Ashes of Death

When the Japanese fishing craft Lucky Dragon (Fukuryu Maru) reached Tokyo Bay on March 14, 1954, the entire crew and 4,000 pounds of tuna onboard were dangerously radioactive. Two weeks earlier, the United States had conducted the first detonation of a hydrogen bomb at Bikini Atoll in the Marshall Islands, deep in the South Pacific.Footnote ¹ The US Atomic Energy Commission (AEC) had previously announced plans to use the islands as a testing ground for nuclear weapons, and it had advised foreign governments to keep ships outside a 50,000-square-mile “danger area” for their safety. This designated danger area proved to be woefully inadequate. The magnitude of the hydrogen bomb’s thermonuclear explosion more than doubled scientific projections, and due to an unforeseen shift in atmospheric conditions, the resulting radiation cloud traveled outside the official danger area to drop a “ghostly white rain, thick with ash” on the Lucky Dragon.Footnote ² When the rain subsided, crew members experienced dizziness, vomiting, and fever – the first visible symptoms of severe radiation exposure. By the time the Lucky Dragon returned to Japan, most of the crew suffered from headaches, skin burns, bleeding gums, and other afflictions.Footnote ³ Six months later, crew member Aikitchi Kuboyama died, and twenty-two others continued to receive hospital treatment for serious injuries.Footnote ⁴ Reflecting on Kuboyama’s untimely passing, a Japanese newspaper inquired, “Has the death of a citizen ever been watched by so many eyes? They are the eyes of a strong anger and protest against the ‘ashes of death.’”Footnote ⁵

The botched nuclear test at Bikini Atoll was among the first in a series of highly publicized environmental disasters that fixed the world’s attention on the growing threat of transboundary pollution. In 1967, the Torrey Canyon, an American-owned and chartered tanker manned by an Italian crew, attracted international infamy when it ran aground on the Seven Stones reef in international waters. Approximately 80,000 tons of crude oil spilled into the Atlantic, befouling resort beaches and decimating sea bird populations along the coast of Cornwall, England.Footnote ⁶ A decade later, another oil spill eclipsed the Torrey Canyon incident when the Amico Cadiz, an American-owned supertanker, collided with a submerged reef and broke in half, dropping nearly all of its 230,000-ton load of crude oil to contaminate the French coastline for hundreds of miles. The year 1986 brought two environmental calamities in quick succession: the Chernobyl nuclear meltdown near Kyiv, Ukraine, which scattered its own “ashes of death” and eventually radiation throughout Europe, and the Sandoz chemical plant fire in Switzerland that sent thousands of liters of highly toxic pesticides down the Rhine to poison wildlife and drinking water in Germany, France, and the Netherlands. In each of these incidents, pollution from one state traversed international borders to inflict grave harm.

Like international disputes over the apportionment of watercourses and fisheries, transboundary pollution presents a classic clash of sovereign entitlements. On the one hand, international law gives states sovereign authority over activities within their territorial jurisdiction – including activities that produce pollution. But on the other hand, international law also entitles states to territorial integrity.Footnote ⁷ This means states are entitled to demand the cessation of transboundary pollution that causes significant harm within their borders.Footnote ⁸

The unilateralist theory of sovereignty would resolve this tension, for all intents and purposes, in favor of the state where transboundary pollution originates. Recall that under the unilateralist theory, states may decide how to exercise their sovereign powers without regard to foreign interests. Accordingly, the unilateralist theory would empower states to engage in practices, such as atmospheric nuclear testing, that generate harmful transboundary pollution. The unilateralist theory would threaten to render illusory the sovereign right to territorial integrity because the enjoyment of the right would depend on the mercy or grace of other states. The “no significant harm” principle from international environmental law would be subordinated to the originating state’s discretion. In an international legal order based on unilateral sovereignty, strong states would be free to do as they please within their borders, while weak states would “suffer what they must.”Footnote ⁹

International environmental law rejects this vision of sovereign unilateralism, embracing an equitable approach that respects and reconciles states’ sovereign interests. Although states have exclusive authority to regulate pollution within their borders, international law requires them to exercise this authority in a manner congenial to other states’ territorial integrity. The primary mechanism for achieving this reconciliation is the requirement that states must exercise due diligence to prevent activities in their territory from producing significant transboundary harm.

This chapter shows how due diligence leads to mandatory cooperation. To satisfy the due diligence standard, states must take into account foreign interests when they regulate activities that may produce transboundary harm. Due diligence also imposes procedural obligations to conduct environmental impact assessments, provide notice to states that may be impacted by transboundary pollution, engage in meaningful consultation and negotiation, and resolve disputes through mediation, arbitration, adjudication, or other third-party dispute resolution. The requirements of mandatory cooperation thus apply to transboundary pollution under international law.

