Indeed, the impact on coastal cities in developed countries may be enormous. The Urban Institute, a nonpartisan think tank, is completing a study for the Environmental Protection Agency on what a three-foot sea level rise would do to Miami. Miami is particularly vulnerable. Not only is it a coastal city, but it is nearly surrounded by water, with the Atlantic to the east, the Everglades to the west, and porous limestone beneath— "one of the most permeable aquifers in the world," says William Hyman, a senior re search associate at the institute. "The aquifer in Miami is so porous that you'd actually have to build a dike down one hundred fifty feet beneath the surface to keep water from welling up." In an unusually severe storm nearby Miami Beach would be swept by a wall of water up to 16 feet above the current sea level.
Image courtesy of US Army Corps of Engineers
Storms are an even greater danger to Galveston, which Leatherman has studied extensively. Given just a couple of feet in sea-level rise, a moderately bad hurricane, of the type that occurs about once every ten years, would have the destructive impact of the type of storm that occurs once a century. And Galveston is typical of a whole range of resort areas on the eastern and Gulf coasts, such as Atlantic City, New Jersey ("almost the whole New Jersey coast, really," says Leatherman); Ocean City, Maryland; and Myrtle Beach, South Carolina. "The point is, all these cities have been built on low-lying sandy barrier islands, mostly with elevations no higher than ten feet above sea level," Leatherman says. "Just a small rise in sea level will result in a lot of complications."
Even as cities become more vulnerable to moderate storms, the intensity of hurricanes may increase dramatically, says Kerry Emanuel, a meteorologist at MIT. Hurricane intensity is linked to the temperature of the sea surface, Emanuel explains. According to his models, if the sea warms to predicted levels, the most intense hurricanes will be 40 to 50 percent more severe than the most intense hurricanes of the past 50 years.
James Titus, director of the Environmental Protection Agency's Sea Level Rise Project, says communities will have two choices: build walls or get out of the way. For cities such as New York or Boston the answer may well he to build walls. But for most other coastal regions, picking up and moving may work out better. One of the first examples of a regional government making a regulation based on the greenhouse effect took place in Maine last year. The state approved regulations allowing coastal development with the understanding that if sea level rises enough to inundate a property, the property will revert to nature, with the owner footing the bill for dismantling or moving structures.
Another worldwide consequence of global warming is increased precipitation: warmer air will mean more evaporation of ocean water, more clouds, and an overall rise in rain and snow of between 5 and 7 percent. But it won't be evenly distributed. One climate model at Princeton University's Geo physical Fluid Dynamics Laboratory predicts that central India will have doubled precipitation, while the centers of continents at middle latitudes--the midwestern United States, for example— will actually have much drier summers than they have now (this summer's drought could, in other words, be a foretaste). Some and areas, including southern California and Morocco, will have drier winters; and winters are when such areas get most of their precipitation. Moreover, the effect may be self-perpetuating: drier soil, says Syukuro Manabe, the climatologist who developed the model, leads to even hotter air.
The changes could be political dynamite for nations that already argue over water resources. A prime example is Egypt and Sudan, both of which draw their lifeblood from the north- flowing Nile. Sudan has been trying to divert a bigger share of the river's water; but downstream, Egypt is experiencing one of Africa's fastest population explosions and will need every drop of water it can get. A string of droughts in the Sudan could make the conflict far worse. The same situation occurs in many other parts of the world.
Not all the tensions will be international. Within nations, local effects of global warming will cause internecine fights for increasingly scarce water. In the United States, for example, western states have long argued over who owns what fraction of the water in such rivers as the Colorado. In California 42 percent of the water comes from the Sacramento and San Joaquin river basins, which are fed by runoff from the Sierra Nevada and other mountain ranges. Most of the water falls as snow in the winter, which melts in the spring to feed the rivers, reservoirs, and subterranean aquifers. The state's normal strategy for water management calls for keeping the reservoirs low in winter, to provide protection against floods, and keeping them as high as possible in summer, to ensure an adequate supply for the giant farming operations in the Central Valley (one of the most productive agricultural regions in the world) and for arid southern California.
Peter Gleick of the Pacific Institute for Studies in Development, Environment and Security, in Berkeley, California, has devised a widely praised model that predicts a dramatic disruption of the state's water supply in the event of global warming, even if total precipitation remains unchanged. It focuses on the Sacramento River basin, which alone provides 30 percent of the state's water and almost all the water for agriculture in the Central Valley.
According to the model, higher temperatures will mean that what falls in winter will increasingly be rain, not snow, and that more of it will run off right away. California may get the same amount of total annual runoff, but the water-distribution system won't be able to deal with it. "California will get the worst of all possible worlds—more flooding in the winter, less available water in the summer," Gleick says. "This will reverberate throughout the state." San Francisco Bay will feel a secondary effect. As freshwater supplies shrink in the summer, seawater, which has already infiltrated freshwater aquifers beneath the low-lying Sacramento Delta, will continue its push inland. Rising sea level will just compound the effect.
Food is another crucial resource that will be affected by the global greenhouse. Taken by itself, a rise in atmospheric carbon dioxide might not be so bad. For many crops more carbon dioxide means a rise in the rate of photosynthesis and, therefore, in growth; and with increased carbon dioxide some plants' use of water is more efficient, according to studies done in conventional glass greenhouses. Also, as the planet gets warmer, crops might be cultivated farther north. But as usual, things are not so simple. A temperature rise of only 3.5 degrees in the tropics could reduce rice production by more than 10 percent.
