 Several replies have been given to this line of argument. First, on an empirical note, we can respond that for several decades now people could not plausibly claim to be excusably ignorant about the effects of greenhouse gas emissions. Of course, specifying precisely when one could no longer claim to be excusably ignorant will be hard, but the salient point is that there are limits to the extent to which one can plead excusable ignorance (Singer 2002:  34).[4]  Second, some argue that it can be fair to ascribe burdens to those who were excusably ignorant of the harms of their emissions if those who emitted benefited sufficiently from the emissions. The rough thought here is that while someone might reasonably complain that it is unfair to penalise them for non-culpably contributing to harm, their case is considerably weakened if it is also the case that the harmful activity also created benefits for them. If they have benefited then making them pay would not be so onerous and might even leave them no worse off than if they had they not emitted. (For discussion of the “excusable ignorance” objection and responses to it see Gosseries [2004: 39–41].)