Headline: Governing Planetary Boundaries: Limiting or Enabling Conditions for Transitions Towards Sustainability?

It seems intuitive to identify boundaries of an earth system which is increasingly threatened by human activities. Being aware of and hence studying boundaries may be necessary for effective governance of sustainable development. Can the planetary boundaries function as useful ‘warning signs’ in this respect? The answer presented in the article is: yes; but. It is argued that these boundaries cannot be described exclusively by scientific knowledge-claims. They have to be identified by science-society or transdisciplinary deliberations. The discussion of governance challenges related to the concept concludes with two main recommendations: to better institutionalise integrative transdisciplinary assessment processes along the lines of the interconnected nature of the planetary boundaries, and to foster cross-sectoral linkages in order to institutionalise more integrative and yet context sensitive governance arrangements. These insights are briefly confronted with options for institutional reform in the context of the Rio + 20 process. If humankind will not manage a transition towards sustainability, its ‘safe operating space’ continues shrinking. Governance arrangements for such ‘systems at risk’ may then be, first, more ‘forceful’ and, second, may run counter to our understanding of ‘open societies’. It is not very realistic that the world is prepared to achieve the first, and it is not desirable to get the effects of the latter. Scholars and practitioners of sustainability may find this a convincing argument to act now.