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, recent progress has been made in applying the theory that some approaching tipping points carry early warning signals. Promising methods are based on detecting ‘critical slowing down’ in the rate that a system recovers from small perturbations.
The question for societies, policymakers and ecosystem managers becomes: What should we do about the prospect of approaching tipping points? Most existing cost-benefit analyses only allow for scientifically-unrealistic representations of tipping points, ignoring the uncertainty around them and the interactions between them. To address this we have put stochastically-uncertain, interacting tipping points into a well-known model of climate and the economy called ‘DICE’, to see how they affect the optimal policy response.
Results/Conclusions
The changing resilience of the climate system can be coupled to abrupt changes in ecosystems. Analysis of observational climate data reveals a pronounced slowing down of North Pacific sea surface temperature fluctuations over the last century, which can help explain well-known marine ecosystem ‘regime shifts’. As surface ocean temperature variability slowed down, marine ecosystems became prone to greater variability, and became more likely to cross tipping points.
The prospect of passing tipping points, including abrupt losses in the value of ecosystem service flows, can radically change optimal policy recommendations. Even when ecosystems and the biosphere are assumed to provide predominantly non-market values, because those services are not substitutable there is a strong incentive to avoid losing them.
Including tipping points in the DICE model increases the social cost of carbon – i.e. the damages expected from emitting one ton of fossil fuel carbon to the atmosphere – by up to 8-fold. This causes the optimal policy to switch from modest mitigation action that lets the climate warm 3C this century to shutting down all net greenhouse gas emission by 2050 and restricting global warming to 1.5C.
Thus even conventional economic theory can arrive at what should be a common-sense result for ecologists.