Paradise Lost

Consisting of thirty-two atolls and one raised coral island dispersed over an area of the central Pacific larger than India, Kiribati (pronounced KIRR-ih-bass) is a breathtaking tropical paradise.Footnote ¹ Straddling the equator, its atolls are surrounded and laced through with shimmering turquoise waters. Towering coconut palm trees and stilt houses line its beaches. Marine life is abundant; fishers in sarongs gather shellfish at low tide. But for Kiribati’s roughly 119,000 permanent residents, the beauty and wonder of their ancestral homeland is overshadowed by impending cataclysm: the devastating impacts of global climate change.

Historically, Kiribati has not suffered from cyclones and similar severe weather, since those rarely occur near the equator.Footnote ² However, climate scientists have long warned of changing weather patterns in the Pacific, predicting that global warming would cause island nations to experience severe weather events with greater frequency.Footnote ³ In 2015, heavy seas and winds from Tropical Cyclone Pam struck Kiribati, flooding and destroying homes and buildings, contaminating freshwater wells, and destroying sea walls.Footnote ⁴ Already reliant on significant development assistance, with the lowest gross domestic product of any sovereign state in Oceania, Kiribati lacks the resources needed to recover from a natural disaster of this magnitude.Footnote ⁵

More terrifying yet for Kiribati is the rising sea. Kiribati is a low-lying nation, with most land less than two meters above sea level.Footnote ⁶ According to the Intergovernmental Panel on Climate Change (IPCC), between 1901 and 1971 the average rate of sea level rise was 1.3mm per year, increasing to 1.9mm per year between 1971 and 2006, and further accelerating to 3.7mm per year between 2006 and 2018.Footnote ⁷ By 2100, the sea level around Kiribati is estimated to rise between 520 mm (20.5 inches) and 910 mm (36 inches), and possibly much more.Footnote ⁸ In 1999, two of Kiribati’s islands, Abanuea and Tebua Tarawa, flooded and disappeared underwater entirely.Footnote ⁹ As these trends continue, much more of Kiribati’s landmass will be lost to the rising sea.Footnote ¹⁰ Nonetheless, the most probable and imminent existential crisis Kiribati faces is the threat of higher seas and more frequent extreme weather combining to render its islands uninhabitable long before they are buried by the sea.Footnote ¹¹

The principal cause of rising sea levels is global warming.Footnote ¹² The IPCC reports global surface temperature reaching 1.1°C above 1850–1900 levels in 2011–2020.Footnote ¹³ Rising global temperature is melting glaciers and ice sheets at an unprecedented pace, all the while expanding the volume of the ocean as sea water warms. The IPCC’s leading headline in its 2023 Synthesis Report is that “[h]uman activities, principally through emissions of greenhouse gases, have unequivocally caused global warming.”Footnote ¹⁴ Greenhouse gases (GHGs) – carbon dioxide, methane, nitrous oxide, and fluorinated gases – trap heat energy from the sun within the earth’s atmosphere, raising global surface temperatures.Footnote ¹⁵ GHGs are created by a host of industrial, agricultural, and consumer practices, including the harvesting, transporting, and burning of coal, oil, natural gas, solid waste, and trees. Past and present GHG emissions have already locked in considerable environmental damage. The IPCC concludes that “[s]ea level rise is unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years.”Footnote ¹⁶ However, the IPCC also affirms that “deep, rapid, and sustained GHG emissions reductions would limit further sea level rise acceleration and projected long-term sea level rise commitment.”Footnote ¹⁷

Put concretely, whether Kiribati faces half a meter, a full meter, or more than a meter of sea level rise by 2100 depends on the extent to which the international community is prepared to mitigate global warming by making “deep, rapid, and sustained” reductions to global GHG emissions. Even under optimistic forecasts, however, Kiribati will need robust sea walls and elevated dwellings to remain habitable. If Kiribati’s citizens are to remain on their islands, the international community must assist Kiribati in implementing adaptation measures to cope with higher sea levels. Mitigation and adaptation are essentially the two policy choices available to address climate change constructively.

Kiribati is sometimes referred to as the “canary in the coal mine” of climate change, since it was among the first countries to suffer devastating harm from global warming, serving as a warning to others.Footnote ¹⁸ Many other countries are now experiencing similar effects, however, and there is mounting evidence that global warming is accelerating faster than expected just a few years ago. Three years after authoring a widely publicized “warning of a climate emergency” that attracted 15,000 expert signatories,Footnote ¹⁹ seasoned climate scientists William Ripple and Christopher Wolf offered this alarming assessment:

These accelerating changes to the global environment pose grave dangers not only for low-lying island nations, such as Kiribati, but also for countries elsewhere, whether they be in the Global North or the Global South. Climate change knows no borders, and its adverse effects threaten to leave no people or territory unharmed.

