  We have not all contributed equally to the problems created by emissions; industrialized nations have contributed historically at much higher levels than those that are still developing. And so we should endorse the guidelines that those who have polluted more should pay more to help redress current problems (The Polluter Pays Principle). However critics argue that this principle unfairly holds some responsible when they did not know they were causing harm, since it was not widely known that greenhouse gases could result in climate change prior to 1990. So on this view, responsibility for emissions prior to 1990 should not conform with the Polluter Pays Principle, even if it is used to allocate costs after 1990. A second principle that is often discussed is The Beneficiary Pays Principle. Those who live in industrialized countries have typically benefited greatly from high levels of emissions so it is not unfair if they are expected to pay a higher proportion of costs. Critics object that a history of benefiting is an insufficiently strong consideration for assigning responsibilities now: in many cases whether or not people benefit is largely outside of their control. According to a third popular principle, The Ability to Pay Principle, the capacity of agents to pay for costs associated with mitigating climate change should be relevant.