Who should pay the costs associated with anthropogenic climate change, how much should they pay, and why? This burden-distribution problem has become the central question of climate justice among scholars and activists, and it remains the primary obstacle to the development of an effective climate regime. The costs are expected to be significant and varied, but can generally be categorized in terms of mitigation'that is, those costs associated with reducing further human contributions toward the increasing atmospheric concentrations of heat-trapping greenhouse gases (GHGs) that cause climate change; and adaptation'that is, those costs that result from attempting to insulate humans from the harms associated with the anthropogenic environmental damage of climate change. Since mitigation actions undertaken by developed countries under the auspices of the Kyoto Protocol are self-financed and mitigation targets accepted by developing countries are widely viewed as contingent upon financing from developed countries, imperatives to reduce GHGs are fundamentally matters of allocating mitigation costs. Adaptation intervenes in the causal chain between climate change and human harm, allowing the former but preventing the latter, but when this is not possible, a third category of compensation costs must be assigned in order to remedy failed mitigation and adaptation efforts. Because the formulas for assessing liability for adaptation and for compensation are identical, and since climate justice requires adaptation efforts that render compensation unnecessary, for the purposes of this essay the category of adaptation shall be understood to include prevention of harm as well as ex post compensation for it. As expected, the 'Copenhagen Accord' that emerged from the Fifteenth session of the Conference of the Parties (COP15) to the 1992 UN Framework Convention on Climate Change (UNFCCC) in December 2009 failed to satisfactorily address this core burden-allocation issue, making its resolution the primary problem to be addressed at the COP16 in CancÃºn, Mexico, at the end of 2010.