This chapter examines the international community’s efforts to address climate change. As we shall see, the overarching method is international cooperation within an institutional framework supplied by international law. States and international organizations have established a supranational framework – the UN Framework Convention on Climate Change (UNFCCC) and related instruments – to articulate and implement mitigation and adaptation policies. We argue that this framework reflects the norms of mandatory cooperation, and it holds cautious promise for equitably allocating the costs of mitigation and adaptation in developing countries hard hit by climate change, such as Kiribati.

Governing the Climate Commons

When the UNFCCC entered into force in 1994, the international community formally recognized that “change in the Earth’s climate and its adverse effects are a common concern of humankind” and that this common concern requires “the widest possible cooperation by all countries and their participation in an effective and appropriate international response.”Footnote ²¹ Similarly, the Convention on Biodiversity identifies the “conservation of biological diversity” (not animal life per se) as “a common concern of humankind.”Footnote ²² By explicitly specifying in treaties that these issues constitute common concerns of humanity, the international community has signaled that states’ sovereign rights to exploit resources within their domestic jurisdiction are coupled with a concomitant obligation to cooperate with other states in advancing sustainable development and preventing catastrophic environmental harm.

In 2015, representatives from 196 states met near Paris to negotiate and draft an agreement that would address climate change at the global level. These talks ultimately produced the Paris Agreement.Footnote ²³ Its principal aim is to promote the adoption of policies that will reduce GHG emissions, thereby “[h]olding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels.”Footnote ²⁴ The scientific consensus is that warming above 2°C will result in “irreversible, catastrophic harm.”Footnote ²⁵

A further objective of the Paris Agreement is “[i]ncreasing the ability to adapt to the adverse impacts of climate change and foster climate resilience and low greenhouse gas emissions development.”Footnote ²⁶ The Global Commission on Adaptation points to several serious harms likely to occur if the international community does not provide adequate resources for adaptation:

• Without adaptation, climate change may depress growth in global agriculture yields up to 30 percent by 2050. The 500 million small farms around the world will be most affected.

• The number of people who lack sufficient water at least one month per year will soar from 3.6 billion today to more than 5 billion by 2050.

• Rising seas and greater storm surges could force hundreds of millions of people in coastal cities from their homes, with a total cost to coastal urban areas of more than USD 1 trillion each year by 2050.

• Climate change could push more than 100 million people in developing countries below the poverty line by 2030.Footnote ²⁷

As the chief means of attaining mitigation and adaptation goals, the Paris Agreement envisions all countries developing and periodically ratcheting up “nationally determined contributions” (NDCs) to address climate change.Footnote ²⁸ NDCs are to be submitted to the United Nations every five years and then used to assess states-parties’ compliance with their ongoing commitments, as we explain in this section.Footnote ²⁹ Sensitive to the concerns of developing countries, the preamble affirms “the principle of equity and common but differentiated responsibilities and respective capabilities.”Footnote ³⁰ More concretely, the Paris Agreement provides a framework for preexisting commitments of USD 100 billion a year in climate finance for developing countries by 2020, and for further financial assistance subsequently.Footnote ³¹ Although the plan’s success depends on the willingness of countries to follow through on these commitments and develop and adhere to meaningful NDCs, we have suggested in previous writing that the obligation of states to do so is explained by the idea that states, severally and jointly, are fiduciaries of humanity.Footnote ³²

Implementation of the Paris Agreement is followed and pursued by the institutional framework inaugurated by the UNFCCC. The centerpiece of this framework is the annual Conference of the Parties to the UNFCCC (COP). The COP is the supreme decision-making body of the UNFCCC. Running concurrently is the Conference of the Parties, serving as the Meeting of the Parties of the Paris Agreement (the CMA series), and the Conference of the Parties to the Kyoto Protocol (the CMP series). So, for example, in Glasgow in 2021 there were concurrent meetings of the 26th session of the Conference of the Parties to the UNFCCC (COP26), the 16th session of the Conference of the Parties serving as the Meeting of the Parties to the Kyoto Protocol (CMP16), and the 3rd session of the Conference of the Parties serving as the Meeting of the Parties to the Paris Agreement (CMA3). The main result of COP26/CMA3 was the Glasgow Climate Pact, an agreement of the 197 Parties in attendance that built on various commitments found in the Paris Agreement.Footnote ³³ Before turning to some of the specific measures in the Glasgow Pact, however, we set out now the numerous ways the legal regime established by the Paris Agreement reflects the norms of mandatory cooperation.

