BOX 3.1 Tipping Points and the Point of No Return

There are sound theoretical reasons to think that tipping points across climatic thresholds exist (Gladwell, 2000; NRC, 2002). Examples of threshold behaviors include thermohaline circulation modifications, ice sheet instabilities, sea ice instabilities, soil-moisture feedbacks, and the onset of high-latitude convection and associated high-level cloud forcing. Hansen et al. (2008) introduced the term “tipping element” to describe subcontinental-scale subsystems of the Earth system that are susceptible to being forced into a new state by small perturbations. Tipping level—the magnitude of climate forcing beyond which, if sustained, abrupt climate change will eventually occur—is differentiated from “point of no return.” If the tipping level is exceeded for only a brief period of time, the original state of the system can be restored. More persistent forcing can push the system to the “point of no return,” where a reduction of the forcing below the tipping level is ineffective in halting the climate shift (Figure 3.1). This irreversibility of the system response is referred to as hysteresis (NRC, 2002).

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FIGURE 3.1 Equilibrium states of a “system” (valleys) in response to gradual anthropogenic CO2 forcing (progressing from dark to light blue). The curvature of the valley is inversely proportional to the system’s response time (τ) to small perturbations. A threshold is reached when the valley becomes shallower and finally vanishes causing the ball to abruptly roll to a new state (to the left).
SOURCE: Lenton et al. (2008), ©National Academy of Sciences, U.S.A.



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