If the higher cost estimates are right, however, and firms did purchase additional permits, the emissions reduction would be less. But the burden on the economy would be limited to a predictable level. Policy can set either price or quantity targets with some assurance, but not both. If the objective is to cap the cost of control, the amount of emissions actually eliminated cannot be guaranteed. If on the other hand the objective is to cut emissions by a certain amount, the cost of doing so cannot be predetermined, either.
Environmentalists prefer the certainty of quantity over the certainty of cost.
However, this preference does not give full weight to the fundamental differences between climate change and most other environmental problems. In this kind of scenario the price of emissions control would start at a low level, giving people time to learn the new rules and develop reliable data on strategies for operating in a low-emissions regime. Over time, the price would be increased as needed to keep the concentrations within the target. One important reason for this gradual acceleration approach is that a faster turnover of capital equipment, like electric generating plants, might be very expensive. Another reason is that a more gradual approach allows greater opportunities for taking advantage of future developments in technology.