Reducing the risk of a collapse of the Atlantic thermohaline circulation

The ability of mitigation to reduce the likelihood of a collapse of the Atlantic thermohaline circulation (THC) is explored given profound uncertainty in our understanding of climate sensitivity and THC processes. At the current time, uniform distributions across the ranges of this uncertainty puts the likelihood of a collapse sometime over the next 200 years at roughly 2 chances in 3 without mitigation from a single baseline emissions trajectory produced by the Nordhaus and Boyer DICE-99 economic model. The subjective likelihood declines with mitigation, and can be influenced by alternative prior distributions, but even the immediate imposition of extremely stringent climate policy would leave a 1 in 4 chance of a THC collapse in the uniform distribution case. Other representations of profound uncertainty are also explored. In all cases, waiting 30 years to act increases the odds of a collapse significantly.