A United Nations special climate report suggests a tax on carbon dioxide emissions would need to be as high as $27,000 per ton at the end of the century to effectively limit global warming. For Americans, that’s the same as a $240 per gallon tax on gasoline in the year 2100, should such a recommendation be adopted. In 2030, the report says a carbon tax would need to be as high as $5,500 — that’s equivalent to a $49 per gallon gas tax. If you think that’s an unlikely scenario, you’re probably not wrong. However, it’s what the Intergovernmental Panel...