The way most people understand the story about global warming and sea level goes something like this: Earth’s atmosphere and oceans are getting warmer, causing ice caps and mountain glaciers to melt. The melt-water then flows into the oceans, causing the water level to rise.
This is true as far as it goes, but there’s a lot more to the story. For one thing, water from land-based ice accounts for only part of the rise in sea level over the past century. Most of the rest is due to the fact that water, like most liquids, expands when it’s heated. The effect, called thermal expansion, is so small that we don’t usually notice it during the course of our everyday lives, except in special circumstances—for example, the liquid that rises in a thermometer when the temperature goes up.
It also turns out that warm water expands faster than cold water, which means that rising temperatures will cause the oceans to expand faster and faster.
But disappearing ice is becoming an important factor in sea-level rise as well. Sea ice—the kind that covers the Arctic Ocean every winter and that polar bears roam around on—isn’t the problem. That ice is already floating, so it doesn’t raise sea level when it melts.
Ice that sits on land, however, especially in the massive ice sheets that cover Antarctica and Greenland, does raise sea level when it melts and flows into the ocean—or even when it falls into the ocean without melting. That second process, which is where icebergs come from, has always gone on. Ice sheets are always in motion, spreading outward toward the ocean under their own weight like a thick round of pizza dough under a rolling pin, feeding glaciers that inch toward the sea. But as temperatures have risen, the glaciers have started moving faster.
For a while, scientists thought this was due to melting water pouring down through cracks in the ice to lubricate the area where a glacier’s underside meets the bedrock, but that’s now looking less convincing. What does seem to be happening is that warming seawater melts tongues of ice where the glacier reaches into the sea, letting the tongues float free from where they had been grinding against ocean floor. When this happens, it’s like easing up on the brakes of a car. The glacier moves faster, dumps more ice into the ocean, and raises sea level.
The vast ice sheets that cover Greenland and Antarctica contain a huge amount of frozen water—more than 70 percent of all the freshwater on the planet. As Earth warms, some of that ice is going to melt (although some of that melting may be offset by increased snowfall), and some of it is going to plop into the sea without melting. This process has already begun, mostly in Greenland, where temperatures aren’t as cold as in Antarctica to start with.
If all of that ice were to go into the ocean, sea level would rise about 200 feet—20 of it from the icecap covering Greenland, 20 or so from the West Antarctic Ice Sheet, and the other 160 from the much larger and thicker East Antarctic Ice Sheet. Scientists know this partly by measuring how much ice there is. They can also estimate potential sea-level rise by studying times in the distant past when there was pretty much no ice on Earth (it happened during the Cretaceous period, for example, about a hundred million years ago). At those times, sea level was indeed hundreds of feet higher than it is now.
But the only way for all of that ice to melt would be for the temperature to rise quite a lot and then stay high for many thousands of years (how many years depends on how high it goes). Just imagine putting an ice cube into an oven, waiting a few seconds, and pulling it out. In that short a time, even at a very high temperature, the ice cube won’t melt much.
Global warming isn’t going to raise the temperature as hot as an oven, of course, and the ice sheets are miles wide at their thickest. Even at the high end of where temperatures might go by the end of the century, most of that ice will still be there. Of course, the temperature might continue to climb after the year 2100, which would speed things up. But even then, and even if the temperature stays high, it would take hundreds of years for all the ice to melt.
Nevertheless, scientists believe that if Greenland’s temperatures rise more than about 4.5°F and stay there, the melting will be unlikely to stop.