You've asked a "worst case scenario" question. Certainly, there were times in the past history of the Earth where there was no ice on land, or very little. The seas were at their highest then. If all the ice on land was to melt, then sea level would rise about 220 feet. Most of this would be from Antarctica (200 ft), where the ice is actually on land and out of the water. But in the Arctic, most of the ice is floating on the ocean.

If you have a glass of ice water that is completely full, will the water overflow when the ice melts? No, because ice displaces its volume, so as it melts, it doesn't cause a change in the water level. So if the ice in the Arctic Ocean melts completely every summer (which is where we are headed within my lifetime), sea level won't be affected by the melt-water. A lot of animals that depend on the sea ice to hunt in the open ocean will certainly go extinct (e.g. polar bears, ring seals), but humans won't have to worry much about rising sea levels. The ice would have to be on land for it to increase sea level as it melted.

The Greenland ice sheet is melting rapidly, and it is on land. When it melts completely (and I believe it will) then sea levels are predicted to rise as much as 20 feet. Sea level will also rise as the ocean slowly gets warmer. Just a small increase in ocean temperatures will cause the water to expand enough to raise sea level by 10-20 inches. Scientists still think that the ice sheets in Antarctica are in no danger of melting, because it is so cold there. There have been some extremely large ice bergs that have calved off the ice sheets and melted, however, so a small increase in sea level beyond what the Greenland ice sheet can cause is possible.

Sea level has already increased enough to flood certain islands completely at high tide. About 11,000 residents living on the Pacific Island of Tuvalu had to evacuate because of rising sea levels, and certainly many more low-lying island nations will have to evacuate in the future. This will create political and social problems as nearby countries will be forced to accept large numbers of refugees. Entire cultures may disappear. All of the fresh water released from melting ice sheets will certainly affect ocean circulation patterns, which depend on both ocean temperature and ocean salt content. Ocean circulation patters have a big effect on both global as well as local climates, so we are all headed for an uncertain future.

If all the ice in the north and south poles melted to water, the world seas would increase by about 61 meters (200 feet).

All of this rise in sea level would come exclusively from the melting of the Antarctic (south pole) ice, which is located on land upon the continent of Antarctica and would flow out to sea and flood the oceans if it melted.

Because the Arctic (north pole) ice is floating entirely upon the Arctic Ocean, it would not displace any more water than it is currently displacing if it melted, and therefore it would not cause a rise in sea level (see explanation below).

Greenland and Alaska, though, have large glaciers on land that are in danger of melting from global warming. If the ice on Greenland were to melt, this would contribute an additional 7 meters (20 feet) to sea level rise.

Right now, the sea level is rising because of global warming, but the majority of this sea level rise is due to the thermal expansion of the water in the oceans, rather than to the melting of glaciers and polar ice sheets. Since the Earth is warming on average due to the greenhouse effect (increasing carbon dioxide in the atmosphere traps heat that is emitted from the Earth and radiates that heat back to the planet rather than allowing it to escape into space), the oceans are also absorbing more heat and are warming as well. When the water in the oceans warms, it expands and the sea level rises.

The melting of glaciers on land (in places like Antarctica, Greenland, and Alaska) and the run-off of this melt-water into the oceans is also contributing to rising sea levels, but to a lesser degree than the expansion of warmer water.

Contrary to popular belief, the melting of the polar ice caps (ice floating on water in the Arctic Ocean and Antarctic Seas) is not contributing directly to any raises in sea level and never will cause any rises in sea level. This is because the volume of water that the ice in the polar ice caps occupies is equal to the volume of water created when this ice melts so that there is no change in the water volume due to melting. In other words, if you place an ice cube in a glass of water and measure the volume of water in that glass, and then let the ice cube melt and re-measure the volume of water in the glass, the volume of water will not have risen (in fact, the water volume may have decreased slightly because the water is colder after the ice melted than before it melted and cold water contracts). However, polar ice floating on the oceans helps keep glaciers and ice sheets on land cool and hold them in place.

The melting of ocean ice in Greenland and Antarctica is thought to increase the rate of glaciers melting on the land behind them and raise the sea level.

The average rate of sea level rise is currently measured at approximately 25-30 cm per century, although this amount varies greatly from place to place since some locations are sinking into the ocean and others are still rising from the ocean (like volcanic islands). Highly uncertain predictions claim that the sea level will rise by about half a meter (50cm) in the next 100 years. Although this sea level rise is significant, there is not considered to be any danger of the ice sheets on Antarctica melting anytime soon due to the year-round below freezing temperatures. However, many large glaciers in Alaska and Greenland are observed to have been shrinking for many years and are certainly in danger of melting and contributing to raising sea levels in the near future.

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GlobalWarming.

Approximately 80 m, I think, but it is highly unlikely that the Antarctic ice sheet will melt as long as the Antarctic continent remains over the south pole. The "round the pole" flow of wind and ocean currents tends to isolate the pole, which makes it very difficult to melt the ice. I think Greenland has 10-20 m worth of ice on it.