Sea Level is the standard height of ocean water measured between high and low tides. It is an important indicator of shorelines, boundaries, and flight altitude level. The ocean's Mean Sea Level rises and falls depending on how much of the Earth's water is liquid, gaseous, or solid. Liquid water in the ocean rises and falls according to the tides and according to the weight of solids floating in it (ice). Gaseous water is atmospheric and contributes to clouds, rain, and snow, and does not affect the sea level until the water changes form from gaseous to solid or liquid. Solid water is ice, and large masses of it float in the colder climates north and south of the equator. The Arctic Ice Cap is the largest floating mass of ice on earth, and has a corresponding effect on sea levels (if it were removed, the Mean Levels would fall), while the Antarctic Ice Cap is partially on land, and so had a lesser effect (the amount of land underneath would still displace water). Ice sheets on Greenland would also contribute to Sea Level rise, as that ice is not currently displacing water in the ocean.
When ice is added or subtracted from the atmosphere to the ice caps in the form of rain or snow, the liquid water is locked in place and does not return to the ocean until the ice melts. As a result, sea levels can rise or fall if the added snowfall on the ice caps is greater or lesser than the amount of ice calving off the main body and entering the ocean. Ice floating in the ocean causes the same rise as liquid water, even while a portion of it rises above the surface, so calved icebergs can be thought of as addition to water displacement after calving.
Recently, there has been a marked increase in calving icebergs associated with the Global Climate Change Theory (formerly Global Warming). The idea is that melting ice will cause the sea levels to rise, thus decreasing land mass and causing ecological disasters as the related local climates change. This theory is supported by the majority of accepted science around the world, and illuminated by the IPCC report presented to the UN in 2001 and 2007. The opposing theory is that melting ice is directly equalized by snowfall in the Arctic and Antarctic Ice Caps, which would keep the water level neutral (see citation).
If both Ice Caps were to melt entirely, the sea levels could rise over two hundred feet, enough to wipe out many sea-level habitation areas. However, at the moment, the changes in the Earth's temperature show variance to a level that makes predictions difficult. Best guess scenarios range from no change (best case, with or without human intervention) to thirty-seven inches (worst case, same).