[strommen]

Yes, It would take a very long time for global warming to cause the sea levels to rise such that cities are underwater.

But the primary way that global warming manifests itself is by causing severe weather events to be more frequent and more severe. (This happens mainly because warmer air holds more water vapor.) So the next Katrina will happen sooner, and it will be worse - because of global warming.

Another problem is that warming will cause lots of problems for agriculture. Those of us growing corn in the Midwest will be okay - but places with marginally fertile land will have famines.

Another problem is that insect-borne illnesses (which already kill millions) are helped in a huge way by warming, even just a few degrees' worth.

These are all bad. But the civilization-threatening part is what happens politically with the people displaced by floods, famine, and plagues of malaria/etc. This kind of stuff causes refugee migrations, political upheaval, revolutions, dictators, etc. The U.S. government is legitimately worried about this stuff [1].

[1] https://www.whitehouse.gov/sites/default/files/docs/National...