I live in NYC. I'm not sure why you are being downvoted, rising oceans are a big concern. That said I think the solution will be pretty simple. When we have to

I live in NYC. I'm not sure why you are being downvoted, rising oceans are a big concern. That said I think the solution will be pretty simple. When we have to, humans are really good at moving dirt. We do it less often now because of the impact on the environment, but if there was a will then it's solveable. I would expect to see a dike ringing Manhattan in my lifetime. The city is currently encircling Manhattan with a "greenway" which is basically a strip of bike lanes & walking paths. Imagine that, but raised 20 feet. I think the same treatment will be useful for Brooklyn & Queens. I would close off some of the rivers that go into the boroughs (e.g. the Gowanus) to make the area that needs to be closed off smaller. That will be a bit disruptive to some industrial sites, but nothing crazy. All of this works because each square mile of NYC is worth so much money and has been built up. I'd worry a lot more about suburban coastal areas where the density doesn't rally support building protection on that level. The urban areas will justify it, but if you live on the beach in NJ or LI, rising sea levels will be a major problem.