provide). Responsiveness to uncertainty: As with cost changes, in a world of uncertainty, it is not clear whether emissions fees or cap-and-trade systems are more efficient—it depends on how fast the marginal social benefits of reducing pollution fall with the amount of cleanup (e.g., whether inelastic or elastic marginal social benefit schedule). Other: The magnitude of the tax will depend on how sensitive the supply of emissions is to the price. The permit price of cap-and-trade will depend on the pollutant market. A tax generates government revenue, but full-auctioned emissions permits can do the same. A similar upstream cap-and-trade system could be implemented. An upstream carbon tax might be the simplest to administer. Setting up a complex cap-and-trade arrangement that is comprehensive has high institutional needs. Command-and-control regulation Command and control is a system of regulation that prescribes emission limits and compliance methods for each facility or source. It is the traditional approach to reducing air pollution. Command-and-control regulations are more rigid than incentive-based approaches such as pollution fees and cap and trade. An example of this is a performance standard which sets an emissions goal for each polluter that is fixed and, therefore, the burden of reducing pollution cannot be shifted to the firms that can achieve it more cheaply. As a result, performance standards are likely to be more costly overall. The additional costs would be passed to end consumers. Trading systems Apart from the dynamic development in carbon emission trading, other pollutants have also been targeted. United States Sulfur dioxide An early example of an emission trading system has been the sulfur dioxide (SO2) trading system under the framework of the Acid Rain Program of the 1990 Clean Air Act in the U.S. Under the program, which is essentially a cap-and-trade emissions trading system, SO2 emissions were reduced by 50% from 1980 levels by 2007. Some experts argue that the cap-and-trade system of SO2 emissions reduction has reduced the cost of controlling acid rain by as much as 80% versus source-by-source reduction. The SO2 program was challenged in 2004, which set in motion a series of events that led to the 2011 Cross-State Air Pollution Rule (CSAPR). Under the CSAPR, the national SO2 trading program was replaced by four separate trading groups for SO2 and NOx. SO2 emissions from Acid Rain Program sources have fallen from 17.3 million tons in 1980 to about 7.6 million tons in