Introduction: The Externality That Ate the Planet
Climate change is, in the standard economic framing, the largest market failure in human history. Every tonne of carbon dioxide released imposes a cost on people who did not consent to it, were not compensated for it, and in many cases have not yet been born. The polluter does not pay. The price of fossil fuels does not reflect the damage they cause. Markets, left alone, will produce too much carbon — not because anyone is behaving badly, but because the price signal is broken.
The economic prescription follows directly from Pigou (1920): attach a price to the externality equal to its marginal social damage. Do this, and the market corrects itself. This is the intellectual foundation of every carbon tax and emissions trading scheme on Earth.
William Nordhaus received the 2018 Nobel Prize for building the models that turn this principle into a number. Nicholas Stern, in the 2006 Stern Review, produced a radically different number using the same framework. Understanding why two rigorous economists reached such different conclusions is the most instructive exercise in environmental economics.
1. The Theory: Externalities and the Pigouvian Solution
Marginal private cost versus marginal social cost
Consider a coal-fired power station. Its marginal private cost (MPC) includes fuel, labour, capital and maintenance. Its marginal external cost (MEC) is the damage its emissions inflict on third parties: heat stress, crop failure, sea level rise, biodiversity loss, health effects from particulates.
MSC = MPC + MEC
The unregulated firm equates price with MPC and produces Qmarket. The socially efficient output equates price with MSC and lies at Qoptimal < Qmarket. The triangle between MSC and MPC over the range (Qoptimal, Qmarket) is the deadweight welfare loss from overproduction.
Notice the crucial and frequently missed implication: the socially optimal quantity of pollution is not zero. Eliminating the last tonne of carbon costs more than the damage it causes. Efficiency requires abating up to the point where marginal abatement cost equals marginal damage — no further.
The Pigouvian tax
A tax set equal to MEC at the optimal quantity internalises the externality perfectly. The firm’s private cost curve shifts up by exactly the external cost. It voluntarily produces Qoptimal. No regulator needs to know each firm’s abatement technology; the firms sort that out themselves.
This is the deep virtue of price instruments over command-and-control regulation. A tax achieves any given emissions reduction at minimum total abatement cost, because it equalises marginal abatement cost across every emitter in the economy. A firm with cheap abatement options abates a lot; a firm with expensive options abates little and pays the tax. Regulation that mandates uniform percentage cuts cannot achieve this.
2. Price versus Quantity: Weitzman’s Enduring Insight
A carbon tax fixes the price of emissions and lets the quantity adjust. A cap-and-trade scheme fixes the quantity and lets the price adjust. Under certainty they are equivalent — any tax has a cap that produces the same outcome.
Under uncertainty they are not. Martin Weitzman established this in “Prices vs. Quantities” (Review of Economic Studies, 1974), and the result governs the entire policy debate.
The rule depends on the relative slopes of the marginal benefit (damage-avoided) and marginal cost (abatement) curves:
- If the marginal abatement cost curve is steeper than the marginal damage curve, quantity instruments (caps) are preferable. Errors in the cap cause modest cost errors.
- If the marginal damage curve is steeper, price instruments (taxes) are preferable.
For carbon dioxide, damages depend on the cumulative stock in the atmosphere, not on any single year’s flow. One year of emissions barely shifts the stock. The marginal damage curve is therefore nearly flat over any single period. Weitzman’s rule then implies that a carbon tax is the theoretically superior instrument for CO₂ — a conclusion most environmental economists accept, and most politicians ignore.
The practical compromise is the hybrid: a cap-and-trade scheme with a price floor and ceiling, which behaves like a tax when prices hit the bounds. The UK’s carbon price floor and California’s price collar both work this way.
3. The Social Cost of Carbon: One Number, Enormous Stakes
The social cost of carbon (SCC) is the present discounted value of the damage caused by emitting one additional tonne of CO₂. It is the number a Pigouvian carbon tax should equal.
Nordhaus’s DICE model (Dynamic Integrated Climate-Economy) — the work recognised by the 2018 Nobel — couples a simple economic growth model to a simple climate model, and solves for the emissions path that maximises discounted social welfare. His 2018 American Economic Journal: Economic Policy presentation of the model produced SCC estimates in the low tens of dollars per tonne, with optimal warming trajectories exceeding 3°C.
The Stern Review (2006) produced SCC figures roughly an order of magnitude higher, and recommended immediate aggressive abatement.
Why the disagreement? The discount rate.
Climate damages arrive in the future. To compare them with abatement costs paid today, we discount. The Ramsey rule decomposes the social discount rate:
r = ρ + η × g
where ρ is the pure rate of time preference (impatience), η is the elasticity of marginal utility of consumption (inequality aversion), and g is the growth rate of per-capita consumption.
- Nordhaus calibrated ρ to observed market interest rates, implying ρ around 1.5%. This is a descriptive approach: use the discount rate people actually reveal.
- Stern set ρ near 0.1%, on the ethical ground that there is no defensible reason to weight a future person’s welfare less than a present person’s, other than the probability of extinction. This is a prescriptive approach.
