Johan Röckstrom recently gave a talk on Planetary Boundaries based on the papers Nature (doi:10.1038/461472a) and Ecology and Society.

In those papers the authors propose propose nine planetary boundaries, beyond which the functioning of the earth system will fundamentally change from the conditions in which human civilization has emerged.  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.

The talk is now up in two parts on YouTube (but the quality is only ok).