Like the UNFCCC, the Paris Agreement acknowledges that mitigating climate change and its adverse effects is “a common concern of humankind.”Footnote ³⁴ Although the Paris Agreement is careful to protect developing countries with the principle of “common but differentiated responsibilities,” all states parties are expected to participate in the Agreement’s regime of international cooperation. Nonetheless, while the Kyoto Protocol establishes legally binding GHG emission-reduction targets for developed countries,Footnote ³⁵ the Paris Agreement does not. The Kyoto Protocol adopts a “top-down” approach that imposes emission-reduction targets on wealthier nations. The Paris Agreement, by contrast, relies on a “bottom-up” method that requires all states to develop NDCs, either by themselves or jointly with others, including through the use of “internationally transferred mitigation outcomes” (typically, a developed country would finance realization of a developing country’s NDC).Footnote ³⁶ The Agreement calls for each party’s successive NDC to ratchet up or “represent a progression beyond” the prior NDC.Footnote ³⁷ States parties are likewise legally required to submit their NDCs to the United Nations and a public registry, report on progress every two years, and have their progress reports reviewed by technical experts and peers.Footnote ³⁸ In fact, each state’s NDCs and progress toward them are transparently displayed and assessed on a publicly available website.Footnote ³⁹ The Agreement further envisions a “global stocktake” every five years to assess collective progress toward its objectives.Footnote ⁴⁰

The Paris Agreement seeks to strike a balance that takes into account state autonomy and self-determination, on the one hand, and the urgent need for a global response to climate change, on the other. State autonomy is respected through the use of NDCs that states are legally entitled to design and implement as they wish. The demands of climate change are reflected in the legal requirement to establish, implement, and report periodically on progress toward NDCs. This activity is wedded to the annual COP meetings under the UNFCCC framework, as described above. In this context, states retain an overarching obligation from the UNFCCC to “protect the climate system for the benefit of present and future generations of humankind,”Footnote ⁴¹ and to do so cooperatively, such as through the negotiated use of “internationally transferred mitigation outcomes.”Footnote ⁴² Thus, explicitly and implicitly, the UNFCCC and the Paris Agreement require states to negotiate with one another in good faith, and on an ongoing basis, for the purpose of developing and implementing NDCs capable of achieving collectively the goals of the Paris Agreement. In the event of a dispute concerning the interpretation or application of the UNFCCC or the Paris Agreement, the controversy is remitted to the International Court of Justice (ICJ), arbitration, or conciliation.Footnote ⁴³ The Paris Agreement thus embraces a form of mandatory cooperation akin to the form established under the UN Convention on the Law of the Sea,Footnote ⁴⁴ where contending parties are obligated to negotiate in good faith, resorting to third-party dispute resolution only if necessary.

The principle of sovereign equality is respected under the Paris Agreement because no state is entitled to dictate terms to another, and every state is ultimately responsible for the content of its NDC. The principle of joint stewardship is put into action by the Paris Agreement’s delegation to states of the responsibility to craft meaningful NDCs to reduce GHG emissions and address the adverse effects of climate change. Joint stewardship assumes an institutional and democratic form through the Paris Agreement’s annual COP and CMA conference series. Every party has one vote, allowing the COP/CMA conferences to speak collectively on behalf of all.Footnote ⁴⁵

