Don, geoengineering is going to have to take place on a massive scale.
I see you mentioned the rising sea levels and what the result would be if all the polar ice caps and glaciers melted. The planet is 70% covered by water right now, how much more would be covered by water if all that ice turned liquid?
To be honest, we have way too much more water on this planet than we need, at some point, the only way to save our coastal cities will be to transport some of that water to space colonies in orbit and on the Moon and Mars (and wherever else we might colonize where water will most definitely be needed.) How quickly would we be able to do this (perhaps developing new technology like space elevators). This is obviously something that will take many decades to develop, perhaps not until 2100 and beyond.