Johan Rockström and colleagues propose a new approach for defining preconditions for human development and predict that crossing certain biophysical thresholds could have disastrous consequences for humanity. They conclude that three of nine interlinked planetary boundaries have already been overstepped.
The “planetary boundaries” this paper proposes define the conditions under which Earth is habitable by human beings—in this sense the paper presents a biophysical understanding of habitability.
The Holocene is Earth’s recent period of environmental stability. That stability has probably been due to the absence of the massive transformations of the planet caused by humans. However, we are now facing a new era in which human actions have become the most important driver of global environmental change. This period is known as the Anthropocene, which is absolutely linked with the ways humans have been inhabiting the planet. Without pressure created by human habitation of Earth, the Holocene would be expected to continue for at least several thousand years.
“Planetary Boundaries” by Felix Mueller – Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons.
I totally agree that defining habitability in terms of a safe operating space with respect to the Earth system remains an important challenge, and I find Rockström et al.’s study rigorous and innovative. As they note, the boundaries they propose represent a new approach to defining biophysical preconditions for human habitability. However, I miss some other ethical considerations related to human behavior that can act as another planetary boundary. I argue that the size and level of development of human society should be also explicitly considered as a planetary boundary. For it is clear that these factors directly and indirectly affect the nine planetary boundaries Rockström et al. mention. This raises some big questions—for example, should we think about restricting population growth as a mechanism to mitigate the ongoing deterioration of Earth’s habitability?
I argue that, looking at the big picture, it is inevitable that we must face a conflict between human development and planetary conservation. The current model of sustainable development, based on the unlimited use of fossil fuels and industrialized forms of agriculture, must be re-defined. I believe solving the current ecological crisis—which is ultimately a crisis of human habitability—requires new interdisciplinary and holistic conceptual approaches. Therefore, a comprehensive audit of the planet not only should include biophysical boundaries that must not be transgressed; it must also involve the ethical and sociocultural considerations inherent to our understanding of how humans should inhabit the planet.
Inhabiting the Anthropocene 
Thanks, Antonio–and welcome to the blog!
I was just cruising around and found a recent post by Andrew Revkin on some recent discussion of the planetary boundaries idea. Rockström’s group apparently have updated them . . . and think a fourth boundary is being crossed. But there have been some suggestions for revisions as well–including one which adds some specifically social justice considerations (scroll down to see about the Oxfam report). And there have also been some critiques–scroll down to see the letter by Ellis et al.,–as well as responses from Steffan and Rockström. A nice supplement to your post!