With climate change as prototype example, this paper analyzes the implications of structural uncertainty for the economics of low-probability, high-impact catastrophes. Even when updated by Bayesian ...learning, uncertain structural parameters induce a critical "tail fattening" of posterior-predictive distributions. Such fattened tails have strong implications for situations, like climate change, where a catastrophe is theoretically possible because prior knowledge cannot place sufficiently narrow bounds on overall damages. This paper shows that the economic consequences of fat-tailed structural uncertainty (along with unsureness about high-temperature damages) can readily outweigh the effects of discounting in climate-change policy analysis.
The "Stern Review" calls for immediate decisive action to stabilize greenhouse gases because "the benefits of strong, early action on climate change outweighs the costs." The economic analysis ...supporting this conclusion consists mostly of two basic strands. The first strand is a formal aggregative model that relies for its conclusions primarily upon imposing a very low discount rate. Concerning this discount-rate aspect, I am skeptical of the Review's formal analysis, but this essay points out that we are actually a lot less sure about what interest rate should be used for discounting climate change than is commonly acknowledged. The Review's second basic strand is a more intuitive argument that it might be very important to avoid possibly large uncertainties that are difficult to quantify. Concerning this uncertainty aspect, I argue that it might be recast into sound analytical reasoning that might justify some of the Review's conclusions. The basic issue here is that spending money to slow global warming should perhaps not be conceptualized primarily as being about consumption smoothing as much as being about how much insurance to buy to offset the small change of a ruinous catastrophe that is difficult to compensate by ordinary savings.
In this article, I revisit some basic issues concerning structural uncertainty and catastrophic climate change. My target audience here are general economists, so this article could also be viewed as ...a somewhat less technical exposition that supplements my previous work. Using empirical examples, I argue that it is implausible that low-probability, high-negative impact events would not much influence an economic analysis of climate change. I then try to integrate the empirical examples and the theory together into a unified package with a unified message that the possibility of catastrophic climate change needs to be taken seriously.
In textbook expositions of the equity-premium, riskfree-rate and equity-volatility puzzles, agents are sure of the economy's structure while growth rates are normally distributed. But because of ...parameter uncertainty the thin-tailed normal distribution conditioned on realized data becomes a thick-tailed Student-t distribution, which changes the entire nature of what is considered "puzzling" by reversing every inequality discrepancy needing to be explained. This paper shows that Bayesian updating of unknown structural parameters inevitably adds a permanent tail-thickening effect to posterior expectations. The expected-utility ramifications of this for asset pricing are strong, work against the puzzles, and are very sensitive to subjective prior beliefs-even with asymptotically infinite data.
The climate system is an angry beast and we are poking it with sticks†.
A critical issue in climate change economics is the specification of the so‐called “damages function” and its interaction with ...the unknown uncertainty of catastrophic outcomes. This paper asks how much we might be misled by our economic assessment of climate change when we employ a conventional quadratic damages function and/or a thin‐tailed probability distribution for extreme temperatures. The paper gives some numerical examples of the indirect value of various greenhouse gas (GHG) concentration targets as insurance against catastrophic climate change temperatures and damages. These numerical exercises suggest that we might be underestimating considerably the welfare losses from uncertainty by using a quadratic damages function and/or a thin‐tailed temperature distribution. In these examples, the primary reason for keeping GHG levels down is to insure against high‐temperature catastrophic climate risks.
Abating climate change is an enormous international public-goods problem with a classical "free-rider" structure. However, it is also a global "free-driver" problem because geoengineering the ...stratosphere with reflective particles to block incoming solar radiation is so cheap that it could essentially be undertaken unilaterally by one state perceiving itself to be in peril. This exploratory paper develops the main features of a free-driver externality in a simple model motivated by the asymmetric consequences of type-I and type-II errors. I propose a social-choice decision architecture, embodying the solution concept of a supermajority voting rule, and derive its basic properties.
The choice of an overall discount rate for climate change investments depends critically on how different components of investment payoffs are discounted at differing rates reflecting their ...underlying risk characteristics. Such underlying rates can vary enormously, from ≈ 1 percent for idiosyncratic diversifiable risk to ≈ 7 percent for systematic nondiversifiable risk. Which risk-adjusted rate is chosen can have a huge impact on cost-benefit analysis. In this expository paper, I attempt to set forth in accessible language with a simple linear model what I think are some of the basic issues involved in discounting climate risks. The paper introduces a new concept that may be relevant for climate-change discounting: the degree to which an investment hedges against the bad tail of catastrophic damages by insuring positive expected payoffs even under the worst circumstances. The prototype application is calculating the social cost of carbon.
Gamma Discounting Weitzman, Martin L.
The American economic review,
03/2001, Letnik:
91, Številka:
1
Journal Article
Recenzirano
By incorporating the probability distribution directly into the analysis, this paper proposes a new theoretical approach to resolving the perennial dilemma of being uncertain about what discount rate ...to use in cost-benefit analysis. A numerical example is constructed from the results of a survey based on the opinions of 2,160 economists. The main finding is that even if every individual believes in a constant discount rate, the wide spread of opinion on what it should be makes the effective social discount rate decline significantly over time. Implications and ramifications of this proposed "gamma-discounting" approach are discussed.
In this brief note (Without holding them responsible for errors, omissions, or interpretations, I am grateful for constructive comments on an earlier version of this note by Joseph Aldy, Severin ...Borenstein, Maureen Cropper, Carolyn Fischer, Meredith Fowlie, Lawrence Goulder, Geoffrey Heal, N. Gregory Mankiw, Michael Mehling, Gilbert Metcalf, Adele Morris, Ian Parry, William Pizer, Simon Quemin, Andrew Schein, Richard Schmalensee, E. Somanathan, Robert Stavins, David Victor, and Gernot Wagner.), I take the initial allocation of carbon emissions as a prototype international public goods problem. Overcoming the free-rider problem in carbon emissions is central to a successful comprehensive international climate-change agreement. Volunteerism alone may go part way, but is unlikely to fully adequately overcome this free-rider problem. (The numerical values of the pledged “Nationally Determined Contributions” under the Paris Agreement are voluntary, although the Paris Agreement itself may help constructively by laying a legal foundation for participation, reporting, verification, transparency, and trust.)
This paper argues that a uniform global tax-like price on carbon emissions, whose revenues each country retains, can provide a focal point for a reciprocal common climate commitment, whereas quantity ...targets, which do not nearly so readily present such a single focal point, tend to rely ultimately on individual quantity commitments. The paper postulates the conceptually useful allegory of a futuristic 'World Climate Assembly' (WCA) that votes for a single worldwide price on carbon emissions via the basic democratic principle of one person, one vote majority rule. A WCA-like uniform price-tax counters self-interest by incentivizing countries or agents to internalize the externality because each WCA agent's higher abatement cost from a higher emissions price is counterbalanced by that agent's extra benefit from inducing all other WCA agents to simultaneously lower their emissions in response to the higher price. The paper derives fresh insights and new simple formulae that relate each emitter's most-preferred world price of carbon to the world 'social cost of carbon' (SCC), and further relates the WCA-voted world price of carbon to the world SCC. Some implications are discussed. The overall methodology of the paper is a mixture of mostly classical with some behavioural economics.
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