The idea that our planet has limits, including the burden placed upon it by human activities, has been around for some time. In 1972, The Limits to Growth was published. It presented a model in which five variables: world population, industrialization, pollution, food production, and resources depletion, are examined, and considered to grow exponentially, whereas the ability of technology to increase resources availability is only linear.
A threshold, or tipping point, is the value at which a very small increment for the control variable (like CO2) produces a very big change, even catastrophic, in the response variable (global warming).
Threshold points are difficult to locate, because the Earth System is very complex. Instead of defining the threshold value you can establish a range and the threshold is supposed to lie inside it. The lower end of that range is defined as the boundary. Therefore it defines a safe space, in the sense that as long as we are below the boundary, we are below the threshold value. If the boundary is crossed, we enter into a danger zone.
Transgressing one or more planetary boundaries may be highly damaging or even catastrophic, due to the risk of crossing thresholds that trigger non-linear, abrupt environmental change within continental– to planetary-scale systems.
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