There are two main reasons why sea level is rising as the world gets warmer. First, as ice sheets and glaciers melt, they send ice and water pouring into the oceans. But another reason is that water, like most substances, expands as it heats up - and as greenhouse gases warm the atmosphere, some of that heat is slowly warming the oceans as well. Scientists understand the expansion of water really well; you can look it up in a textbook. It is much harder to predict what will happen to the ice, though. It is not just the melting ice that scientists have to contend with -- it is also the fact that tidewater glaciers (glaciers that flow into the sea) and ice sheets move downhill faster with warming. In places like Greenland, that means they drop chunks of ice into the sea at a greater rate than they have in the past -- and adding ice to the sea faster than it can melt drives sea level higher.

Scientists now project that three or four feet of sea-level rise is likely by the century's end, if emissions of greenhouse gases continue to grow at current rates. For millions of people who live in low-lying coastal areas, that's a direct threat. It is a threat to many millions more because surges of water from storms will push seas even further inland. Coastal marshes tend to absorb the energy of waves and surges. If they disappear under the rising sea, that buffer will be gone, making the land more vulnerable to flooding and erosion. And rising saltwater could get into underground supplies of fresh water. That could threaten drinking-water supplies, disrupt coastal agriculture, and destroy ecosystems. Finally, when heavy rains on land send water gushing down rivers, the rivers will start backing up and flooding low-lying land sooner. A sea-level rise of three feet might not sound like a lot, but it could do enormous damage.