Carbon, Natural Capital and the Option Values of Climate Policies

We develop a unified cost-benefit framework that allows for a better understanding of nature conservation and climate policies under risk and uncertainty. We derive modified Hotelling rules from a social planner’s welfare optimization. They reveal four forces that jointly determine market design for climate and nature conservation: First, discounted marginal climate damages enter the social cost of carbon (SCC) and marginal ecosystem services the social value of nature (SVN). Second, climate and nature are coupled, which raises both prices: degradation of ecosystems increases the SCC, while climate damages raise the SVN. Third, a climate-nature beta quantifies additional hedging components of policies against fat tails, when we consider a stochastic setting with exogenous random shocks. The climate-nature beta summarizes the option values for abatement, adaptation, ecosystem restoration and carbon dioxide removal. Fourth, Markov markups quantify tipping risks, which we capture by extending the model to a constrained Markov decision process with state-contingent transition probabilities. Thereby, we endogenize tipping points: the likelihood of moving into a high-damage regime becomes a function of the atmospheric carbon stock and natural capital, which depend on policy choices. Thus, hazard risks are a policy-sensitive component of the system’s dynamics. The model yields state-contingent asset-pricing formulas for carbon prices, restoration subsidies, land charges, and capacity payments. We propose institutions at the level of the European Union that could implement Pigouvian taxes and subsides as well as new types of SCC- and SVN-indexed bonds to share non-diversifiable risks arising from Earth's changing climate and the degradation of its biosphere.