How much will sea level rise?


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. (By contrast, the melting of ice that was in the sea all along, like the ice pack that covers the Arctic Ocean in winter, does not make sea level rise at all. If you have a glass of water with ice cubes in it filled nearly to the top, it does not overflow as the ice melts. But dump more ice into that glass and see what happens).

In part because of that uncertainty, and also because of insufficient information about the relationship between melting and sea level in the past, the authoritative Intergovernmental Panel on Climate Change, or IPCC, counted only the expansion of seawater and the increased melting in its latest report. It left the motions of glaciers out entirely. The IPCC knew this could be important, but did not have enough information to estimate it. Thus, everyone knew that the IPCC’s projection that sea level would rise between half a foot and two feet by the end of this century was not the whole story.

Since that report came out in early 2007, though, scientists have gotten a better handle on the uncertainties. They now project that three or four feet of sea-level rise is likely by 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.

How do we know?

The sea surface moves constantly up and down due to tides and ocean waves, but scientists can still measure average sea level to an accuracy of millimeters or less. They do it with satellites, including one called TOPEX/Poseidon, and, since 2005, another called Jason-1 and most recently Jason-2. These satellites bounce radar waves off the surface thousands and thousands of times each year, all over the world, to figure out how sea level is changing. Another set of satellites, named Terra and Aqua, use remote sensors to measure sea surface temperatures.