Over a 200-year horizon, the difference between 1.5% and 0.1% compounds into a factor of roughly twenty. The entire Stern–Nordhaus disagreement about how much to spend on climate reduces to a philosophical judgement about intergenerational ethics, dressed in the notation of economics. This is worth stating plainly in an exam, because it demonstrates that you understand what the model is doing rather than merely what it outputs.
Weitzman (2009), in his “Dismal Theorem,” pushed further: if the probability distribution of climate sensitivity has sufficiently fat tails, expected damages may be unbounded, and the optimal policy is driven not by best-guess damages but by catastrophic tail risk. Cost-benefit analysis, on this view, is the wrong framework altogether — the problem is closer to insurance against ruin.
The US Environmental Protection Agency’s 2023 revision of its SCC methodology, incorporating updated damage functions and lower discount rates, produced values substantially above the figures used in prior US regulatory analysis — moving official practice closer to the Stern end of the range.
4. Case Study: The Swedish Carbon Tax
Sweden introduced a carbon tax in 1991 and has escalated it steadily; it is now among the highest carbon prices in the world, exceeding €100 per tonne.
Julius Andersson, writing in American Economic Journal: Economic Policy (2019), evaluated the tax’s effect on transport-sector emissions using the synthetic control method. He constructed a weighted combination of comparison countries that tracked Sweden’s pre-1991 emissions trajectory closely, then compared actual Swedish emissions with this synthetic counterfactual after the tax.
Andersson found a substantial reduction in CO₂ emissions from transport relative to the synthetic control. He also found something theoretically important: consumers responded more strongly to the carbon tax than to an equivalent change in the market price of petrol. The tax elasticity exceeded the price elasticity.
This should not happen under standard theory — a dollar is a dollar. Andersson’s interpretation invokes salience and permanence: a legislated tax is understood to be durable and is publicly discussed, whereas a market price movement may be transitory. The finding connects directly to Chetty, Looney and Kroft’s (2009) work on tax salience.
Sweden’s experience also refutes a common political objection. Between 1990 and the present, Swedish GDP grew substantially while territorial emissions fell substantially — decoupling growth from carbon in a way that the “carbon taxes kill the economy” argument does not readily accommodate.
5. Case Study: The EU Emissions Trading System
The EU ETS, launched in 2005, is the world’s largest carbon market. Its history is a masterclass in mechanism design failure and repair.
Phase I (2005–2007): the over-allocation collapse
Member states allocated permits based on self-reported baselines. Firms over-reported. Permits were over-issued. When verified emissions data arrived in April 2006 revealing the surplus, the permit price collapsed toward zero. Phase I permits could not be banked into Phase II, so they became worthless.
Lesson: a cap-and-trade scheme is only as good as the cap, and a cap set by the regulated parties will be set too loosely.
Phases II–III: the surplus persists
The 2008 financial crisis cut industrial output and therefore emissions, but the cap was fixed. A vast permit surplus accumulated. Prices languished in single digits for years — far below any plausible social cost of carbon. The scheme was pricing carbon, but at a level that changed almost no behaviour.
Phase IV and the Market Stability Reserve
The EU introduced the Market Stability Reserve in 2019, which automatically withdraws permits from circulation when the surplus exceeds a threshold. This gave the cap a degree of price responsiveness it had previously lacked. EU carbon prices subsequently rose dramatically, moving into a range where they materially affect the merit order of electricity generation — that is, where coal becomes uneconomic relative to gas.
The Carbon Border Adjustment Mechanism (CBAM), phased in from 2023, addresses the remaining structural weakness: carbon leakage. If the EU prices carbon and China does not, emissions-intensive production migrates rather than abates, and global emissions may not fall at all. CBAM taxes the embodied carbon of imports at the EU price. It is the most consequential experiment in unilateral climate policy currently running, and its interaction with WTO law is unsettled.
6. Coase, Property Rights, and the Limits of the Bargaining Solution
Ronald Coase, in “The Problem of Social Cost” (Journal of Law and Economics, 1960), demonstrated a result that continues to be misunderstood. If property rights are clearly assigned and transaction costs are zero, private bargaining will achieve the efficient allocation regardless of who initially holds the rights. The initial assignment determines the distribution of the surplus, not the efficiency of the outcome.
Coase’s own point, however, was the opposite of the one usually attributed to him. He was not arguing that government intervention is unnecessary. He was arguing that transaction costs are the central object of study, and that in their presence, the assignment of rights matters enormously for efficiency.
For climate change, transaction costs are effectively infinite. The parties number in the billions, span centuries, include the unborn, and have no mechanism for enforceable agreement. The Coase theorem tells us exactly why the atmosphere cannot be handled by private bargaining — a negative result of considerable practical importance.
7. Distributional Effects and Political Economy
Carbon taxes are typically regressive in their direct incidence: low-income households spend a larger share of income on energy, and energy is carbon-intensive. This is the central political obstacle.
