First, as the time when we could prevent dangerous climate change slips away, the time for costly investments to protect ourselves has arrived. Second, for some cities, less well situated or less wealthy than New York, protection is going to be extremely challenging—and in some cases perhaps impossible.

Speaking at the Brooklyn Navy Yard, which last fall was inundated by the storm surge from Superstorm Sandy, Bloomberg warned his audience that by the middle of this century, storm tides may be riding on a sea that is two feet higher than today. According to new maps released by the Federal Emergency Management Agency this week, a quarter of the city—where some 800,000 people now live—will lie in flood zones. High