Planetary boundaries: Guiding human development on a changing planet.

The planetary boundaries framework defines a safe operating space for humanity based on the intrinsic biophysical processes that regulate the stability of the Earth system. Here, we revise and update the planetary boundary framework, with a focus on the underpinning biophysical science, based on targeted input from expert research communities and on more general scientific advances over the past 5 years. Several of the boundaries now have a two-tier approach, reflecting the importance of cross-scale interactions and the regional-level heterogeneity of the processes that underpin the boundaries. Two core boundaries--climate change and biosphere integrity--have been identified, each of which has the potential on its own to drive the Earth system into a new state should they be substantially and persistently transgressed. [ABSTRACT FROM AUTHOR]