The Glasgow Climate Pact

Six years after the signing of the Paris Agreement, COP26 in Glasgow gave the international community an opportunity to assess progress, grapple with challenges, and strengthen NDCs. Held during the COVID-19 pandemic, COP26 demonstrated states’ willingness to take new decisions and strengthen commitments. Over fifty formal decisions were adopted.Footnote ⁴⁶ In addition, COP26 secured near-global net zero NDCs from 153 countries, such that over 90 percent of world gross domestic product and around 90 percent of global emissions are now covered by net zero commitments.Footnote ⁴⁷ Significantly, the Glasgow Pact cemented 1.5°C as the primary global temperature ceiling, with the more severe impacts of 2°C warming emphasized.Footnote ⁴⁸ The Pact also emphasized near-term action aimed at 2030, such as “reducing global carbon dioxide emissions by 45 per cent by 2030 relative to the 2010 level and to net zero around mid-century as well as deep reductions in other greenhouse gases.”Footnote ⁴⁹ The Paris Agreement, by contrast, only refers to achieving net-zero “in the second half of this century.”Footnote ⁵⁰ The Glasgow Pact also invited the Parties to “consider further actions to reduce by 2030” methane and other non-carbon dioxide GHGs, whereas the Paris Agreement is silent on methane and other non-carbon dioxide GHGs.Footnote ⁵¹ Possibly the most significant novelty in the Pact, however, is its call on the Parties to accelerate “efforts towards the phasedown of unabated coal power and phase-out of inefficient fossil fuel subsidies.”Footnote ⁵² This is the first time an international instrument has expressly called for a reduction in the use of coal, which alone is responsible for roughly 40 percent of all current carbon dioxide emissions.Footnote ⁵³

The many joint commitments in the Glasgow Pact are the product of negotiation and cooperation that aims for consensus within the COP/CMA framework. Crucially, these multilateral decisions and commitments include major GHG producers, such as China, India, Russia, and the United States. In the case of the provision on coal, China and India resisted the original language that called for a phasing out of coal power but did agree to a progressive “phasedown.”Footnote ⁵⁴ This is consistent with mandatory cooperation’s norm calling on states to negotiate matters of common concern in good faith without prescribing predetermined outcomes.

Mandatory cooperation in this context assumes that states wear two hats. One is the hat of their people’s representative, a role that requires them to bargain zealously for their people’s welfare. The other hat is worn as a joint steward of humanity, where they are called on, with other states, to safeguard the earth’s environment. As their people’s representative, they are entitled to seek reasonable accommodations consistent with “the principle of equity and common but differentiated responsibilities.”Footnote ⁵⁵ As a steward of humanity, they are tasked with ensuring that equity and reasonableness frame the claims made by individual states, including their own. Plausibly, China and India’s insistence on “phasedown” rather than “phase out” falls within the margin of maneuver they enjoy as both their people’s representatives and good stewards of humanity.

Some might be surprised that China and India compromised at all. Why did they not just walk away and refuse to set up any standard for measuring the reduction of coal power? Let us assume that China and India believed that it was in their interest to compromise, either for the sake of reputational gain or because they themselves were being affected by climate change or were otherwise concerned about its possible effects on them. Whatever their actual motivation, the presence of a self-interested reason for compromising does not imply that they did not have an obligation to negotiate in good faith. By agreeing to take part in the COP/CMA process, they agreed to negotiate and participate in good faith, and thereby incurred an obligation to do so. Moreover, they would likely present themselves and their interlocutors as having an obligation to negotiate in good faith, since good faith is the least that can be reasonably expected of contending parties who must negotiate a collective approach to a common concern. Accordingly, for Parties within the COP/CMA framework, good faith and mandatory cooperation are the overarching practices to which they are committed.

It is equally clear from the record of proceedings and negotiations that the Parties view the provisions of the Glasgow Pact as binding and authoritative. The Parties’ self-understanding of the Pact is noteworthy because, unlike the Paris Agreement, which is formally a treaty within the UN system, the Glasgow Pact is not a treaty but rather a “soft law” instrument of international law. As a decision of COP26/CMA3 (Decision 1/CMA.3), it resembles in form a resolution of the UN General Assembly. Now, many of the Pact’s provisions recall and seek urgent implementation of articles from the Paris Agreement. For example, paragraph 26 “[e]mphasizes the urgent need for Parties to increase their efforts to collectively reduce emissions through accelerated action and implementation of domestic mitigation measures in accordance with … the Paris Agreement.”Footnote ⁵⁶ In these cases, the Pact can piggyback on the Agreement by serving as an authoritative interpretation of the relevant articles of the Agreement, which unquestionably are legally binding under international law.

The open-textured nature of many of the Agreement’s provisions means that even the novel and specific commitments from the Pact, such as the coal phasedown, may be read as giving content to those provisions. For example, the coal phasedown may be understood as part of the commitment from Article 4(1) of the Agreement to “reach global peaking of greenhouse gas emissions as soon as possible.”Footnote ⁵⁷ Moreover, even where the Glasgow Pact enshrines commitments that cannot be brought into the four corners of the Paris Agreement, Joanna Depledge, Miguel Saldivia, and Cristine Peñasco rightly observe that “COP decisions still shape the obligations and expectations of parties and stakeholders, and have the advantage of immediate and global applicability, since they do not require domestic ratification to take effect.”Footnote ⁵⁸

