A number of resilience researchers, and many others, have proposed the concept of planetary boundaries in a new paper A safe operating space for humanity in Nature (doi:10.1038/461472a).
Johan Rockstrom and others propose nine planetary boundaries, beyond which the functioning of the earth system will fundamentally change. They argue that we have crossed the climate, nitrogen and extinction boundaries, and need to change the course of our civilization to move back into conditions which provide a safety for human civilization.
Nature has a special feature on Planetary Boundaries. It has also published seven independent essays by experts who reflect upon each of the defined boundary (two of the nine were not defined due to a lack of information), and their blog Climate Feedback is also hosting a discussion of the article.