Why do oceans rise if polar ice caps melt?

Answer 1

If it's the NORTH polar ice cap melting, the oceans DO NOT rise. This is a scientific fact. The North polar ice cap is FLOATING on the ocean. When floating ice melts, it does not change the level of the water it is floating in. This fact can be easily confirmed by a simple scientific experiment. Put a solid block of ice in a clear glass of water so that the ice floats. Mark the level of the water on the outside of the glass. Wait for the ice to melt and check the level of the water again. It will be exactly the same as it was before the ice melted.

But, if you are talking about the SOUTH polar ice cap, it's a different story. This ice cap is not floating, but is suspended above the continent of Antarctica. As it melts, it drips down into the ocean, causing the ocean to rise. However, the fact of the matter is that, though ice is constantly melting at least along the edges of the Antarctic ice cap, the amount of ice being ADDED to the Antarctic ice sheet (by snowfall) more than compensates for any loss due to melting. Not only is the additional ice in Antarctic enough to compensate for the melting in Antarctic, it's also enough to compensate for the melting of all other suspended glaciers in the world, such as those over Greenland, Alaska, and northern Canada. So, on average, the melting of polar ice caps is NOT causing the oceans to rise.

The primary reason that the oceans are "rising" is tectonic activity causing the land masses to sink. Another contributing factor is erosion, depositing sediments on the ocean floor. But the melting of ice caps is not contributing to the rise in sea level.

However, the effect of the coefficient of thermal expansion of the ocean water itself is likely to result in the greatest contribution to seal level increases. Water is most dense at 4 degrees Celsius. Above and below this temperature, the density of water decreases (the same weight of water occupies a bigger space).

In 1995 the Intergovernmental Panel on Climate Change issued a report which contained various projections of the sea level change by the year 2100. They estimate that the sea will rise 50 centimeters (20 inches) with the lowest estimates at 15 centimeters (6 inches) and the highest at 95 centimeters (37 inches). The rise will come from thermal expansion of the ocean and from melting glaciers and ice sheets. Twenty inches is no small amount -- it could have a big effect on coastal cities, especially during storms.

Answer 2

Because the sheer weight of ice on top of Antarctica is pushing the continent down. If you take this weight off, it will "spring" back up. But keep in mind that this "springing" back up (called Post-Glacial Rebound), is not instantaneous at all, but takes thousands of years.

Post-Glacial Rebound is occurring right now in the Great Lakes region of North America. All of the Great Lakes are "tilting" to the south, as the lake floor under the northern edges of the lakes (more recently covered by glaciers) rise more rapidly than under the southern edge. It has been 10,000 years since this area was covered by glaciers.