As you know if we put a cube of ice on a glass of water because of floating force it will be floated. if we measure the height of water in that time it has no difference with the time that ice is melted. so why scientists say that melting ices of north pole and south pole will affect the height of seas and oceans?
2 Answers
Melting of the Arctic cap would, indeed, have no effect. However, the Greenland ice and the Antarctic cap do not (for the most part) float. If these melt, all of the water produced will go to raising the mean sea level.
The Greenland ice sheet, plus a few glaciers and such, totals about 2.85 million cubic kilometers. All of that water running into the oceans would produce an increase in global sea level of about 7 meters.
The Antarctic ice, at about 27 million cubic kilometers, is mostly above sea level. The exception is the West Antarctic ice sheet, which does not float but rests on bedrock as much as 2500 feet deep, so melting the below-sea-level portion would not raise the sea level. This portion is fairly small, though, and melting the Antarctic ice would raise sea level by about 60 meters.
Because not all iceshelf is floating water: Groenland is kilometers thick of ice laying on terrain, same for most of Antartic.