The study looked at how different taxes on oil, natural gas, and coal can help Spain reduce carbon emissions. They found that just a carbon tax isn't the best approach for mild environmental goals. Heavy taxes on coal are crucial, while subsidizing oil and natural gas could be helpful. Oil should be taxed less than natural gas and coal. As environmental goals get tougher, it costs more to reduce CO2 emissions. Lastly, a carbon tax brings in more money than the optimal tax mix, which could be a problem for policymakers.