Our lonely little sliver of biosphere on our sole habitable planet (so far) has its limits. But what are they? How can we know? From the September 2009 edition of Nature:
The framework presented is an attempt to look holistically at how humanity is stressing the entire Earth system. Provocatively, they go beyond the conceptual to suggest numerical boundaries for seven parameters: climate change, ozone depletion, ocean acidification, biodiversity, freshwater use, the global nitrogen and phosphorus cycles, and change in land use. The authors argue that we must stay within all of these boundaries in order to avoid catastrophic environmental change.
... But even if the science is preliminary, this is a creditable attempt to quantify the limitations of our existence on Earth, and provides a good basis for discussion and future refinement.
The actual article, "A safe operating space for humanity" is worth a read if you have access. Interesting concept, even if the numbers themselves are incredibly preliminary.