Sustainability & Population
What is "ecologically sustainable" is a function of behavior and population. With a large enough population, it is very difficult (or impossible) to achieve sustainability.
There are of course natural and built-in population controls, such as ecological disasters, epidemics or wars (all either naturally cyclical or brought about by over-population).
Still, it is difficult to talk about "sustainable design" as long as the global human population counts more than 6 billion. We will either need an extremely sophisticated approach to sustainable design (regenerative rather than just minimizing the harm), or a less sophisticated (more realistic) approach and a significantly reduced population.
We often think of over-population as more serious in poor countries, but it may - ironically - be a far more serious problem in richer countries where the lifestyle of each individual has greater impact (impact = lifestyle + population).