Climate Tipping Points

The concept of a “tipping point” refers to a critical threshold at which even a small perturbation triggers a significant response causing a system to move from one equilibrium state to another. A glass on the verge of tipping over provides a good visualisation of the issue at hand. When held at any angle below the critical value and released, the glass tends to move back to a stable upright position. However, when balanced at the required threshold it is extremely sensitive to even the lightest touch and a small force applied in the correction direction results in a rapid transition to a new stable state. This concept can also be applied to the climate system, where a small change in a driving parameter such as mean global temperature can lead to a disproportionate response from a number of components within the system. The new equilibrium state, or even the transition to that state, of such components tends to result in large scale impacts on the earth system which may manifest, for instance, as significant sea level rise, an increased incidence of extreme weather events and widespread forest dieback. Therefore the assessment (Lenton et al., 2008, hereafter L08), or reassessment (Smith et al., 2009), of likely so-called tipping elements, their respective thresholds and probabilities (Kriegler et al., 2009), and possible prediction or early warning of their arrival (Scheffer et al., 2009; Lenton, 2011) have recently received much discussion in the literature. In the following sections this work shall first review which tipping elements have been identified in the climate system and then go on to discuss the current state of our ability to provide early warnings of their onset as global temperatures continue to rise and we approach their respective predicted thresholds. The tipping elements focused on in this work are those discussed in L08 and therefore follow their definition of being potentially policy-relevant. That is, each element meets the inequality given in their point 1, is at least subcontinental in scale (∼ 1000 km) and is considered as policy-relevant if: