Challenges arise when trying to define quantitatively the boundary position for each global process. First, a control variable is identified as a parameter that regulates each process, and then we try to identify the point at which science indicates that we are at risk of crossing a tipping point. Safe operating zones are indicated in green, uncertainty in yellow, and danger in red. The planetary boundary framework takes a normative stance by positioning the boundary at the lower end of uncertainty as a precautionary principle. For processes without global tipping points, the threshold is indicated at the point where the feedback could push, for example, the climate system across the tipping point. We are in the red for climate change, biodiversity loss ,and the nitrogen cycle, and this means that we may be inducing tipping points and may have created change at an irreversible scale. This framework is continuously being built upon and developed.
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