Regrettably, not everything on which states agreed prior to Glasgow has come to pass, exposing the relative frailty of UNFCCC framework. The Glasgow Pact notes “with deep regret” the failure of developed countries to meet the goal previously set of mobilizing jointly USD 100 billion per year by 2020 to support mitigation in developing countries, while welcoming increased transparency and pledges that surpass this goal.Footnote ⁵⁹ The Pact also calls on developed countries “to at least double their collective provision of climate finance for adaptation of developing parties from 2019 levels by 2025.”Footnote ⁶⁰ It is far from clear that even this apparently ambitious goal is adequate. The International Energy Agency estimates that “[b]y the end of the 2020s, annual capital spending on clean energy in [developing] economies needs to expand by more than seven times, to above USD 1 trillion, in order to put the world on track to reach net-zero emissions by 2050.”Footnote ⁶¹ While price-tags such as this are eye-watering, the Global Commission on Adaptation has calculated that investing USD 1.8 trillion globally in five adaptation areas – early warning systems, infrastructure resiliency, dryland crop production, protecting mangroves, and water resource management resiliency – would produce total net benefits of USD 7.1 trillion.Footnote ⁶² The challenge for policymakers will be harnessing the ambition underlying the Glasgow Pact to convince developed countries to invest jointly in mitigation and adaptation measures that are in their long-term economic as well as climate interest.

COP28 and the First Global Stocktake

Held in Dubai in December 2023, COP28 brought together more than 150 heads of state and government and some 85,000 participants, including representatives of national delegations, civil society organizations, business, indigenous peoples, and international organizations. The major outcome from the conference was the first global stocktake on climate matters prescribed by the Paris Agreement.Footnote ⁶³ In what the United Nations hailed as “a demonstration of global solidarity,” nearly two hundred parties reached consensus on the stocktake, which provides assessments of mitigation and adaptation efforts to date, the state of the climate, and prescriptions on what needs to done.Footnote ⁶⁴ The headline-grabbing novelty was the stocktake’s call for “[p]hasing out inefficient fossil fuel subsidies” and “[t]ransitioning away from fossil fuels.”Footnote ⁶⁵ Expressing “serious concern that 2023 is set to be the warmest year on record,” the agreement calls on parties to implement “deep, rapid and sustained reductions in global greenhouse gas emissions of 43 per cent by 2030 and 60 per cent by 2035” so as to limit warming to 1.5°C with limited overshoot.Footnote ⁶⁶ Reaffirming “equity and the principle of common but differentiated responsibilities,” the stocktake recognizes that time frames for peaking temperatures in different countries “may be shaped by sustainable development, poverty eradication needs and equity and be in line with different national circumstances.”Footnote ⁶⁷ Mitigation prescriptions include “[t]ripling renewable energy capacity globally and doubling the global average annual rate of energy efficiency improvements by 2030.”Footnote ⁶⁸ The stocktake likewise affirms ambitious adaptation measures to contend with the effects of climate change, especially on the most vulnerable states, such as Kiribati. For example, the Parties acknowledge “that climate change impacts are often transboundary in nature and may involve complex, cascading risks that require knowledge-sharing and international cooperation for addressing them.”Footnote ⁶⁹ In fact, the agreement devotes an entire section to “International Cooperation,” recognizing “the importance of international cooperation, including transboundary cooperation, for contributing to progress towards the goals of the Paris Agreement.”Footnote ⁷⁰

At COP29 in 2024, states established a new climate finance goal to increase the scale of international assistance to developing countries from the previous goal of USD 100 billion annually to 300 billion annually by 2035.Footnote ⁷¹ At COP30, in 2025, governments have committed to present new NDCs that are “economy-wide, cover all greenhouse gases and are fully aligned with the 1.5°C temperature limit.”Footnote ⁷² On our view, states bear a legal obligation to both their people and humanity to advance these goals through good faith cooperation. We consider now the argument in favor of viewing their cooperation as mandatory.

