 There is a third challenge. What if persons need to engage in activities which emit greenhouse gases in order to enjoy a decent minimum standard of living? Many hold that it is unfair to make extremely poor people pay the cost of emitting greenhouse gases where doing so would push them beneath a decent standard of living. More generally, one might argue that if people are entitled to a certain standard of living (which need not be specified according to sufficientarian criteria, but could also be specified according to egalitarian or prioritarian or some other criteria) then it would be wrong to make them pay if doing so entails that they cannot enjoy that standard of living.