the height of the last ice age.

The real question is: Is the ice melting faster than falling snow is adding new ice?

This is certainly the case with many glaciers around the world, mostly in regions outside the Arctic and Antarctica. It could be true of Greenland, but no one expects all of Greenland's ice to melt in this century. If all of Greenland's ice melted, it would cause sea levels to rise by about 21 feet, or 6.5 meters.

Melting of Antarctic ice would supply the rest of the 215 feet of sea level rise from melting polar ice.

Ice definitions

Glacier: A mass of natural ice on land that forms over many years when more snow falls than melts each year. The ice in the glacier moves.

Iceberg: Ice floating in the water that broke off from a glacier, ice cap, ice sheet or ice shelf. Icebergs are made of fresh water.

Ice cap: A dome-shaped glacier spreading out in all directions, usually covering less than 19,300 square miles.

Ice sheet: A mass of glacial ice that covers more than 19,300 square miles of land.

Ice shelf: An extension of an ice sheet or ice cap formed when the ice pushes out over water to float. Pieces break off as icebergs.

Sea ice: Ice floating on an ocean that formed when sea water froze, unlike an iceberg. As sea ice freezes, salt is forced out, making the ice less salty than the water.

Source: National Snow and Ice Data Center, Boulder, Colo.


The good news is that this seems highly unlikely, certainly for next few thousand years. In its 2001 report on climate change, the Intergovernmental Panel on Climate Change said that melting all of Antarctica's ice would require global temperatures to be about 36 degrees F (20 C) higher than now.

This is more than three times the greatest warming seen as possible this century and is "a situation that has not occurred for at least 15 million years and which is far more than predicted by any scenario of climate change currently under consideration," the report says.(Related: The 2001 IPCC report)

Warming of the Arctic has been very much in the news recently with publication of an international report on how the climate there is changing. (Related: Report warns of rapid Arctic warming)

The report says quite a bit about Arctic ice melting. The big concern is about Arctic sea ice, not land-based ice like the Greenland Ice Sheet, which would increase sea levels if it melted.

Since sea ice is already floating on the ocean, it does not raise sea levels when it melts, but this does not mean we shouldn't worry about it.

For one thing, less sea ice means that the ocean absorbs sunlight that the ice would have reflected away. This warms the water. Also, as the report points out, less sea ice means animals such as polar bears and seals have a harder time.

Many of the reports I've seen on the melting of Arctic sea ice, both in print and on television, have maps showing the decrease in Arctic sea ice. But, most of them have not made the point that the figures show the average summer extent of the sea ice.

Normally, ice covers about 5.8 million square miles of the Arctic Ocean in mid-February, when the sun never rises over much of the ocean and is very low in the sky over the rest. As the sun comes up in the spring, ice begins melting and by mid-September would normally cover only about 3.5 million square miles.

This melting is normal. The report talks about additional melting.