by Jenna Beck
If you mark the water level on a glass filled with water and ice, you’ll notice the level doesn’t change as the ice melts. So when the Intergovernmental Panel on Climate Change released its full Fourth Assessment Report on September 18th, why did it warn that sea levels will rise when the polar ice caps melt? What makes the globe different from a glass of ice water?
The key to picturing the global warming meltdown is knowing the difference between the ice that floats on the sea and the ice that rests on the continents.
Sea ice is made of frozen seawater and floats on the ocean’s surface in the form of icebergs. Since ice is less dense than liquid water, icebergs float on the ocean’s surface like ice cubes in a drink. The ice and the water are in equilibrium, so the submerged portion of the ice takes up as much space as the water from the ice cube would if it were to melt. From satellite images, NASA estimates that the north and south poles are covered in 25 million square kilometers of sea ice—roughly the same surface area as the North American continent. Sea ice may be supporting Santa’s workshop on the North Pole, but if all 25 million square kilometers were to melt tomorrow, global coastlines wouldn’t rise a centimeter.
Continental ice, such as alpine glaciers and the ice sheets that cover Antarctica and Greenland, does have the potential to raise the sea level. It’s made of compacted freshwater snowfall that collects on the continents. And because continental ice is held above the sea by land, it would introduce new water into the ocean if it were to slide off that land. Continental ice melt running into the ocean would be like adding more water to the glass of ice water. NASA estimates that continental ice covers 15 million square kilometers—about ten percent of the earth’s land surface area. While alpine glaciers aren’t large enough to significantly alter sea levels, if the massive ice sheets that cover Antarctica and Greenland were to melt completely, sea levels could rise by 75 meters (246 feet).
So melting continental ice raises the sea level, while melting sea ice doesn’t. But while sea ice can’t directly change coastlines, it can change the rate at which the continental ice melts. Sea ice reflects sunlight and provides insulation to the polar regions. So when it melts, the surrounding area warm even faster.
There are also floating ice shelves to consider—frozen ice planes that extend from continental ice sheets into the ocean. Like sea ice, the ice shelves float on the water and does not alter sea levels per se. But if the ice shelves were to disappear, the unfrozen sea water that would come into contact with the continental ice sheets could trigger landslides of continental ice into the ocean. This would happen if water seeped in-between the ice sheet and the land. In both Greenland and Antarctica, the ice shelves are in contact with unfrozen, warming seawater and have begun to melt. But that’s a story for another time.