Disappearing beaches
Grim examples
Role of beaches
Solving beach erosion?
Global warming and sea levels
Right: Today's view of the beach at Southampton, N.Y.
Below, right: Here's what would be left if the sea rises one meter. Realtors beware: A storm could obliterate the ritzy barrier beach.
Courtesy of National Environmental Trust.
Ice melting off the Greenland ice cap, shown here, could raise sea level. But the Antarctic ice cap is the big enchilada in sea-level doomsday scenarios.
Image courtesy of the United States Coast Guard.
Warming ocean = rising ocean?
After the last ice age, the rapid melting of glaciers rapidly raised sea level. That melting tapered off about 6,000 years ago, and sea level -- compared to land -- became fairly stable. However, over the past century, sea level over much of the United States has risen by 25 to 30 centimeters relative to land, according to Jim Titus, the Environmental Protection Agency's project manager on sea level rise. Even that figure is a guesstimate, Titus says. "We only know that sea level last century rose more than average over the last several thousand years."
The warming of the atmosphere caused by increases in greenhouse gases is melting glaciers and causing ocean water to warm and expand thermally. Both effects increase the volume of the ocean, raising its surface level. Need a refresher on global warming?
How much will these factors add to the existing trend toward sea-level rise? The Why Files asked Titus what to expect. He did his best to answer the question, but he started by throwing cold water on our desire to learn the absolute rise in the ocean surface. In other words, how far has the average ocean surface moved from the center of the Earth? Unfortunately, nobody has made that measurement consistently, so we must settle for records of relative sea level rise, which tells us what's happened to sea level in comparison to a certain hunk of coastal real estate.
Windiness aside, what did he say?
Now come the caveats. This calculation, Titus stresses, "makes no sense" because it amounts to adding a guesstimate of future rise to a range of historic rise. Projections, he says, are "even less certain" than historic data. And remember that nobody knows the exact trend in carbon dioxide emissions, which drive the accelerated greenhouse effect.
At any rate, EPA says there's a 5 percent chance that global warming will not augment the existing trend toward a sea rise. But there's also a 5 percent chance that the sea will rise an extra two feet, and a 1 percent chance that it will rise an extra three feet, above the existing trend. Stabilizing global carbon dioxide emissions would cut the rise in half, the EPA reported.
Those numbers don't seem too intimidating, until you look at the consequences. Under the worst scenario, island nations, coastal cities and beaches alike would be threatened with obliteration, especially if the rise continued for more than one century. Groundwater aquifers could be polluted by salt water. Tunnels, harbors and coastal wetlands could be soaked with salt water. The 30-centimeter increase over the existing rate of rise, after all, is greater than the rise over the past century, which caused all the ruckus we've been describing.
But nobody expects the prediction to be right on the money. The obstacle to making accurate predictions, Titus says, is this: "We've never done this before -- pumped greenhouse gases into the atmosphere at this rate. If we do this a couple of times, then we can have more confidence in our predictions. This is an experiment, and unexpected things can happen."
The short answer is that a 1995 EPA study projects about a foot -- about 30 centimeters -- of extra relative rise over the next century or so for the U.S. coast. Since the "background" rise is about 25 to 30 centimeters per century, the total relative rise comes to between 55 and 60 centimeters over a century. But remember that this depends on location -- in areas where the land is actually rising, relative sea level might not rise at all.
The big dunk
Methane madness. The continental shelves harbor huge amounts of stored methane, a potent greenhouse gas in its own right. If coastal waters warm enough, this methane could be released, causing greatly increased warming. Similarly, warming in the Arctic tundra could release vast stores of carbon dioxide, with similar effects.
Ice-sheet insanity. Melting glaciers are but a trickle of water on the planetary scale. They're small change compared to the West Antarctic Ice Sheet, which holds 3 million cubic kilometers of fresh water. Were it to melt, sea level would rise 20 feet, and coastal cities, not to mention beaches, would be in tough shape.
Eventually, if greenhouse warming continues, big changes could be in store for our planet. These so-called "doomsday scenarios" result from feedback effects -- networks of cause and effect that amplify the warming caused by the original increase in carbon dioxide. We'll mention just a couple of possibilities.
Let me make one thing perfectly obscure
But when ice cascades off land, it does increase the volume of the oceans. Ten years ago, glaciologists were worried this might happen if the Ross Ice Shelf off the Antarctic coast continued melting, thus releasing massive ice flows from the West Antarctic Ice Sheet into the Southern Ocean. The ice shelf, they thought, served as an obstacle holding back the enormous ice sheet. Calculations warned of a massive collapse of the ice sheet that could raise the sea level by 20 feet in a century.
Ice shelves melt all the time, "calving" icebergs that, no matter how enormous, do not effect sea level. Because floating ice displaces the same volume of water as melted ice, iceberg that formed from floating ice on Antarctica's vast ice shelves add no volume to the ocean.
Yow!
The current opinion among glaciologists is a lot more reassuring. Charles Bentley, a retired University of Wisconsin-Madison geologist who has spent 40 years studying the frozen continent, says the mathematics of the old calculations artificially accentuated the possibility of collapse, and also oversimplified the movement mechanism.
To risk oversimplification ourselves, here's the new view. Most of the ice sheet rests on land that's below sea level. At a point called the "grounding line" it starts floating, thus displacing its own weight in water, so the important question is to locate the grounding line. And as it turns out, the line may not move much because the flow of the ice streams seems to be restrained by friction against rocks at the bottom and sides rather than the ice shelf. The streams, says Bentley, "don't particularly care whether there is an ice shelf there or not." So if the ice shelf melts, the flow of the streams should not change appreciably.
And since the volume added to the ocean depends on how much ice moves from land to water -- as determined by the grounding line -- the upshot seems to be relative stability. "The ice streams do not appear to be susceptible to the kind of unstable retreat once envisaged," says Bentley. "Their flow is largely insensitive to the presence of the ice shelf so the grounding line would remain the same."
Instead of possibly collapsing in 100 years, as was considered possible 10 years ago, Bentley says the West Antarctic Ice Sheet is more likely to collapse -- if at all -- in perhaps 5,000 years at the soonest.
Talk about breathing room!
Not yet drenched in a rising tide of data? Then check out our coastal-erosion bibliography.
