Chapter 15 — The Economics of the Environment and Climate Change

Chapter 11 introduced externalities and previewed climate change as the largest externality in human history. This chapter gives it the full treatment.

Climate change is an economic problem in the most fundamental sense: it involves tradeoffs between present costs and future benefits, between one country's emissions and another country's damages, between the standard of living we want now and the planet we leave to future generations. The physics of climate change is the domain of climate science. The question of what to do about it — how much to spend, when to spend it, who should pay, which technologies to deploy, how to distribute the costs — is economics.

By the end of this chapter, you should understand the economic framework for analyzing climate change, the central policy tools (carbon tax, cap-and-trade, regulation, green industrial policy), and the most consequential economic debate about climate policy: the discount rate.

15.1 Climate change as an externality

When a coal-fired power plant generates electricity, it emits carbon dioxide. The CO₂ enters the atmosphere, where it persists for centuries, trapping heat. The warming changes rainfall patterns, raises sea levels, intensifies extreme weather, disrupts agriculture, and threatens ecosystems and human health around the world.

None of these costs is borne by the power plant or its customers. The electricity price reflects the cost of coal, labor, and capital. It does not reflect the cost of the climate damage the CO₂ causes. The gap between the private cost (what the plant pays) and the social cost (what the planet bears) is the externality — and it is the largest such gap in the history of human economic activity.

Three reasons climate is the hardest externality problem

1. It's global. A ton of CO₂ emitted in China has the same warming effect as a ton emitted in Iowa. The atmosphere doesn't respect borders. This means any effective solution requires international coordination — and no enforcement mechanism exists for international agreements. The Paris Agreement (2015) relies on voluntary national pledges; there is no penalty for non-compliance.

2. It's intertemporal. The costs of reducing emissions are borne now. The benefits (avoided climate damage) accrue over decades and centuries. This temporal mismatch is the core of the economic challenge, and it brings us to the discount rate debate (§15.3).

3. It's irreversible (on human timescales). CO₂ persists in the atmosphere for hundreds to thousands of years. Once emitted, it can't be taken back (at least not cheaply). The economic term is path dependence: today's emissions lock in future warming. If we wait too long to act, some consequences become unavoidable regardless of future actions.

These three features together make climate change qualitatively different from the local externalities in Chapter 11. The Walden Creek pollution case could be solved by a local Pigouvian tax or Coase bargaining among 300 fishing-club members. Climate change cannot be solved by any local mechanism. It requires global policy at a scale that has no historical precedent.

15.2 The social cost of carbon

To set a carbon price (whether through a tax or a cap-and-trade system), we need to estimate the social cost of carbon (SCC) — the total economic damage caused by emitting one additional ton of CO₂. This includes:

• Agricultural losses (reduced crop yields, changed growing seasons)
• Sea-level rise damages (flooding, coastal infrastructure destruction, forced migration)
• Extreme weather damages (more intense hurricanes, floods, droughts, wildfires)
• Health effects (heat-related mortality, disease vector changes, air pollution)
• Ecosystem damages (coral reef loss, species extinction, forest dieback)
• Productivity losses (outdoor workers less productive in extreme heat)

The U.S. government has published official estimates of the SCC. The Obama administration set it at about $50 per ton. The Trump administration lowered it to about $7 (by using a higher discount rate and excluding damages to other countries). The Biden administration set an interim value of about $51, and a 2023 update raised it to about $190 — reflecting newer climate models and a lower discount rate.

The range — from $7 to $190 — is not a sign that economists don't know what they're doing. It is almost entirely driven by one variable: the discount rate.

15.3 The discount rate debate: Stern vs. Nordhaus

The discount rate is the rate at which you value future benefits and costs relative to present ones. A high discount rate means future damages count for less in today's decision-making. A low discount rate means future damages count for almost as much as present damages.

This matters enormously for climate policy. The damages from climate change are concentrated in the future (2050, 2100, 2200). The costs of reducing emissions are concentrated in the present. The discount rate determines how you trade off present costs against future benefits.

Nordhaus — the higher-discount-rate view

William Nordhaus (Yale, Nobel Prize 2018) developed the DICE model (Dynamic Integrated Climate-Economy) — the most widely used integrated assessment model for climate economics. Nordhaus uses a discount rate in the range of 3–5% per year, which he argues reflects the actual rate of return on capital in the economy. The logic: if you can invest a dollar today and earn 4% per year, then a dollar of climate damage fifty years from now is worth about $0.14 in today's terms (discounted at 4%).

With this discount rate, Nordhaus's model recommends a gradual approach to climate policy: a modest carbon tax now (starting around $30–50 per ton), rising slowly over time as the economy decarbonizes. Aggressive near-term action is not justified because the costs of rapid decarbonization are high and the benefits are heavily discounted.

