Live on the ocean
Both are solved by one thing!
If we find a way to create cities on the ocean - it does not matter if floating or on a platform - we solve both issues - we have the space for population growth and we have the space for when the sea levels rise.
Two thirds of the space we have is ocean!
Why the hell do we even bother with trying to reverse climate change at all - yes, we may well be responsible for climate change, but we are "a drop in the ocean" compared to what nature can and will do in the next 100 years. So forget fixing this cause of problems and start on handling the result of the problem instead - two birds with one stone.
P.S. Please do take this as just a wild rant to start a bit of debate - such issues need a lot more serious discussion. P.P.S. I was drunk when I posted it :-)