Planetary boundaries: Estimating a safe operating space for humanity
Will Steffen
Background/Question/Methods

The present geological epoch, the 10,000-year old Holocene, has provided an accommodating and supportive environment for the development of the human enterprise, from the advent of agriculture to the contemporary societies we enjoy today. But we are now leaving the Holocene and entering the Anthropocene, a new epoch driven by a wide range of human activities that are fundamentally changing our environment at the global scale. There is evidence that many of these global changes may well be deleterious to human well-being, prompting concern that we should limit our activities to ensure that the Earth remains within the Holocene environmental envelope. The concept of planetary boundaries provides a new framework for developing effective policy for stewardship of the Earth System.
Results/Conclusions Planetary boundaries are based on intrinsic features of the Earth System, in particular the fact that large sub-systems can undergo threshold/abrupt change behaviour which can push the sub-system to a new state. These potential alternate states are outside of human experience and outside of the Holocene environmental envelope; many would lead to serious impacts on human societies. We have identified nine critical Earth System processes or sub-systems that either are subject to threshold/abrupt change behaviour or act as slow variables that influence the resilience of the Earth System as a whole.  For each we propose a planetary boundary, a quantitative estimate of where the “no-go” zone lies; beyond this boundary the risk of undesirable change becomes unacceptably high. Collectively, this set of nine planetary boundaries defines a safe operating space for humanity, within with we can continue to evolve and develop.