When talking about sustainability one can't separate population from
consumption. The US, and much of the 'developed' world has been at or near
ZPG for quite a while. At the same time, however, our rate of consumption
has increased so our total impact on the planet has increased, even if our
populations stable. What's more is that the type lifestyle enjoyed by those
in the 'developed' world simply cannot be shared by the 'undeveloped' or
'developing' world because there simply isn't enough stuff. One small
example: the US consumes 25% of annual oil production and yet we are only
1/20th of the world's population.

I've often thought it would be interesting to calculate the total cost in
kilo joules necessary to sustain an 'average' American vs., say, an
'average' Bangledeshi or Indian. I bet that if were to multiply our
respective populations by the appropriate number we would find that the 250
million inhabitants of the US have a much larger environmental impact that
the 1.2 billion or so inhabitants of India and Bangledesh. Further more I
would wager that this impact is increasing at a higher rate in the US than
in India and Bangledesh despite the fact that our population is stable and
their's is exploding.