Back in 2013, I led some research that critiqued the ‘Planetary Boundaries‘ concept (my refereed paper, Does the terrestrial biosphere have planetary tipping points?, appeared in Trends in Ecology & Evolution). I also blogged about this here: Worrying about global tipping points distracts from real planetary threats.
Today a new paper appeared in the journal Science, called “Planetary boundaries: Guiding human development on a changing planet“, which attempts to refine and clarify the concept. It states that four of nine planetary boundaries have been crossed, re-imagines the biodiversity boundary as one of ‘biodiversity integrity’, and introduces the concept of ‘novel entities’. A popular summary in the Washington Post can be read here. On the invitation of New York Times “Dot Earth” reporter Andy Revkin, my colleagues and I have written a short response, which I reproduce below. The full Dot Earth article can be read here.
Steffen et al (2015) revise the “planetary boundaries framework” initially proposed in 2009 as the “safe limits” for human alteration of Earth processes(Rockstrom et al 2009). Limiting human harm to environments is a major challenge and we applaud all efforts to increase the public utility of global-change science. Yet the planetary boundaries (PB) framework – in its original form and as revised by Steffen et al – obscures rather than clarifies the environmental and sustainability challenges faced by humanity this century.
Steffen et al concede that “not all Earth system processes included in the PB have singular thresholds at the global/continental/ocean basin level.” Such processes include biosphere integrity (see Brook et al 2013), biogeochemical flows, freshwater use, and land-system change. “Nevertheless,” they continue, “it is important that boundaries be established for these processes.” Why? Where a global threshold is unknown or lacking, there is no scientifically robust way of specifying such a boundary – determining a limit along a continuum of environmental change becomes a matter of guesswork or speculation (see e.g. Bass 2009;Nordhaus et al 2012). For instance, the land-system boundary for temperate forest is set at 50% of forest cover remaining. There is no robust justification for why this boundary should not be 40%, or 70%, or some other level.
While the stated objective of the PB framework is to “guide human societies” away from a state of the Earth system that is “less hospitable to the development of human societies”, it offers little scientific evidence to support the connection between the global state of specific Earth system processes and human well-being. Instead, the Holocene environment (the most recent 10,000 years) is assumed to be ideal. Yet most species evolved before the Holocene and the contemporary ecosystems that sustain humanity are agroecosystems, urban ecosystems and other human-altered ecosystems that in themselves represent some of the most important global and local environmental changes that characterize the Anthropocene. Contrary to the authors’ claim that the Holocene is the “only state of the planet that we know for certain can support contemporary human societies,” the human-altered ecosystems of the Anthropocene represent the only state of the planet that we know for certain can support contemporary civilization.