Interesting question. I don’t think it’s as simple as it appears. Density is good – less cars needed, less sprawl, and it’s easy to integrate mass transportation. Air conditioning will be a real issue. I suppose it depends on what the alternative is – if people are moving here from temperate regions, then this is an overall environment killer. However, if the alternative is small spread-out offices (or in the case of condos single-family homes) in the same region – then this will save energy. You’d have less surface area and therefore less solar loads and conductive heat gain. You can have larger equipment (centrifugal chillers) – which is much more efficient than small equipment.

Sustainable? No. Not without at least something like solar power to run your air conditioning and lighting. More sustainable than the alternative? Probably.