Towards a “fair-efforts” metric for climate pledges

It took twenty years of international negotiations for national governments to formally acknowledge our common responsibility for mitigating climate change.However, this success came at the expense of a key dimension of this responsibility enshrined in the UN Framework Convention of Climate Change, to differentiate countries' responsibilities based on capabilities and circumstances.To the contrary, the Paris Agreement deliberately side-steps issues of "fairshares" [1], leaving countries to voluntarily submit Nationally Determined Contributions (NDCs) based on self-serving notions of fairness.Following the Glasgow Climate Pact, pledges from 83 countries, including by 51 mostly industrialized countries, aim for net zero by 2050 [2].However, according to equitable principles of international environmental law, industrialized countries should pledge net zero by 2030 to achieve the Paris goals [1].Instead, the 2030 targets are weak and push out deep cuts to mid-century.Furthermore, net zero pledges include those from low-and middle-income countries (LMIC) such as India and Nigeria, which have per capita incomes that are a tenth of European countries, albeit to be achieved after 2050.Although largely symbolic, the Glasgow experience may portend pressure on LMIC governments to ratchet ambition as the climate crisis worsens, public pressure to act intensifies, and as mitigation investments need to increase from the present level of hundreds of billions to the trillions that are required to achieve the Paris goals [3].The urgency of achieving the Paris goals dictates that low-carbon investments must happen roughly contemporaneously around the world in all sectors to avoid lock-in to fossils and achieve global net zero not long after 2050 [4].Yet, even political realism dictates that industrialized countries, including China, absorb technology risk of deep decarbonization by undertaking R&D and be early adopters of new low-carbon technologies [5] in applications that still lack commercially viable alternatives, such as in industrial process heat, long-term electricity storage, and freight transport.The combination requires global cooperation, and in turn a fairness benchmark that encourages stronger actions in industrialized countries but also maximize efforts in LMIC.What metrics should then guide assessments of countries' fair efforts towards the Paris Goals?I argue here that the allocation principles that have dominated two decades of burdensharing proposals are unsuitable for international agreements in part because they allocate shares of global mitigation targets to countries based on principles without accounting for the true costs of mitigation, which evolve with technology, or non-monetary social benefits.A key principle relates to historical responsibility, whose measure has been countries' cumulative emissions since industrialization.In essence, polluters should pay for the mess they created.Seen otherwise, industrialized countries reaped the benefits of abundant fossil-based energy without bearing its true cost, part of which were transferred through climate change to future