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A group of renowned Earth-system and environmental scientists define boundaries for the biophysical processes that determine the Earth's capacity for self-regulation . The framework 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 set based on existing data. For some processes, such as anthropogenic climate change and human modification of the nitrogen cycle, the report says that we may already have stepped out of safety zone, and need to back-pedal quickly. For others, such as ocean acidification, we are rapidly approaching a brink beyond which there may be abrupt and non-linear changes .
For the most part, the contributors to the report. agree that the ‘planetary boundaries’ framework is a useful and worthwhile endeavour. But they also issue cautions in choosing upper limits on environmental degradation. Some such as climatologist Myles Allen warns that setting a limit on long-term atmospheric carbon dioxide concentrations at 350 parts per million may distract from the much more immediate near-term challenge of limiting warming to 2°C. Whatever the long-term target, keeping temperatures to no more than 2°C above pre-industrial values will require substantial emissions reductions over the coming decades, says Allen. Without knowing what future generations might do, and without a good understanding of the long-term behaviour of the carbon cycle, the report argues that perhaps we should focus on what’s achievable in the near-term.
Furthermore, boundaries don't always apply on a global scale, even for processes that regulate the entire planet. Local circumstances can ultimately determine, for instance, how soon water shortages or biodiversity loss reach a critical verge.
Nontheless, the planetary boundaries concept and its first estimate of numeric values provides us with an important wake-up call of how close we are to overstressing the Earth.
Reference: Nature’s Special: Planetary Boundaries