Abstract

The term tipping point usually refers to a critical threshold at which a tiny perturbation can qualitatively alter the state and development a system. The term tipping element has been introduced to describe large-scale components of the Earth System that may pass a tipping point. Of particular interest are those elements in the climate system that may be tipped by human activities this century and undergo a qualitative change before the next millennium. We have critically reviewed these potential policy-relevant tipping elements, draw- ing on the fast-increasing pertinent literature and an international workshop, in order to compile a master list and assess where their tipping points lie. For an important subset, their sensitivity to global warming and the uncertainties in this are ranked using the results from an expert elicitation exercise. The Greenland Ice Sheet emerges as the tipping element with the nearest threshold and the least uncertainty in this. The majority of causal connections identified between tipping elements are positive where tipping one element encourages tipping another. This raises the alarming possibility of human activities triggering domino dynamics in the climate system. The implications for human societies and climate policy are profound. In principle, early warning systems could be established to detect the proximity of some tipping points, but in practice the necessary long time-series of high resolution observations are lacking. Recognizing the nonlinearity in damage costs associated with passing a tipping point fundamentally alters the minimisation problem for the combined costs of mitigation and adaptation, shifting temperature targets accordingly. On the other hand, the accessibility of certain tipping points in the socio-economic system may even provide no-regret options for triggering the transition to a low carbon economy.