Stern — the lower-discount-rate view

Nicholas Stern (London School of Economics) authored the Stern Review on the Economics of Climate Change (2006), commissioned by the UK government. Stern uses a discount rate close to 1.4% per year — much lower than Nordhaus. The logic: we have no ethical justification for valuing future people's welfare less than our own. The discount rate should reflect only the (small) probability that future generations might not exist, not a preference for the present over the future.

With this discount rate, the Stern Review argues for urgent, aggressive action: a high carbon price now ($50–100+ per ton), rapid decarbonization, and immediate large-scale investment in clean energy. The present costs of aggressive action are more than justified by the enormous future damages that would be avoided.

Who is right?

Both positions are internally consistent. They disagree about a value judgment — how much should we discount the welfare of future people? — that economics alone cannot resolve.

Nordhaus's discount rate is descriptively accurate: it reflects the rate of return the market actually pays on investment. If you can earn 4% on your money, then a dollar today is worth more than a dollar in the future, and decisions should reflect that.

Stern's discount rate is normatively motivated: we should not value future people's welfare less than our own just because they haven't been born yet. Applying a market discount rate to intergenerational welfare is ethically questionable because future people don't get to participate in today's market.

The debate is not about the physics of climate change (both accept the IPCC projections). It is not about the economics of carbon pricing (both support a price on carbon). It is about a moral question — how much do we owe to future generations? — that masquerades as a technical question about the "right" discount rate.

Most climate economists have converged on a position somewhere between Stern and Nordhaus — a discount rate of 2–3% — which implies a social cost of carbon in the range of $100–200 per ton. This is the range that the Biden administration's 2023 update adopted. It implies a carbon price substantially higher than any currently in effect anywhere in the world.

15.4 Three policy approaches

Approach 1 — Carbon tax

A tax per ton of CO₂ emitted. The most straightforward application of the Pigouvian tax from Chapter 11.

How it works: every ton of CO₂ emitted by a power plant, factory, vehicle, or other source is taxed at a rate equal to (or approaching) the social cost of carbon. The tax raises the effective price of fossil fuels relative to clean alternatives, shifting production and consumption toward lower-carbon options.

Advantages: efficient (lets the market find the cheapest reductions); generates revenue (which can be rebated to consumers, used for clean-energy investment, or used to reduce other taxes); simple to implement (piggybacks on existing energy tax infrastructure); transparent (the price is known).

Disadvantages: politically very difficult ("tax" is a toxic word in many democracies); doesn't guarantee a specific quantity of emissions (it sets a price, not a cap); can be regressive (low-income households spend a larger share of income on energy).

About 90% of economists surveyed by the IGM Forum support a price on carbon in some form. The political support is much weaker — carbon taxes have been adopted in only a few jurisdictions (British Columbia, several Nordic countries, parts of Canada).

Approach 2 — Cap-and-trade

Sets a total emissions cap and distributes tradable permits. The acid rain program (Chapter 11) is the model.

How it works: the government sets a declining cap on total greenhouse gas emissions. Permits equal to the cap are distributed (by auction or free allocation). Firms that can reduce emissions cheaply sell their unused permits to firms that can't. The permit price reflects the marginal cost of the last ton reduced.

Advantages: guarantees a specific quantity of emissions (the cap is certain); efficient (firms find the cheapest reductions); can be linked across jurisdictions (the EU ETS trades across 30 countries).

Disadvantages: price volatility (the permit price fluctuates with economic conditions, making planning harder); initial allocation is politically contentious (free permits are windfall gifts to polluters; auctioned permits are de facto taxes); monitoring and enforcement are required.

The EU Emissions Trading System is the world's largest carbon market. It covers about 40% of EU greenhouse gas emissions and has gradually become more effective as the cap has tightened.

Approach 3 — Regulation and green industrial policy

Direct mandates and subsidies. This is what most countries actually do, because it's politically easier than carbon pricing.

Examples: - Fuel economy standards (CAFE standards in the U.S.) - Renewable energy mandates (requiring utilities to source a percentage of electricity from renewable sources) - Coal plant phase-outs (several countries have announced phase-out dates) - Clean energy subsidies (the U.S. Inflation Reduction Act of 2022, which provides about $370 billion in clean-energy tax credits and subsidies)

Advantages: politically feasible (subsidies are easier to pass than taxes); directly targets specific sectors; can drive technological change through R&D investment.

Disadvantages: less efficient than carbon pricing (the government picks winners rather than letting the market find the cheapest reductions); expensive (the IRA's cost is estimated at $370B–$1.2T over a decade); can create long-term subsidy dependence.

The IRA as a case study in green industrial policy

The Inflation Reduction Act of 2022 is the largest climate investment in U.S. history. It passed without any Republican support and was designed as a subsidy program (rather than a carbon tax) specifically because subsidies were politically feasible and taxes were not.

The IRA provides tax credits for: - Electric vehicles ($7,500 per qualifying vehicle) - Solar panels, wind turbines, and battery storage (production and investment credits) - Heat pumps and energy-efficient home upgrades - Clean hydrogen production - Carbon capture and storage - Clean manufacturing (batteries, solar panels, wind turbines made in the U.S.)

