Yes, sea levels are slowly rising as climate change takes place. Future storms are likely to be worse and cause much more damage than in the past. New York is heavily populated and it will remain so for a long time. If nothing is done, much property will have to be destroyed to account for worse weather.
Hurricane Sandy clearly illustrated the impact of global warming. It appears that as a country, we have decided to accept this new reality, including rising seas levels and warmer oceans that generate strong storms farther north, instead of making significant changes to the ways we produce energy. As a result, protection is needed for the impacted areas. The metropolis of New York is not going anywhere, and neither is global warming, so it only makes sense to provide protection to the area.
Experts are predicting that, as global warming increases, sea levels are going to rise, which means that future storms are only going to mean more trouble for a low-lying city like New York that is surrounded by water. If we truly care as much about national defense as we say we do, then we should also care about defense from a natural disaster. Few cities go wrong by building needed infrastructure.
Despite the recent events that have occurred in New York City, I do not believe that New York should build sea gates. At this point it is not financially or logistically possible to build sea gates in the city. Also Hurricane Sandy is said to be a once and a life time catastrophe, so it isn't an urgent matter.