Global warming forecasts predict a dangerous rise in global sea level. But the flood threat for low-lying areas may lie closer to home.
According to oceanographer John Millman, the problem is not so much that the ocean is going up but that the land is going down. This subsidence often is caused by unwise development, he says. That means it is a risk that people can learn to control.
Pumping out ground water from beneath urban coastal areas can make land sink much faster than the global ocean rises. Damming or diverting rivers whose silt deposits offset land subsidence exacerbates the problem. Killing coral reefs with pollution puts coral islands at risk because the upward growth of the coral has been fast enough to offset the 1 to 2 millimeter per year global sea level rise in this century.
These are the kinds of effects due to thoughtless development that Professor Millman cites. Based at the College of William and Mary at Gloucester Point, Va., he explained his concern during briefings on new horizons in science held last month in Roanoke, Va., by the Council for the Advancement of Science Writing.
Natural effects, in addition to human influence, cause land to rise and sink. This can happen where the crustal plates that carry both continents and the ocean floor bump together or move apart. Sometimes the encounter makes land rise. Other times, when one plate is depressed, coastal land can sink. The danger of flooding is enhanced when man-made subsidence adds to natural sinking.
"I truly do not believe that, over the next 50 to 100 years, much of the world is going to be affected by ocean-level rise," Millman said. Instead, he says, "let's worry about those areas that are subsiding."
Egypt's northern coastal region is a case in point. Last summer, Daniel Stanley of the Smithsonian Institution and Glenn Goodfriend of the Carnegie Institution - both in Washington - reported geological studies showing that Egypt's northeastern Nile delta has been sinking for thousands of years. It's now only about 1 meter above sea level in that region. In the journal Nature, the scientists note that this makes "the Northern Suez Canal and the coastal cities of Port Said and Port Fuad (combined population nearly 500,000) highly vulnerable."
In his briefing, Millman reiterated geophysicists' warnings that the Aswan Dam now prevents the Nile from delivering its former annual sediment load to the coast. This sediment deposit has offset natural subsidence and erosion. Now the delta is both sinking and eroding. "Egypt, as we know it, may not be around in 100 years," he warned.
Millman sees the greatest flooding danger in Southeast Asia. This is where more than half of the world's megalopolises - 13 urbanized areas with more than 5 million people each - are located. All but Delhi, India, lie at least partly on coastal lowlands and river deltas. Cities such as Bangkok, Bombay, Calcutta, Dhaka, and Manila are at risk. Large areas of Bangladesh's fertile coastal zone already suffer catastrophic flooding during intense storms.
Millman says that, in many of these areas, development means damming or diverting rivers. This not only deprives river deltas of silt, it may also force urban areas to pump out ground water to make up for lost water.
Climate-change models predict a range of possible sea-level effects. Sea level has been rising mainly because of the thermal expansion of sea water as climate has warmed about a degree Centigrade this century. Continued global warming could accelerate that annual sea-level rise from 1 to 2 millimeters to as much as 6 millimeters.
What people at risk of inundation can do, Millman says, is rethink development and land use. "If they manage lands correctly ... manage rivers correctly," he says, "they probably can minimize the effects of ocean-level rise."