Ice and snow occur in various forms in cold regions around the world. Each kind of frozen water has its own relationship to global warming and sea level. For example, when viewed from space, one of Earth’s most obvious features, is, at times, the huge white band of snow covering northern Asia, Europe and North America. Global warming will reduce the amount of snow cover. However, the reduction will have virtually no impact on sea level. The volume of water in all the snow in the world is infinitesimal compared to the vast volume of the oceans.
After snow, the next most visible form of frozen water on Earth is sea ice. As its name implies, sea ice is seawater that has frozen on the surface of the oceans, generally near the poles. Like snow, the amount of sea ice changes with the seasons. At its maximum extent, it covers about 10 million square miles, about the size of ten Australias. The amount of sea ice in the Arctic has been declining steadily for at least as long as systematic satellite monitoring began in the late 1970s. Many researchers predict that the Arctic will be virtually free of sea ice in summers before the middle of this century and perhaps much sooner. The disappearance of sea ice will not alter sea level (though it will have other impacts such as changing the reflectivity of the polar regions). That’s because sea ice floats, just like ice cubes in a drink. When the ice in glass of soda melts, the level of the surface remains unchanged.
The huge glaciers in Antarctica and Greenland, known as continental ice sheets, don’t float like sea ice. They rest on land. As a result, when they melt and the water flows into the ocean, sea level rises. Although they cover less area than sea ice or snow cover, they are substantially thicker (up to nearly three miles thick). Therefore, they contain much more water; enough to raise sea level by about 250 feet. Mountain glaciers, also melting, raise sea level too as the recede. They contain about 1 percent as much water as the polar ice sheets.