On Jan. 20, Joe Biden will be inaugurated as the 46th president of the United States. He will face an unprecedented set of challenges, including global climate change—one of four stated policy priorities of his administration (along with the coronavirus pandemic, economic recovery and racial equity)—in addition to the immediate issue of the looming Senate trial of President Trump and ongoing threats of violence from extremist supporters. Because climate change is a global commons problem and international cooperation is necessary to limit free-rider incentives, President-elect Biden has pledged to immediately initiate the process of rejoining the Paris Agreement (from which President Trump withdrew the United States on Nov. 4, 2020—the earliest date permitted by the agreement). Thirty days after the necessary paperwork is filed with the United Nations, the United States will again be a party to the agreement. That’s the easy part. The hard part is coming up with a quantitative statement of how and by how much U.S. emissions of greenhouse gases will be reduced over time.

To fully appreciate the challenge the new administration will face, it is helpful to reflect on the history of international negotiations that brought us to this point. At the Earth Summit in Rio de Janeiro in 1992, the U.N. Framework Convention on Climate Change (UNFCCC) was first negotiated, committing parties to achieve stabilization of greenhouse gas concentrations in the atmosphere at a level that would “prevent dangerous anthropogenic interference with the climate system.” Three years later in Berlin at the first annual Conference of the Parties, it was agreed that the wealthier countries (listed in UNFCCC Annex I) would commit to targets and timetables for emission reductions, but not the other 129 (largely developing) countries. This was an attempt to provide for distributional equity among nations —recognizing that the industrialized countries were responsible for the lion’s share of accumulated greenhouse gases in the atmosphere, and by virtue of their wealth were more capable of taking action. Two years after that, in 1997, the Kyoto Protocol was enacted, codifying these objectives with quantitative targets for Annex I countries only.

The Clinton administration negotiated the protocol with considerable enthusiasm under the leadership of Vice President Gore, but it did not submit the protocol to the Senate for possible ratification, knowing that the protocol’s lack of any emissions-reduction responsibility for the large emerging economies (China, India, Brazil, Korea, South Africa, Mexico and Indonesia) meant it would fail in the Senate. This was a reasonable assumption, given that the Byrd-Hagel Resolution, which said as much, had passed the Senate by a vote of 95-0 just four months before the Kyoto conference.

The Kyoto Protocol was highly flawed. First, the Annex I countries alone could not reduce global emissions, despite a particularly severe target for the U.S., as the significant growth in emissions came from the emerging economies. Second, because the protocol excluded most countries (in particular, developing countries with relatively low costs of emissions mitigation), the costs were vastly greater than need be—four times the cost-effective level by conservative estimates. Third, it was questionable whether distributional equity was even achieved, given that 50 non-Annex I countries had greater per-capita income than the poorest of Annex I nations. So, the United States never ratified Kyoto, and eventually Australia, Canada, Japan and Russia dropped out, leaving the European Union and New Zealand as the only Annex I parties participating (together accounting for 14 percent of global emissions).

Almost two decades after Kyoto, a fundamentally different approach to international climate cooperation was taken by the Paris Agreement of 2015, which was developed under the joint leadership of the U.S. and China during the Obama administration....