Depends how much the water rises, as Ethilrist discusses.
At 1m - some regional catastrophe. There’s usually about more than 3 feet between current sea level and structures. Problems will be more catastrophic in Hurricane Sandy situations, where water surges well above normal. As time goes on, more and more cities will compensate for that with dams and barriers, or by closing off certain flood-prone areas. Countries like Netherlands and Bengaladesh will have the bigger challenge, as will islands - I’m thinking of some low-lying pacific islands, and the Maldives where the 2006 tsunami went across the island basically flooding everything.
At 3 - 4M, things get very serious. Many fairly safe areas of cities are flooded, the coastline changes, the ecology of coastlines also changes - dead waterlogged trees instead of sandy beaches, etc.
However, the Egyptian pyramids for example are up on a limestone shelf well above the city of Cairo (and suburb or Giza) which itself is about 60 or more miles upriver from the sea. As long as climate change does not bring pounding torrential rains every day, they will survive. The bigger catastrophe would be the Nile delta. It’s a vast fertile area that feeds a substantial portion of Egypt. If much of it is flooded or regularly washed with salt water, things will be worse.
venice has surge gates, but they are for occasional storms, not to be a permanent dyke. Rivers empty into the lagoon. That water would have to be pumped out somehow… eventually a losing proposition. I’ve walked across the Lido - it may be a block for storm surges, but I don’t recall it being very high - so it’s not a dam to protect Venice from 3m of higher water. I’ve been in Venice During aqua alta, and without those barriers they would get a foot or two of water in Piazza San Marco, the lowest area, already. Building a 20-foot dam around everything, several dozen miles, would be prohibitively expensive.
Even something like the Statue of Liberty - yes, it’s on a pedestal uphill, it should be fine from 1m, but at 3-4m it’s probably a small island or concrete pier rising out of the bay.
The big problem is - what will be protected and how? There is so much that would need protecting, not just from 3m of high water, but from the potential storm surges that would result with a higher water level. 4m raised sea level means potential surges 6 or 7m above current sea level. Who would want to live behind that wall in active weather areas?
As for “uninhabitable”, that depends on weather trends. Would stronger warming imply bigger clouds, heavier rain, and more cloud cover mitigate temperatures? The same sunshine that gives us 50C or 130+F temperatures in India or Saudi Arabia does not seem to be anywhere near as serious in Brazil, for example.
I’m a bit of a skeptic on global warming. Check the latest sunspot data, and we seem to be heading to a global low and another little ice age. We’re also due for a real ice age anytime in the next millennium or three. What humans can do is paltry compared to mother nature. However, pretending that dumping huge amounts of CO2 in the atmosphere and ignoring the consequences because “we can’t be sure” is certainly not a clever move. There’s only one way to find out for sure what will happen, and is it a good idea to try to find out?