Early evidence (2023–2025) suggests the IRA is driving substantial clean-energy investment. The Rhodium Group estimates that the IRA will reduce U.S. greenhouse gas emissions by 32–42% below 2005 levels by 2030 (compared to about 25–30% without the IRA). Whether this is enough to meet U.S. commitments under the Paris Agreement is debated.

The IRA's approach — subsidize clean energy rather than taxing dirty energy — is less efficient in theory (it doesn't directly price the externality) but more politically durable in practice (nobody hates receiving a tax credit). Whether this tradeoff — efficiency for political feasibility — is worth it depends on whether you believe a carbon tax was actually feasible in 2022 (almost all political observers say it was not).

15.5 The political economy of climate policy

Why has the world been so slow to address climate change, given that the economics is clear and the science is even clearer?

1. Present bias (behavioral). The costs of action are immediate and visible (higher energy prices, economic disruption, lifestyle changes). The benefits are distant and diffuse (avoided climate damage in 2050, 2100). Chapter 10's present bias predicts exactly this pattern: humans systematically under-weight the future relative to the present. Climate policy asks people to accept present costs for future benefits — the hardest kind of policy to sell.

2. Concentrated costs, diffuse benefits. The fossil fuel industry bears concentrated costs from climate policy (their product becomes more expensive or regulated). The benefits are spread across all of humanity over centuries. The concentrated losers organize; the diffuse winners don't. This is the same political dynamic that explains trade protectionism (Chapter 9).

3. Free-riding. Each country benefits from other countries' emission reductions without having to reduce its own. The global externality + free-riding = collective action problem of the most severe kind. No enforcement mechanism for international agreements exists.

4. The just transition problem. The communities most dependent on fossil fuels (coal towns in Appalachia, oil-producing regions, petrochemical workers) face concentrated economic disruption from decarbonization. A transition that doesn't address their welfare will generate political backlash that delays or reverses climate policy. The phrase "just transition" captures this: the economic transformation must be fair to the workers and communities that lose, or it will not be politically sustainable.

15.6 The discount rate, revisited: what you owe the future

The Stern-Nordhaus debate isn't just a technical argument about a parameter in an economic model. It is a moral argument about what living people owe to people who haven't been born yet. This makes it one of the most consequential ethical questions of the 21st century.

The honest framing: we do not have a neutral, value-free way to determine the "right" discount rate. Every choice of discount rate embeds a value judgment about intergenerational equity. If you choose Nordhaus's rate (3–5%), you are saying that future people's welfare matters less than present people's — roughly half as much per generation. If you choose Stern's rate (~1.4%), you are saying that future people's welfare matters almost as much as present people's. If you choose a rate of 0%, you are saying that future people matter exactly as much as we do — and the cost-benefit analysis changes dramatically (almost any climate action is justified).

The reader's job is not to pick a number from a table. It is to understand what the number means — what value judgment it embeds — and to decide for themselves how much weight the future should get. The economics gives you the framework for thinking about it. The answer is yours.

15.7 Where this is going

Chapter 15 is the last of the "market failures that matter" chapters before the information-asymmetry chapter that closes Part III. The externality framework from Chapter 11, the surplus framework from Chapter 8, the behavioral framework from Chapter 10, and the inequality framework from Chapter 13 all converge here. Climate change is the topic where all of the micro toolkit meets the biggest question.

In Chapter 32 (Fiscal Policy), we'll see carbon tax revenue as a fiscal policy tool. In Chapter 39 (Where Economists Agree), we'll see that ~90% of economists support carbon pricing — one of the strongest areas of professional consensus in the field.

Key terms recap: social cost of carbon — the estimated dollar damage from one additional ton of CO₂ discount rate — the rate at which future costs and benefits are valued relative to the present Stern Review — the 2006 report arguing for urgent aggressive action (low discount rate) Nordhaus DICE model — the leading integrated assessment model (higher discount rate, more gradual action) carbon tax — a Pigouvian tax on CO₂ emissions cap-and-trade — tradable emission permits under a declining cap carbon border adjustment — a tariff on imports from countries without carbon pricing (to prevent "carbon leakage") stranded asset — a fossil fuel asset that loses value as the world decarbonizes just transition — ensuring that workers and communities dependent on fossil fuels are not left behind Inflation Reduction Act (IRA) — the 2022 U.S. law providing ~$370B+ in clean-energy subsidies and tax credits Paris Agreement — the 2015 international climate accord based on voluntary national pledges

Themes touched: Markets power+imperfect (the largest externality in history), Tradeoffs (present costs vs. future benefits, efficiency vs. equity, Stern vs. Nordhaus), Disagreement (the discount rate debate), Behavioral (present bias on climate), Affects daily life (energy prices, extreme weather, food costs, sea-level rise).