The economics of the solution is straightforward. A carbon tax raises revenue. Recycling that revenue as uniform lump-sum dividends makes the combined policy strongly progressive — because low-income households emit less carbon in absolute terms, they receive back more than they pay. British Columbia’s carbon tax was designed as revenue-neutral from its inception in 2008 and survived politically for over a decade; France’s 2018 fuel tax increase, which recycled no revenue to households, triggered the gilets jaunes protests and was withdrawn.
The lesson generalises. The efficiency case for carbon pricing is settled among economists. The design question that determines whether a carbon price survives contact with democracy is what happens to the revenue.
Additional design considerations:
- Double dividend: Goulder and others have examined whether recycling carbon revenue into cutting distortionary labour taxes yields a second efficiency gain. The theoretical literature finds the “strong” double dividend elusive — the tax-interaction effect erodes much of the gain — but a “weak” double dividend, in which revenue recycling is superior to lump-sum rebates on efficiency grounds, is robust.
- Grandfathering versus auctioning: Free permit allocation transfers rents to incumbent emitters without improving efficiency, and forgoes the revenue that could fund recycling.
8. Exam Technique
Cambridge A-Level
- Draw the MPC/MSC diagram correctly, labelling the deadweight loss triangle and the optimal tax as the vertical distance between MSC and MPC at Qoptimal — not at Qmarket. This is a very common error.
- For evaluation, always address: (i) information requirements for setting the tax, (ii) regressivity and revenue recycling, (iii) carbon leakage and international coordination, (iv) elasticity of demand for energy.
- Cite a real scheme. Sweden, the EU ETS and British Columbia are all examiner-friendly.
AP Microeconomics / Macroeconomics
- Know that the optimal level of pollution is positive, not zero.
- Be able to explain why a tax achieves a given abatement target more cheaply than a uniform mandate — the equalisation of marginal abatement costs.
- Understand tradable permits as achieving the same equalisation via a market rather than a price.
Summary
Carbon pricing is the rare policy on which the economics profession is close to unanimous, and on which political systems have found it hardest to act. The theory — Pigou, Coase, Weitzman, Nordhaus — is a century old and internally coherent. The empirics — Sweden, British Columbia, the EU ETS — show that carbon prices reduce emissions without collapsing economies.
What remains genuinely contested is not whether carbon should be priced but at what level, which turns on how much weight we place on the welfare of people not yet born. That is not a question economics can answer. It is a question economics can only make precise.
Exercises for Further Thought
1. Nordhaus and Stern used the same model structure, the same physics, and reached recommendations differing by an order of magnitude — almost entirely because of the pure rate of time preference. Nordhaus argued the discount rate should be inferred from observed market behaviour; Stern argued it should be chosen on ethical grounds. Which position is correct, and why? In your answer, consider whether the observed market interest rate can meaningfully be said to express a preference about people who do not yet exist and cannot participate in that market.
Suggested reading: Nordhaus, W. D. (2007). “A Review of the Stern Review on the Economics of Climate Change.” Journal of Economic Literature, 45(3), 686–702. Read it alongside Chapter 2 of the Stern Review itself, and note precisely where the two authors’ arguments stop being about economics.
2. Andersson (2019) found that Swedish consumers responded more strongly to a carbon tax than to an equivalent increase in the market price of petrol. Standard theory says this is impossible. Propose at least two mechanisms that could generate this result, and design an empirical test that would distinguish between them. What would it mean for climate policy if the “announcement effect” of a tax were a substantial part of its impact?
Suggested reading: Andersson, J. J. (2019). “Carbon Taxes and CO₂ Emissions: Sweden as a Case Study.” American Economic Journal: Economic Policy, 11(4), 1–30.
References
- Andersson, J. J. (2019). Carbon Taxes and CO₂ Emissions: Sweden as a Case Study. American Economic Journal: Economic Policy, 11(4), 1–30.
- Chetty, R., Looney, A., & Kroft, K. (2009). Salience and Taxation: Theory and Evidence. American Economic Review, 99(4), 1145–1177.
- Coase, R. H. (1960). The Problem of Social Cost. Journal of Law and Economics, 3, 1–44.
- Goulder, L. H. (1995). Environmental Taxation and the Double Dividend: A Reader’s Guide. International Tax and Public Finance, 2(2), 157–183.
- Nordhaus, W. D. (2007). A Review of the Stern Review on the Economics of Climate Change. Journal of Economic Literature, 45(3), 686–702.
- Nordhaus, W. D. (2018). Projections and Uncertainties about Climate Change in an Era of Minimal Climate Policies. American Economic Journal: Economic Policy, 10(3), 333–360.
- Pigou, A. C. (1920). The Economics of Welfare. London: Macmillan.
- Stern, N. (2007). The Economics of Climate Change: The Stern Review. Cambridge University Press.
- Weitzman, M. L. (1974). Prices vs. Quantities. Review of Economic Studies, 41(4), 477–491.
- Weitzman, M. L. (2009). On Modeling and Interpreting the Economics of Catastrophic Climate Change. Review of Economics and Statistics, 91(1), 1–19.