Cap and Trade
What is Cap and Trade?
Cap and trade is a system that limits total pollution and lets firms buy and sell permits to emit within that cap.
The government sets a cap and issues tradable permits; firms that cut emissions cheaply can sell permits to those that can't. It puts a market price on pollution and achieves a target at the lowest total cost, addressing a negative externality.
Cap and Trade: a worked example
Two plants each emit 40 tons of a pollutant, so baseline emissions are 80 tons. The regulator caps emissions at 60 tons and hands each plant 30 permits, one permit per ton. Plant A can cut emissions for 20 dollars a ton, while Plant B faces 50 dollars a ton. Without trading, each must cut 10 tons: A pays 10 times 20, or 200 dollars, and B pays 10 times 50, or 500 dollars, for 700 dollars overall. Now allow trading. A cuts the full 20 tons for 20 times 20, or 400 dollars, dropping to 20 tons of emissions, then sells its 10 spare permits to B at 35 dollars each and collects 350. A ends up paying 50 dollars net instead of 200, and B pays 350 for permits instead of 500 to abate. Emissions still total 60 tons, reached for 400 dollars of real resources rather than 700.
The mistake students make with cap and trade
Students hear buy and sell permits and conclude that wealthy firms simply purchase the right to pollute more, so total emissions climb. The trade is zero sum in tons. Every permit B buys is a ton A no longer emits, and the number of permits is fixed before any trading happens, so aggregate pollution cannot exceed the cap regardless of who ends up holding them. A second slip counts the 350 dollars B pays as part of the cost of hitting the target. That payment is a transfer between two firms. Only the 400 dollars of abatement consumes real resources.
Cap and Trade questions
How does cap and trade actually reduce pollution?
The cap does the reducing, not the trading. Regulators issue a fixed number of permits below current emissions, and a firm may release only as many tons as it holds permits for, so total emissions cannot exceed the cap whoever ends up owning them. Trading then decides the distribution of the cuts. Firms with cheap abatement options cut deeper and sell their surplus, firms facing expensive cuts buy instead, and the environmental target is hit at the lowest total abatement cost.
What is the difference between cap and trade and a carbon tax?
Cap and trade fixes the quantity of emissions and lets the market discover the permit price. A carbon tax fixes the price per ton and lets firms choose how much to cut, so the final quantity stays uncertain. Choosing between them comes down to which uncertainty is more costly. When an environmental threshold is sharp, controlling the quantity is safer. When firms need predictable costs before committing to long-lived equipment, a published tax rate is easier to plan around.
Why do permits have a price if the government hands them out free?
Permits command a price because they are scarce and useful. A firm short of permits must cut a ton of emissions instead, and cutting costs money, so it will pay up to its own marginal abatement cost for one. A seller will not part with a permit for less than the abatement cost it avoids by keeping it. Bidding settles somewhere between the highest and lowest marginal abatement costs in the market, which is why free allocation still produces a real market price.
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