Every rational person knows that our planet is under ecological stress, and that human survival on planet Earth over the next several centuries is not assured. What constitutes a sustainable environment for human civilization on planet Earth? Geologists call the last 10,000 years — since well before the start of agriculture — the Holocene epoch. This period was characterized by much warmer temperatures and far less thermal variability than the other 400 thousand years about which we have reliable information. These unusually mild and stable environmental conditions may be necessary for the survival of human civilizations.

Eight years ago, 29 eminent environmental scholars (led by Johan Rockström of Stockholm University and Will Steffen of Australian National University) proposed nine quantitative planetary boundaries that, if not violated, would enable indefinite continuation of Holocene-like conditions (stockholmresilience.org). These nine planetary boundaries defined what these scholars called "a safe operating space for humanity." The nine planetary boundaries concerning these environmental conditions are: (1) climate change, (2) biosphere integrity (e.g. species extinction and genetic diversity), (3) land-system change (e.g. destruction of tropical, temperate and boreal forests), (4) freshwater use, (5) biogeochemical flows (e.


g. phosphorus and nitrogen pollution), (6) ocean acidification, (7) atmospheric aerosol loading (particles suspended in the atmosphere), (8) stratospheric ozone depletion and (9) introduction of novel entities that have undesirable physical or biological effects (e.g. excessive plastic, heavy metals, chlorofluorocarbons).

The environmental scholars defined three risk zones for each of these nine environmental conditions: a safe operating zone, a zone of uncertainty associated with increasing environmental risk, and a crisis zone characterized by high risk and serious danger. The nine planetary boundaries were originally located on the borders between the safe operating zone and the zone of uncertainty. However, information about these planetary boundaries was updated three years ago and published in the February 2015 issue of the prestigious journal Science.

According to the 2015 information, three of the nine ecological domains have not yet exceeded their respective planetary boundaries and are thus within safe operating zones: (4) freshwater use, (6) ocean acidification and (8) stratospheric ozone depletion. On the other hand, four of the nine domains have already surpassed their planetary boundaries: (1) climate change, (2) biosphere integrity, (3) land-system change and (5) biogeochemical flows. Climate change and land-system change still function within the zone of uncertainty, but biosphere integrity and biochemical flows are now within their hazardous crisis zones. Information about (7) atmospheric aerosol loading and (9) novel entities is currently insufficient to assign reliable global measures, but regional data about these conditions are troublesome.

The scholars who advance the planetary boundaries perspective resolutely maintain scientific objectivity and avoid unprofessional alarmism. Nevertheless, they emphasize that their nine environmental conditions are by no means independent and interact strongly with each other. These interactions could produce an unanticipated non-linear (e.g. exponential) response that might propel planet Earth far beyond the Holocene conditions that made human civilization possible. The scholars express particular apprehension about interactions involving climate change and biosphere integrity, which could each have truly disastrous consequences.

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