Recalcitrant States

Arguably, the international community has elected to place climate change policy under joint stewardship precisely because a healthy global environment is a non-excludable public good, and so the reduction of GHG pollution and adaptation poses a serious collective action problem. Self-interested states would prefer to free ride, letting others bear the cost of mitigation measures, while they themselves avoid those costs while benefitting from a relatively healthy environment. The logic of the prisoner’s dilemma encourages states to pursue their narrow self-interest, since whether or not other states shoulder the burden of climate mitigation, a state will typically do better economically by avoiding the burden. But, of course, if a plurality of heavily polluting states adopted this policy, climate mitigation would be severely threatened if not doomed. The mandatory cooperation of the Paris Agreement and Glasgow Pact addresses this collective action problem by providing a framework for joint stewardship that makes the development and implementation of NDCs transparent, with the hope that national policymakers may be held accountable. Of course, this approach depends on policymakers caring about climate change and the Agreement’s multilateral process, or at least caring about the consequences of refusing to participate in good faith. The quality of states-parties’ participation in the COP/CMA framework, their ability to reach consensus on deeply contentious and high budget matters, and their public commitment to the goals of mitigation and adaptation suggest that, in general, most states at most times care a lot about climate change and want to be known as good global climate actors. But, regrettably, not all states at all times.

Pursuant to his “America First” policy, US President Donald Trump indicated early in his first term that the United States would withdraw from the Paris Agreement.Footnote ⁷³ Article 28 of the Agreement provides that a party may give notice of withdrawal “any time after three years from the date on which the Agreement has entered into force” for that party, but the withdrawal takes effect only one year after notification.Footnote ⁷⁴ The United States submitted its withdrawal notification on November 4, 2019, and formally withdrew from the Paris Agreement on November 4, 2020, one day after the presidential elections in which Joseph Biden prevailed. Withdrawal made the United States a free rider on the mitigation efforts of others, and was a brazen assertion of unilateralism in breach of the United States’ joint stewardship obligations. The assertion was especially brazen because, as noted, the Paris Agreement does not subject states-parties to emission-reduction quotas or targets. States must merely develop some climate change mitigation policy in nonbinding (but good faith) negotiations with others and subject the policy’s implementation to public review. Withdrawal from the Paris Agreement was therefore an outright rejection by the United States of international legal norms requiring mandatory cooperation in the field of climate change. This jeopardized collective action on the environment, in part because the United States is one of the world’s largest carbon emitters, and in part because defections from collective action regimes and open free riding breed resentment and may prompt further defections.

The United States’ withdrawal did not have grave practical implications in the short term because the Biden administration signaled during and immediately after the 2020 election that it would rejoin the Paris Agreement.Footnote ⁷⁵ Nonetheless, upon his return to the White House in January 2025, President Trump initiated a second US withdraw from the Paris Agreement,Footnote ⁷⁶ potentially prompting other states to follow.Footnote ⁷⁷ It is worth reflecting, therefore, on whether a recalcitrant state is entitled to go it alone and do as it pleases with respect to climate policy.

We suggest that the substantive and procedural norms of mandatory cooperation are binding independent of their codification in the Paris Agreement or the Glasgow Pact. The rationale for the theory proceeds from the premise that states’ GHG emissions spill over into the global atmosphere possessed in common by all. As discussed in Chapter 6, transboundary environmental harm triggers duties of mandatory cooperation, such as duties to share information, negotiate in good faith, demonstrate due regard for other states and their nationals, and reduce harm where practicable.Footnote ⁷⁸ In the climate change context, where transboundary pollution harms all states, the duties of mandatory cooperation become omnilateral. A practical way to specify and satisfy these duties within a regime that respects sovereign equality and avoids unilateralism is to authorize states to serve as coequal joint stewards of the atmosphere within an institutional framework such as the UNFCCC. For this charge to be legally meaningful, states must have some obligation to take it seriously.

The legal obligation to cooperate with other states in addressing the harmful effects of climate change derives in part from the issue’s long-standing status under the UNFCCC, the Convention on Biodiversity, the Paris Agreement, and the Glasgow Pact as a “common concern of humankind.” This status is evidence of a customary rule that states have an obligation to work together in good faith to mitigate climate change because the environment is an important and nonexcludable public good on which humanity depends. Further evidence of such a customary rule is discernible from the relationship between a state and its people, on the one hand, and the state and humanity, on the other. As fiduciaries of their people and joint stewards of humanity, states owe both their people and humanity an obligation to do what is reasonably necessary to safeguard the environment. Because climate change is quintessentially a collective action problem, addressing it effectively necessarily requires states to negotiate mitigation and adaptation measures together in good faith.

