Some of the most powerful effects will be seen on flatter coastal land abutting the shallows of large water bodies. If these areas also are prone to landfall by tropical cyclones, the effects will be further intensified. Among the hardest hit will be tropical and sub-tropical river deltas – broad fans of sediment and waterways where rivers meet the sea. Because such deltas often are the sites of port cities, large human populations will be exposed to significantly higher risk. Hot spots include the U.S. East Coast and Gulf Coast, Asia, and islands.
The risk comes not only from rising sea levels due to ice-melt, and the expansion of ocean water as it warms, but to increasing storm surges and high-tide flooding. As seen in recent years in New Orleans and along the Gulf Coast, storm surges are amplified by sea-level rise, causing them to hit higher water levels and allowing the surges to reach farther inland.
Beyond 2100, the consequences of sea-level rise could well force an inland retreat by human civilization to higher elevations. By 2150, storm surges likely will be twice as high, or higher, than they are today. And in general, after 2100, rising sea levels in the 3 to 6.5-foot range (1 to 2 meters) will cause widespread damage to coastal areas. Strategic adaptation will help at first – raising coastal structures and building extensive seawalls. But as the problem worsens, the continuing impact to society will be greater and the cost of responding will increase.