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In the 1970s, the physicist James Lovelock wrote about the planet Earth in relation to what he termed the Gaia hypothesis.

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The main idea was that Earth is a self‒regulating system that will seek to maintain equilibrium, initially including an implicit assumption that human acti‒ vities were insignificant in relation to Earth's homeostatic mechanisms.

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It was criticized already during its time, but suffered a more severe blow due to the observations of the Earth’s atmosphere, which yielded the discovery of the ozone hole over Antarctica in 1985.

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Chemicals produced by humankind did indeed change the atmosphere in ways that affect life on Earth in fundamental ways.

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The evidence of anthropogenic climate change also shows that the magnitude of power human society has over nature is large enough to be conceived as a geological force.

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More recently the discourse on global change has incorporated not only the notion of major gradual change, but also attention to thresholds or tipping points.

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These are abrupt changes in biophysical or social systems that can be the result of many interacting drivers of change.

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The term tipping point is often, but not always, used to describe a change that is difficult to reverse.

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It is a regime shift into a new stable state.

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Tipping points have been studied for some time in local ecosystems, such as lakes turning from clear to turbid.

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In the Arctic, there is increasing interest in understanding the risk for thresholds in both ecosystems and in linked social‒ecological systems.

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One could argue that the loss of Arctic sea ice in the summer, in and of itself and in terms of its consequences, is an example of the Arctic having passed an irreversible threshold.

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How‒ ever, given the complexity of the systems involved, we may not fully grasp for many years the significance of what is happening now.