Mandatory cooperation in a context in which the United States has withdrawn from the Paris Agreement would require the United States to take good faith steps to mitigate climate change and provide for adaptation. The United States would enjoy wide discretion to design and implement mitigation measures, much as states-parties to the Paris Agreement enjoy broad authority over the development and implementation of NDCs. The United States would also, however, be under an obligation to exercise that discretion in a way that did not subvert the collective mitigation efforts of other states, and in a manner that demonstrated due regard for other states touched by its GHGs. In addition, mandatory cooperation would entail procedural obligations of investigation, consultation, negotiation, and possibly third-party dispute resolution. Thus, even if the United States were to again withdraw from the Paris Agreement, and for a lengthier duration, it may still be bound by substantive and procedural customary norms of mandatory cooperation that call on the United States to have due regard for the environmental interests of other states, including Kiribati and others suffering climate harms now.

This again raises the question of compliance. What if the United States withdraws and explicitly refuses to recognize the validity or applicability of any international legal obligations in this sphere? Such a move could have far-reaching, and potentially disastrous consequences for global efforts to combat climate change. Purely as a conceptual matter, however, the compliance problem does not appear any more or less pressing here than in other areas of international law. Moreover, issues of compliance go to states’ motivations for complying with international norms and the likelihood of their compliance with them, whereas our project has been to theorize norms that are already part of international legal practice. While practical concerns counsel publicists to take compliance seriously, elaborating the nature and implications of legal principles, as we have tried to do, is a separate inquiry.

A further issue raised by the prospect of customary norms of mandatory cooperation concerns whether the persistent objector doctrine would apply, such that unwilling states can legally opt out of multilateral regimes. In our view, these states would be in the same legal position as states that attempt to negotiate a settlement of a dispute in good faith but simply cannot reach amenable terms with the counterparty. Like these states, persistent objectors would remain subject to norms that prohibit them from adopting measures or positions that undermine the efforts of other states to develop an equitable multilateral framework. They would also remain subject to the obligation to continue to pursue good faith efforts to reach a mutually acceptable outcome. We recognize that, as a general matter, holding states to a customary obligation to seek third-party dispute resolution over climate change mitigation measures might appear, at this point in the evolution of international law, more prescription than description. However, it is a prescription wholly consonant with the principles of sovereign equality and joint stewardship that underlie mandatory cooperation’s other norms. And, as we explain now, it is also a prescription that international courts and tribunals appear ready to endorse, just as they have affirmed duties of mandatory cooperation for other matters of international concern, such as maritime delimitation and transboundary pollution.

Adjudicating Climate Change Mitigation

Within the past several years, international courts and tribunals have received several requests for advisory opinions on states’ international obligations concerning climate change mitigation. In December 2022, the Commission of Small Island States on Climate Change and International Law (COSIS), an organization that represents climate-vulnerable states like Kiribati, requested an advisory opinion from the International Tribunal for the Law of the Sea (ITLOS).Footnote ⁷⁹ COSIS asked the tribunal to specify states’ obligations to preserve and protect the marine environment under the UN Convention on the Law of the Sea (UNCLOS) in light of the deleterious impact of GHGs on the global climate.Footnote ⁸⁰ A month later, Chile and Colombia requested that the Inter-American Court of Human Rights (IACtHR) prepare an advisory opinion concerning states’ climate-related obligations under the American Convention on Human Rights.Footnote ⁸¹ The request for an advisory opinion specifically invited the Court to clarify, inter alia, states’ “cooperative obligations” for mitigating climate change.Footnote ⁸² On March 29, 2023, the UN General Assembly adopted resolution 77/276 with the support of 150 states, requesting an advisory opinion from the ICJ on “the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases for States and for present and future generations.”Footnote ⁸³ Within just four months, these requests for advisory opinions vaulted states’ obligations with respect to climate change to the center stage of international adjudication.

The ITLOS tribunal, the IACtHR, and the ICJ have now delivered landmark advisory opinions concerning climate change. The ITLOS tribunal unanimously affirmed that states-parties to UNCLOS bear duties to “prevent, reduce, and control” the emissions that contribute to oceanic warming and acidification.Footnote ⁸⁴ States’ duties to curb GHGs are not limited to their voluntarily determined contributions under the Paris Agreement; to satisfy UNCLOS, states must “take all necessary measures to prevent, reduce and control marine pollution from anthropogenic GHG emissions, including measures to reduce such emissions.”Footnote ⁸⁵ The measures states take to prevent, reduce, and control GHG emissions must be reasonably calculated to protect the marine environment, consistent with the requirement of due diligence under customary international law.Footnote ⁸⁶

