The Social and Policy Implications of Climate Tipping Points

A ‘tipping point’ occurs when a small change in forcing triggers a large and abrupt response from a system, qualitatively changing its future state.  At the scale of ecosystems, some tipping points have already been observed, and more are anticipated in future, triggered by multiple drivers including climate change.  At the global-scale several ‘tipping elements’ have been identified in the Earth system that may pass a tipping point due to climate change this century.  Our capacity to forecast such abrupt, non-linear changes has historically been poor.  However, much interest has recently been generated by the theory that some approaching tipping points carry early warning signals. 

I will explore the prospects for gaining early warning of approaching climate tipping points.  Promising methods are based on detecting ‘critical slowing down’ in the rate that a system recovers from small perturbations, and on accompanying changes in the statistical distribution of its behaviour.  I will show examples of early warning signals prior to past abrupt climate changes, and in models being gradually forced past climate tipping points.  Then I will show an example from observational climate data – a pronounced slowing down of North Pacific sea surface temperature fluctuations over the last century.  This has important implications for well-known marine ecosystem ‘regime shifts’ in the North Pacific.  As surface ocean temperature variability slowed down, marine ecosystems became prone to greater variability, and became more likely to cross tipping points.

The question for societies and policymakers becomes: What should we do about the prospect of approaching climate tipping points?  Most existing cost-benefit analyses of climate change only allow for simple and scientifically unrealistic representations of climate tipping points, ignoring the uncertainty around them and the interactions between them.  To address this we have put the possibility of multiple interacting tipping points into a well-known model of climate and the economy called ‘DICE’, to see how they affect the optimal policy response.  DICE is one of the models used to set the US Federal social cost of carbon – i.e. the damages expected from emitting one ton of fossil fuel carbon to the atmosphere.  I will show that including tipping points in the DICE model increases the social cost of carbon 8-fold.  Hence the prospect of climate tipping points should be leading to much more decisive global action to tackle the causes of climate change.