In the latest issue of Nature, a group of leading academics argue that humanity must stay within defined boundaries for a range of essential Earth-system processes to avoid catastrophic environmental change (Nature 461, 472–475; 2009). In proposing the concept of 'planetary boundaries', Johan Rockström of the Stockholm Resilience Centre and co-authors present a new framework for measuring stress to the Earth system and define a safe operating space for human existence on this planet.
Rockström and co-authors suggest preliminary boundaries for the following indicators of environmental change: climate, ocean acidification, stratospheric ozone depletion, freshwater use, biodiversity, the global cycles of nitrogen and phosphorus, and land-use change. They propose that for three of these — the nitrogen cycle, the rate of loss of species and anthropogenic climate change — the maximum acceptable limit has already been transgressed. In addition, they say that humanity is fast approaching the boundaries for freshwater use, for converting forests and other natural ecosystems to cropland and urban areas, and for acidification of the oceans. Crossing even one of these planetary boundaries would risk triggering abrupt or irreversible environmental changes that would be very damaging or even catastrophic for society. Furthermore, if one boundary is transgressed, then there is a more serious risk of breaching the other boundaries.
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