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, this uncertainty puts likelihood of a collapse sometime over the next 200 years to be more than 2 chances in 3 without mitigation from a single baseline emissions trajectory produced by the Nordhaus and Boyer DICE-99 economic model. The likelihood declines with mitigation, but the immediate imposition of extremely stringent climate policy would leave a 1 in 4 chance of a THC collapse. Waiting 30 years to act increases the odds to more than a 1 chance in 3. Key Words: climate policy, thermohaline circulation collapse, profound uncertainty2 The collapse of the Atlantic thermohaline circulation (THC) is the primary example of possible non-linear impacts of climate change that were highlighted by the Intergovernmental Panel on Climate Change as a significant “source of concern ” for

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