Global climate change is a big problem caused by greenhouse gases like CO2. To control these emissions, there are two main ways policymakers can use: either setting a limit on emissions (cap-and-trade) or putting a tax on carbon emissions. Both methods would make fossil fuels more expensive for consumers. Choosing between them depends on whether the aim is to control emissions or costs. A study compares these options and suggests that they both could work if the market was perfect, but since it's not, the choice comes down to what specific outcome is desired.