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Premium experiment • a pollution cap with tradable permits

ECONOMICS • ENVIRONMENTAL ECONOMICS

Cap and Trade, Permit Prices & Emissions Abatement

Explore cap and trade, permit prices and emissions abatement. Tighten the cap and see how firms with different costs share pollution reductions.

● CAP AND TRADEemissions cap → permit price → abatement
Total emissions
Required abatement
Permit price
Abatement cost

Reasoning inspector

live model state
Total emissions—
Required abatement—
Permit price—
Total abatement cost—
Firms—
Emissions cap—

Experiment with the mechanism

A cap-and-trade system fixes total allowable emissions and lets firms trade permits. Firms with low abatement costs have an incentive to reduce more and sell permits, while high-cost firms prefer to buy permits until marginal abatement costs become aligned.

LEARN WITH THE SIMULATION

Understand Cap and Trade, Permit Prices & Emissions Abatement by experimenting.

START HERE

Cap and trade is a market-based way to control total pollution. A regulator sets an overall emissions cap and issues a limited number of permits. Firms can trade those permits, so companies with cheap abatement have an incentive to reduce more pollution while companies facing expensive abatement can buy permits instead.

BUILD THE IDEA

What you should understand first

  • Separate the emissions cap from the permit price.
  • Explain why firms with lower marginal abatement costs reduce more emissions.
  • Understand how trading can meet a fixed emissions target at lower total cost than identical firm-level limits.

TRY IT YOURSELF

What happens when the emissions cap becomes tighter?

  1. Record total emissions, required abatement and permit price at the default cap.
  2. Lower the cap while keeping baseline emissions and cost conditions fixed.
  3. Replay the firm-level abatement bars.
  4. Compare required total abatement, the permit price and total abatement cost.
Read the visual:

A tighter cap requires more total abatement. Firms must move further up their abatement-cost schedules, so the common permit-price signal rises.

WHY IT WORKS 01

Why trading shifts abatement toward low-cost firms

Suppose one firm can eliminate a unit of pollution for much less than another. If permits can be traded, the low-cost firm can reduce an extra unit and sell a permit, while the high-cost firm can buy that permit instead of making an expensive reduction. Both firms can benefit from the trade. The environmental result is unchanged because total permits still equal the emissions cap.

WHY IT WORKS 02

Why marginal abatement costs tend to converge

If one firm's marginal abatement cost is below the permit price, reducing another unit of emissions and selling the freed permit is profitable. If another firm's marginal abatement cost is above the permit price, buying a permit is cheaper than making that reduction. Trading continues until those opportunities largely disappear, pushing marginal abatement costs toward the shared permit-price signal.

WHY IT WORKS 03

Why the cap and the price play different roles

The regulator chooses the emissions quantity through the cap, while the permit market helps discover the price needed to make firms collectively respect that quantity. Tightening the cap makes permits scarcer and requires more abatement, which tends to raise the permit price. This is different from a carbon tax, where the regulator fixes the price and lets the resulting emissions quantity adjust.

DO NOT MIX THESE UP

Common misconception

Cap and trade does not mean firms are allowed to pollute without limit. The cap fixes the total quantity of emissions allowed. Trading only changes who performs the reductions. The permit price is also not chosen directly in a competitive permit market; it emerges from scarcity and firms' marginal abatement costs.

PRACTICE CHALLENGE

Can two firms reduce very different amounts of pollution and still face the same permit price?

  • Use several firms with heterogeneous abatement-cost slopes.
  • Replay the allocation of abatement.
  • Compare low-cost and high-cost firms.
  • Explain why different physical reductions can still be consistent with a common market permit price.
Reveal the reasoning

Yes. Firms with flatter abatement-cost schedules can reduce more emissions before their marginal cost reaches the permit price. Firms with steeper cost schedules reach that same marginal-cost level after less abatement. The efficient trading outcome therefore does not require equal pollution cuts across firms; it requires the marginal cost of the last unit reduced to be aligned around the common permit price.

MODEL BOUNDARIES

What this simulation assumes

Every simulation simplifies reality or a mathematical system. These are the assumptions and limits to keep in mind while interpreting the results.

Model assumptions

  • A fixed baseline emissions level is distributed across heterogeneous firms.
  • Firms have rising marginal abatement costs and can trade permits freely.
  • The emissions cap is binding when it is below baseline emissions.

Model limitations

  • Banking, borrowing, market power, uncertainty, monitoring and transaction costs are omitted.
  • The model focuses on cost-effective allocation of a fixed emissions target, not full climate policy design.

CONNECT IT

Where this matters

  • Carbon markets
  • Air pollution policy
  • Environmental regulation
  • Market-based emissions control

WORDS THAT MATTER

Key vocabulary

Emissions cap
The maximum total emissions allowed by the program.
Permit
A tradable authorization to emit a specified amount.
Marginal abatement cost
The cost of reducing one additional unit of emissions.
Cost effectiveness
Achieving a given environmental target at the lowest total abatement cost.