Q: How much has sea level risen?

A: About 17 centimeters (7 inches) over the last 100 years. Much of this is due to the expansion of water as it warms.

Q: Is that a lot?

A: That depends what you mean by "a lot." Before the last century, sea level had risen much more slowly. And 1 centimeter (0.4 inches) of sea-level rise translates to 100 centimeters (40 inches) of shoreline loss.

Q: Is this the start of even faster sea-level rise?

A: People on low-lying islands are already starting to see some effects. But there's a lot of uncertainty about future sea level. Different ways of predicting future sea-level rise lead to different estimates, ranging from 18 centimeters (7 inches) to more than a meter (about 40 inches) by 2100.

History at Risk

Many historic U.S. cities will feel the impact if sea level rises significantly. Much of downtown Boston, for instance, is built on landfill and sits at very low elevation. This means even a meter of rise could have disastrous social and economic consequences.

Not-so-slow motion

Ice sheets—thick blankets of ice like the ones that currently cover Greenland and Antarctica—are always in motion, but scientists don't completely understand how the ice moves. That makes it tricky to predict how ice sheets will respond to warming. This false-color satellite image—red is land, white is ice—shows the outlet (circled, above) that drains the fast-flowing Jacobshavn glacier in Greenland. Greenland is losing about 200 cubic kilometers (50 cubic miles) of ice a year.

Since 1993, sea-level rise has picked up speed. What's next?

Today, ice sheets are melting much faster than scientists had assumed. What's going to happen to all that water? Sea level will rise. If the Greenland ice sheet were to melt completely, for instance, about 7 meters (23 feet) worth of water would be added to the world's ocean. If emissions continue to grow at the present rate, this outcome is likely to occur—but scientists don't know when. What we do know is that current warming means some increase in sea level is already locked in. Hundreds of millions of people will likely feel the impact.

On the Edge

The primary effects of sea-level rise are increased flooding during storm surges and permanent submersion of coastal land. Most vulnerable to these effects are coastal deltas—low-lying areas where rivers meet the sea. Some of the world's largest, most densely populated cities are located in these regions; indeed, 634 million people live within ten meters (33 feet) of sea level. Also at risk are small Pacific islands such as those in the tiny island nation of Tuvalu.