Near-optimal and Paris-compliant pathways robust to physical uncertainty, economic paradigms, and intergenerational equity
Thomas Bossy
Abstract
Integrated Assessment Models (IAMs) are used to explore the potential impacts of different policy options and socioeconomic developments on a range of outcomes, such as greenhouse gas emissions, climate change, and human well-being. They are therefore crucial for policy guidance.
The mitigation scenarios explored by the IPCC to keep climate warming below 2°C are products of modelling paradigms, such as the shape of cost functions or the optimisation methoddecision criteria, which require conceptual choices.
Although these paradigms are well identified, the impact robustness of conceptual choices on the scenarios promoted by the scientific community has only been partially explored.
Here, we treat a large ensemble of CO2 emission pathways, generated with a simplified carbon model, that are geophysically compatible with the Paris Agreement in a thousand different representations of the Earth system’s physics. We then construct and calibrate a generic cost function using IPCC data, which can reproduce most of the existing cost functions from aggregated IAMs. We then define near-optimal pathways and assess the consequences of conceptual choices on both the method of optimisationdecision criterion and the shape of the cost function.
We find that about half of the near-optimal pathways are common to all conceptual approaches and can be called economically robust. If we choose Net Present Value (NPV) as the decision criterion to optimise, almost 90% of the near-optimal pathways are common. The shape of the cost function becomes more discriminating when a minimax approach is chosen as the decision criterion for optimisation and the key factor whether or not inertia is taken into account.On the other hand, using a minimax approach as the optimisation paradigm discriminates near-optimal paths depending on whether inertia is considered or not. In this case, taking inertia into account reduces intergenerational inequity.
Interestingly, we show that it is harder to be “generationally” robust than economically robust as only 7% of the near-optimal pathways for the current generation (2021-2060) are also near-optimal for the future generation (2061-2100). Fortunately, these pathways are also economically robust to conceptual choices in the two modelling paradigms. They are characterised by early mitigation starting in 2023 and net negative emissions appearing later and to a reasonable extent, similar to or slightly more optimistic than comparable IPCC scenarios.
Overall, our results highlight the importance of considering conceptual choices when using aggregated IAMs to develop climate change mitigation policies. The consideration of physical uncertainty appears to be less critical as we find robust pathways in all configurations.