How do carbon tax and cap-and-trade differ as policy instruments?

• A. Both policies are identical in mechanism and outcomes.
• B. A carbon tax sets a quantity limit; cap-and-trade sets a price ceiling.
• C. A carbon tax subsidies fossil fuels; cap-and-trade bans emissions entirely.
• D. A carbon tax sets a price on emissions; cap-and-trade sets a cap on total emissions and allows trading permits; both aim to reduce emissions.

Answer: (D)

The main idea is that carbon tax and cap-and-trade differ in how they set the constraint on emissions and how price signals are created. A carbon tax puts a price on carbon by charging a tax per ton of CO2 emitted. Firms respond by reducing emissions when the cost of abatement is cheaper than the tax, so emissions fall in response to the tax level. But the exact amount of emissions isn’t fixed; it depends on how much abatement occurs at that price.

Cap-and-trade, on the other hand, imposes a hard limit on total emissions by issuing a limited number of permits. Each firm must hold enough permits to cover its emissions, and permits can be bought and sold, letting reductions occur where they’re cheapest. This guarantees a specific emissions outcome (the cap) but the price per ton can fluctuate with market conditions.

Both tools use market signals to cut emissions cost-effectively, but only one guarantees a particular cap while the other guarantees a price. The other options don’t capture this distinction: a carbon tax does not set a quantity limit, cap-and-trade does not fix a price, and subsidies or bans are not what these instruments implement.