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
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
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.
For more on this topic: