I'm actually not much of an environmentalist (BIOFUELS BITCHES!!!). Not that I don't care or align myself with the broader movement, just that it isn't my thing. When I think about land use issues I'm not really primarily concerned with carbon emissions, but rather sensible affordable lifestyle choices.
But, yes, to the extent that population growth happens around existing population centers instead of in new ones, the choice is basically infill or more sprawl. Expanding exurbia even farther involves even longer commuting times to the extent that jobs don't travel there too, and infill will face understandable resistance from current residents, though some of it will happen anyway. It's why people like advocate for better infill development, the kind of development which won't change the fundamental development patterns for most of the population. All we're really talking about is dense development around transit stations, moderate density around transit corridors, and better pedestrian integration with surrounding existing communities so that current residents, too, can easily board the SUPERTRAIN.
