| Summary: | One of the most controversial conclusions to emerge from many of the first generation of integrated assessment models (IAMs) of climate policy was the perceived economic optimality of negligible near-term abatement of greenhouse gases.Typically, such studies were conducted using smoothly varying climate change scenarios or impact responses. Abrupt changes observed in the climatic record and documented in current models could substantially alter the stringency of economically optimal IAM policies. Such abrupt climatic changes-or consequent impacts-would be less foreseeable and provide less time to adapt, and thus would have far greater economic or environmental impacts than gradual warming.We extend conventional, smooth IAM analysis by coupling a climate model capable of one type of abrupt change to a well-established energy-economy model (DICE).We compare the DICE optimal policy using the standard climate sub-model to our version that allows for abrupt change-and consequent enhanced climate damage-through changes in the strength (and possible collapse) of the North Atlantic thermohaline circulation (THC). We confirm the potential significance of abrupt climate change to economically optimal IAM policies, thus calling into question all previous work neglecting such possibilities-at the least for the wide ranges of relevant social and climate system parameters we consider. In addition, we obtain an emergent property of our coupled social-natural system model: "optimal policies" that do consider abrupt changes may, under relatively low discount rates, calculate emission control levels sufficient to avoid significant abrupt change, whereas "optimalpolicies" disregarding abrupt change would not prevent this non-linear event. However, there is a threshold indiscount rate above which the present value of future damages is so low that even very large enhanced damages in the 22nd century, when a significant abrupt change such as a THC collapse would be most likely to occur, do not increase optimal control levels sufficiently to prevent such a collapse. Thus, any models not accounting for potential abrupt non-linear behavior and its interaction with the discounting formulation are likely to miss an important set of possibilities relevant to the climate policy debate.
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