Among other things, the DEP was concerned by the damage storm surges might inflict on a city surrounded by water. Although city officials declined to discuss concrete solutions for this article saying they were still in the "assessment" phase, scientists foresee potential fixes ranging from raising key infrastructure and building dikes, to flood gates and temporary seals over tunnel entrances. One group proposes raisable flood barriers large enough to protect all of Manhattan Island.
Sea levels have risen almost a foot in the past century, partly because of ice melt and thermal expansion (warmer water has more volume), and partly because of naturally occurring land subsidence of the Northeast. In the same period, area temperatures have risen nearly 2 degrees F. About two-thirds of that increase occurred in the past 30 years and sea-level rise has accelerated in the past decade. "The core body of knowledge has solidified" on climate change, says Cynthia Rosenzweig, the lead GISS scientist on the climate-change task force. "We're moving into a solution phase."
But possible solutions – and how to pay for them – are still "big question marks," says Gary Heath, director of bureau operations and environmental analysis at the DEP. Although antiflooding technologies are basic and well established, implementing them in a city as old and crowded as New York is no simple task.
Elevating roads, for example, sends more runoff into subway grates. Water pumped out of subway tunnels – already some 14 million gallons daily – goes into sewer systems that might be overtaxed by rainwater. "You solve one problem and you create another," says Madan Naik, chief structural engineer of New York City Transit. "It's got to be a collaborative effort, whatever we do."