Big$-$A Autos sells gigantic SUVS that are modeled after military style vehicles, and only get about $5$ miles per gallon. Gas prices are currently low, and thus many consumers are purchasing Big$-$A$\text{'}$s SUVs. Many environmentalists argue that these consumers should have to pay an extra tax to purchase vehicles like this, because their poor gas mileage contributes to climate change. If the emissions from these vehicles are actually contributing to climate change $,$ but this societal cost is not reflected in the price that consumers are paying for the vehicles and their fuel, in economic terms the climate change cost borne by society but not by the purchasers of these vehicles is an example of a