Antonin Pottier

A NEW THEOREM TO FIND BERGE EQUILIBRIA

This paper examines the existence of Berge equilibrium. Colman et al. provide a theorem on the existence of this type of equilibrium in the paper [Colman, A. M., Korner, T. W., Musy, O. and Tazdait, T. [2011] Mutual support in games: Some properties of Berge equilibria, J. Math. Psychol. 55, 166–175]. This theorem has been demonstrated on the basis of a correspondence with Nash equilibrium. We propose to restate this theorem without using Nash equilibrium, and deduce a method for the computation of Berge equilibria.

BERGE–VAISMAN AND NASH EQUILIBRIA: TRANSFORMATION OF GAMES

In this paper, we reconsider the concept of Berge equilibrium. In a recent work, Colman et al. [(2011) J. Math. Psych.55, 166–175] proposed a correspondence for two-player games between Berge and Nash equilibria by permutation of the utility functions. We define here more general transformations of games that lead to a correspondence with Berge and Nash equilibria and characterize all such transformations.

Why are climate policies of the present decade so crucial for keeping the 2 °C target credible?

Decision-makers have confirmed the long term objective of preventing a temperature increase greater than 2 °C. This paper aims at appraising by means of a cost-benefit analysis whether decision makers’ commitment to meet the 2 °C objective is credible or not. Within the framework of a cost-benefit type integrated assessment model, we consider that the economy faces climate damages with a threshold at 2 °C. We run the model for a broad set of scenarios accounting for the diversity of “worldviews” in the climate debate. For a significant share of scenarios we observe that it is considered optimal to exceed the threshold. Among those “non-compliers” we discriminate ”involuntary non-compliers” who cannot avoid the exceedance due to physical constraint from ”deliberate compliers” for whom the exceedance results from a deliberate costs-benefit analysis. A second result is that the later mitigation efforts begin, the more difficult it becomes to prevent the exceedance. In particular, the number of ”deliberate non-compliers” dramatically increases if mitigation efforts do not start by 2020, and the influx of involuntary non-compliers become overwhelming f efforts are delayed to 2040. In light of these results we argue that the window of opportunity for reaching the 2 °C objective with a credible chance of success is rapidly closing during the present decade. Further delay in finding a climate agreement critically undermines the credibility of the objective.

Disentangling the Stern/Nordhaus Controversy: Beyond the Discounting Clash

The Stern/Nordhaus controversy has polarized the widely disparate beliefs about what to do in order to tackle the climate challenge. To explain differences in results and policy recommendations, comments following the publication of the Stern Review have mainly focused on the role played by the discount rate. A closer look at the actual drivers of the controversy reveals however that Stern and Nordhaus also disagree on two other parameters: technical progress on abatement costs and the climate sensitivity. This paper aims at appraising the relative impacts of such key drivers of the controversy on the social cost of carbon and climate policy recommendations. To this end, we use the flexible integrated assessment model RESPONSE which allows us to compare very diverse worldviews, including Stern and Nordhaus’ ones within the same modelling framework and map the relative impacts of beliefs on the three key drivers of the controversy. Furthermore we appraise quantitatively, by means of a linear statistical model, the impacts on results of an extended set of core parameters of RESPONSE. We show that beliefs on long term economic growth, technical progress, the form of the climate damage function and the climate sensitivity have an impact as important as beliefs on pure time preference. Hence, we can qualify the role played by the discount rate in the Stern/Nordhaus controversy and more broadly in the definition of climate policies.

The Comparative Impact of Integrated Assessment Models’ Structures on Optimal Mitigation Policies

This paper aims at providing a consistent framework to appraise alternative modeling choices that have driven the so-called “when flexibility” controversy since the early 1990s, dealing with the optimal timing of mitigation efforts and the social cost of carbon (SCC). The literature has emphasized the critical impact of modeling structures on the optimal climate policy. We estimate within a unified framework the comparative impact of modeling structures and investigate the structural modeling drivers of differences in climate policy recommendations. We use the integrated assessment model (IAM) RESPONSE to capture a wide array of modeling choices. Specifically, we analyse four emblematic modeling choices, namely the forms of the damage function (quadratic vs. sigmoid) and the abatement cost (with or without inertia), the treatment of uncertainty, and the decision framework, deterministic or sequential, with different dates of information arrival. We define an original methodology based on an equivalence criterion to compare modeling structures, and we estimate their comparative impact on two outputs: the optimal SCC and abatement trajectories. We exhibit three key findings: (1) IAMs with a quadratic damage function are insensitive to changes of other features of the modeling structure, (2) IAMs involving a non-convex damage function entail contrasting climate strategies, (3) Precautionary behaviors can only come up in IAMs with non-convexities in damage.

Climate Damage on Production or on Growth: What Impact on the Social Cost of Carbon?

Recent articles have investigated with integrated assessment models the possibility that climate damage bears on productivity (TFP) growth and not on production. Here, we compare the impact of these alternative representations of damage on the social cost of carbon (SCC). We ask whether damage on TFP growth leads to higher SCC than damage on production ceteris paribus. To make possible a controlled comparison, we introduce a measure of aggregate damage, or damage strength, based on welfare variations. With a simple climate-economy model, we compare three damage structures: quadratic damage on production, linear damage on growth and quadratic damage on growth. We show that when damage strength is the same, the ranking of SCC between a model with damage on production and a model with damage on TFP growth is not unequivocal. It depends on welfare parameters such as the utility discount rate or the elasticity of marginal social utility of consumption.