We here augment the climate model in MIND from an impulse-response model ( 17) to a three-layer ocean model with much-improved representation of ocean heat uptake (see Materials and Methods). The macroeconomic and energy system model is coupled to an impulse-response climate model that simulates the global surface temperature response to anthropogenic greenhouse gas emissions ( 17, 18). Hence, our model does not allow for negative emissions in optimal mitigation strategies. For this prototypical study, we do not include the controversial technology option of carbon capture and storage. The individual investment paths represent MIND’s control variables. Various energy technologies are represented by individual capital stocks, including learning by doing, and would allow for individual investment paths. The energy system module resolves energy technologies in terms of fossil fuels, modern renewable energy sources, and traditional nonfossil energy sources. A Ramsey-type module of centennial economic growth ( 16) is coupled to an energy system module. The original MIND with cost-effectiveness analysis (CEA) has been developed and used to address the climate mitigation problem. We have substantially augmented the climate physics of the optimizing climate-energy-economy model MIND. A similar numerical analog method has previously been used to investigate targets for atmospheric CO 2 concentration ( 13). Overall, we have a well-posed method to introduce a numerical analog of a temperature target. As the time of correspondence, we choose the year 2200, which marks the end of the time horizon considered here, stressing that we focus on long-term SLR. Thus, we ask what maximum SLR would have been accepted by the proponent of the 2.0° and 1.5☌ targets within cost-effective decision-making and fix this as the new SLR target. To fairly compare the mitigation costs of SLR targets to temperature targets, we present a procedure by which the maximum allowable SLR of an SLR target is deduced from the implied SLR of a temperature target. An SLR target would directly connect to impacts of long-term SLR and thus be more directly relevant to coastal planning and adaptation measures related to SLR. In contrast to previous work, we investigate SLR targets with an integrated assessment model. Mitigation costs are determined by both the allowable cumulative carbon emissions and the timing of future carbon emissions. This further suggests that a temperature target is not only insufficient to limit long-term SLR but might also be insufficient to minimize the mitigation costs from the point of view of someone who is primarily interested in SLR as global warming impact. Furthermore, the two assumed emissions pathways could lead to different economic costs of mitigation. This difference between different emissions pathways arises because later emissions have a smaller effect on the near-term SLR but a larger effect on the future SLR due to its delayed response to radiative forcing. The former emissions pathway would lead to a larger SLR by the end of the emissions but to a smaller SLR rate after the end of the emissions, whereas the latter emissions pathway would lead to a smaller SLR by the end of the emissions but to a larger SLR rate after the end of the emissions. The two emissions pathways would approximately lead to the same surface warming at the end of the emissions. The contrast between SLR and surface warming in response to anthropogenic carbon emissions is illustrated by the follow thought experiment: We assume two emissions pathways with the same cumulative emissions, one with higher earlier emissions and lower later emissions and the other with lower earlier emissions and higher later emissions. The long-term SLR due to the melt of ice sheets and mountain glaciers is delayed by the thermal inertia and is determined by the time integral of surface warming temporal profile from present to future ( 10). SLR due to thermal expansion relates to warming over the full depth of the ocean and thus is mainly determined by the time integral of radiative forcing and hence the time profile of carbon emissions, while surface warming has been related to the upper ocean temperature and thus to the instantaneous radiative forcing and hence the cumulative carbon emissions ( 9). The response time scale of each SLR contributor to external forcing is much longer than those of surface warming ( 4, 6– 8). The present and future SLR is mainly driven by thermal expansion of the oceans, by the melt of glaciers and small ice caps, and by the melt of the Greenland and Antarctic ice sheets. SLR is an integrated index of climate response to external radiative forcing. SLR behaves qualitatively differently from surface warming in response to atmospheric radiative forcing.
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