Significantly, the ITLOS tribunal also offered a resounding endorsement of mandatory cooperation for climate change mitigation. The tribunal emphasized that “the duty to cooperate is a fundamental principle in the prevention of pollution of the marine environment under [both UNCLOS] and general international law.”Footnote ⁸⁷ The “core” of this obligation, the tribunal explained, is states’ duty to cooperate in “developing a common regulatory framework ‘for the protection and preservation of the marine environment.’”Footnote ⁸⁸ Under UNCLOS, states must “endeavour to harmonize” their regulatory responses to climate change, exercising “stringent” due diligence to adopt common rules, standards, and recommended practices and procedures to protect and preserve the marine environment from the adverse impacts of GHGs.Footnote ⁸⁹ Due diligence requires states to expend substantial effort, consult with one another in good faith,Footnote ⁹⁰ and cooperate in undertaking and promoting scientific research and exchanging information and data about oceanic pollution.Footnote ⁹¹ Perhaps most striking, the tribunal declared that these obligations of mandatory cooperation are not fully exhausted by the UNFCCC and the Paris Agreement:

Thus, mandatory cooperation may require the international community to take more aggressive measures now and in the future to prevent, reduce, and control the escalating impacts of GHGs on the global environment.

The advisory opinions of the IACtHR and the ICJ were released as this book went to press and go further still. Linking state obligations to address climate change to human rights enshrined in the American Convention of Human Rights, the IACtHR frames its analysis as responding to “the climate emergency” of “climate change, pollution, and biodiversity loss.”Footnote ⁹³ Breaking new ground, the IACtHR finds that states are subject to a peremptory or jus cogens norm of international law to safeguard humanity against “anthropogenic conducts with irreversible impacts on the vital equilibrium of the planetary ecosystem,” since these conducts “undermine the conditions required sine qua non for the enforceability of fundamental human rights that are already protected under international law by peremptory provisions.”Footnote ⁹⁴ Further, the IACtHR dedicates a lengthy section to “The Obligation of Cooperation,” finding that “[t]he obligation of States to cooperate in good faith forms part of customary international law” and that “cooperation is a cardinal principle established in the OAS Charter.”Footnote ⁹⁵ The Court observes that, as a general matter, “the obligation of cooperation has special relevance in all contexts in which the international community pursues common objectives or faces problems that require collective solutions,” and that this is “precisely the case in relation to addressing the causes and impacts of climate change.”Footnote ⁹⁶

The ICJ’s Advisory Opinion is likewise unequivocal in its prescription of mandatory cooperation to address climate change.Footnote ⁹⁷ The Court observes at the outset that states-parties to the UNFCCC, the Kyoto Protocol, the Paris Agreement, UNCLOS and other environmental treaties have interlocking treaty-based obligations of mitigation and adaption to address climate change.Footnote ⁹⁸ The ICJ also finds that states have a duty under customary international law “to prevent significant harm to the environment” that applies “to global environmental concerns.”Footnote ⁹⁹ And, just as significantly, the Curts affirms “that the duty of States to co-operate for the protection of the environment is a rule whose customary character has been established.”Footnote ¹⁰⁰ Emphasizing the customary status of the duty to co-operate, the Court notes that the “duty to co-operate is intrinsically linked to the duty to prevent significant harm to the environment, because unco-ordinated individual efforts by States may not lead to a meaningful result.”Footnote ¹⁰¹ The customary duty to cooperate “is applicable to all States,” the Court says, although its demands are sensitive to “the common but differentiated responsibilities and respective capabilities principle.”Footnote ¹⁰² The customary duty applies “regardless of whether a State is a party to the climate change treaties.”Footnote ¹⁰³ The ICJ concludes that “[c]o-operation is not a matter of choice for States but a pressing need and a legal obligation.”Footnote ¹⁰⁴

Conclusion

It is far beyond the remit of this chapter to offer a forecast of our climate future. However, let us return for a moment to Kiribati, the “canary in the coal mine.”Footnote ¹⁰⁵ Should the worst or even the fairly bad come to pass in the near future, the world will have to choose between very costly adaptation measures or providing space and status elsewhere for a people who will have become climate refugees.Footnote ¹⁰⁶ For now, the President of Kiribati, Taneti Maamau, seeks funding from developed countries “in the billions” to physically raise some portion of Kiribati’s land mass to escape the encroaching sea.Footnote ¹⁰⁷ “The wealthy countries are after all responsible for what we are facing,” President Maamau says.Footnote ¹⁰⁸ Whether developed countries step up may depend, in part, on whether they recognize and embrace their obligation to cooperate on fair terms as joint stewards of the environment.