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### Emissions control rate

The emissions control rate is the fraction of emissions that are reduced or controlled by a climate change policy. For example, if a climate change policy limits emissions to 60% of what they otherwise would have been, the control rate is 40%. Under the business as usual scenario there are no controls on emissions of CO2, or the emissions control rate is set to zero. If you choose to impose a carbon tax or a treaty, or if you allow the computer to determine the optimal level of emissions reductions, the emissions control rate will be positive as some portion of emissions will no longer occur either because of the tax or because it is prohibited by the treaty. When the emissions control rate is equal to 1, emissions have ceased altogether and there is no further climate change. As we reduce emissions more, costs are assumed to go up: we choose the lowest cost reductions first and then move to higher and higher cost reductions. You can choose the rate at which these costs go up by choosing a value for abatement costs, with higher values meaning that costs go up faster as we reduce more. more

The emissions control rate $\mu(t)$ is raised to the power $\theta_{2}$ to represent increasing marginal costs of abatement. The rate at which marginal costs increase is determined by the user. The default model sets this exponent to 2.8, but the user can adjust it between 2 and 4.