No TS is right, overpopulation is a myth. Climate change is driven by resource consumption which is largely driven by the more developed countries. The problem isn't the amount of people, its the Western development model that leads to massive per capita strain on the environment.
If anything developed countries have low birth rates so there's not enough people being born.
A lot of the empty land in the West are federally owned reserves and they're in very dry areas where it'd be hard to develop.
However we do waste land by having single use, single family zoning in suburbs around our cities when we could allow for more mixed use, high density urban development. Tokyo has a population that is 50% larger than that of the NYC metro area while taking up 1/4th of the land area so its 8x as dense. If all our major urban centers were like that we'd have a lot more available space and housing prices would be lower as well as having less obesity and smog related health problems