There is a provacative series running in Nature about planertary boundaries and a “safe operating space for humanity”. Everything in the series is free/open access here. There is a main article and a series of essays and editorials about the approach. Note, none of this is peer-reviewed science. I have mixed feeling about it. At first, it seemed like a useful framework, at least to scare people. But after reading the paper and thinking about the boundaries (see the table below) the whole exercise seems arbitrary and subjective. But when you are a gray-beared elder scientist, with your own institute in Stockholm, I suppose you can say anything you want in Nature or Science.
In this issue of Nature, a group of renowned Earth-system and environmental scientists led by Johan Rockström of the Stockholm Resilience Centre sets out to define boundaries for the biophysical processes that determine the Earth’s capacity for self-regulation (see page 472). The framework presented is an attempt to look holistically at how humanity is stressing the entire Earth system. Provocatively, they go beyond the conceptual to suggest numerical boundaries for seven parameters: climate change, ozone depletion, ocean acidification, biodiversity, freshwater use, the global nitrogen and phosphorus cycles, and change in land use. The authors argue that we must stay within all of these boundaries in order to avoid catastrophic environmental change.
The boundaries are based on existing data. For some processes, such as anthropogenic climate change and human modification of the nitrogen cycle, we may already have crossed the line, and need to back-pedal quickly. For others, such as ocean acidification, we are rapidly approaching a threshold beyond which there may be abrupt and nonlinear changes.
The exercise requires many qualifications. For the most part, the exact values chosen as boundaries by Rockström and his colleagues are arbitrary. So too, in some cases, are the indicators of change. There is, as yet, little scientific evidence to suggest that stabilizing long-term concentrations of carbon dioxide at 350 parts per million is the right target for avoiding dangerous interference with the climate system. Focusing on long-term atmospheric concentrations of the greenhouse gas is perhaps an unnecessary distraction from the much more immediate target of keeping warming to within 2 °C above pre-industrial levels. Nor is there a consensus on the need to cap species extinctions at ten times the background rate, as is being advised.