Good Neighbors

Less than ten miles north of the Canada-US border lies Trail, a modest mining town in British Columbia. For over a century, Trail has hosted the world’s largest zinc and lead smelter, a sprawling complex adjacent to the Columbia River. Celebrated as “the jewel of the British Columbian mining industry,” the Trail smelter has served as a “locus of regional growth, a pillar of provincial development, and an instrument of Canadian economic policy.”Footnote ¹⁰ For environmentalists and international lawyers, Trail has also become synonymous with industrial pollution. In the late 1920s, the Trail smelter provoked international controversy when it became apparent that the sulfur dioxide fumes emitted in prodigious quantities from its 409-foot smokestack were devastating timber lots, orchards, gardens, pastures, and farmland in neighboring Washington State.Footnote ¹¹ Aggrieved American homesteaders demanded that the smelter cut back its emissions to protect them from further harm. When private negotiations between the smelter and the homesteaders stalled, attention shifted toward international diplomacy. Canada and the United States eventually concluded a bilateral treaty, which provided for the smelter to compensate private claimants for harm incurred before 1932 while referring questions about subsequent injuries to arbitration.

The arbitral tribunal delivered its final decision in 1941. Citing national court decisions that held sovereigns responsible for private transboundary pollution, the tribunal reasoned that “under the principles of international law … no State has the right to use or permit the use of its territory in such a manner as to cause injury by fumes in or to the territory of another or the properties or persons therein, when the case is of serious consequence and the injury is established by clear and convincing evidence.”Footnote ¹² This affirmation of the prohibition against transboundary pollution has become a canonical cornerstone of international environmental law.Footnote ¹³

Trail Smelter envisions an international legal order in which states must interact as good neighbors, consistent with principles of equity and sovereign equality.Footnote ¹⁴ As Günther Handl has explained,

Being a good neighbor, in this sense, means states must live with some inconveniences that are inevitable features of communal life. It would be unreasonable, for example, to expect states to exercise such complete control over their territory as to eliminate all risk of transboundary harm. But being a good neighbor also means that states may not simply play territorial sovereignty as a trump card to avoid taking responsibility for serious transboundary harm. They must exercise due diligence, using the resources at their disposal – including legal regulation – to ensure that dangerous activities do not inflict significant injury abroad. When transboundary harm does occur, they must take steps to curtail further injury and provide compensation.Footnote ¹⁶ These obligations are necessary to respect the principle of sovereign equality under international law.

Elementary Considerations of Humanity

Canada and the United States used the Trail Smelter arbitration to bolster their shared commitment to good neighborly relations. However, the legal principles articulated in Trail Smelter would soon receive broader application when the ICJ extended these principles to a dispute between embittered geopolitical adversaries on the other side of the Atlantic.Footnote ¹⁷

On October 22, 1946, two British destroyers struck submerged mines in the Corfu Channel off the coast of Albania. The ensuing blast caused major damage to the destroyers and claimed the lives of over three dozen crew members. The incident marked the culmination of a series of tense standoffs between Albania and the United Kingdom, which had squared off on opposite sides of the nascent Cold War. Earlier the same year, an Albanian battery had fired in the direction of two British cruisers passing through the channel without connecting. The British made a formal diplomatic protest and, in further defiance, sent four warships steaming through the channel. When two of these ships – the Saumarez and the Volage – struck mines, suffering casualties and major damage, the United Kingdom initiated proceedings against Albania in the ICJ.Footnote ¹⁸

A central question in the Corfu Channel Case was whether Albania could be held responsible for the mine blasts. The United Kingdom did not offer any direct evidence to establish that Albania had planted the mines in the channel.Footnote ¹⁹ Nonetheless, the Court concluded that Albania must pay compensation for the United Kingdom’s injuries due to a combination of factors. First, by all accounts, the minefield had been laid in Albanian territorial waters not long before the deadly explosions. Second, during the relevant period, Albania exercised exclusive jurisdiction and continuous oversight over the waters, such that the mines could not have been placed without its knowledge.Footnote ²⁰ Third, Albania did not warn the United Kingdom about the minefield.Footnote ²¹ Under these circumstances, the Court concluded, Albania had a duty to warn the United Kingdom of the impending danger. Albania’s duty to provide notice rested “on certain general and well-recognized principles, namely: elementary considerations of humanity, even more exacting in peace than in war; the principle of the freedom of maritime communication; and every State’s obligation not to allow knowingly its territory to be used for acts contrary to the right of other States.”Footnote ²² Having failed to notify the United Kingdom that its navy risked serious harm by traversing the Corfu Channel, Albania assumed legal responsibility for injuries suffered by the Saumarez, the Volage, and their crews.

By holding that Albania was obligated to provide notice of threats emanating from its territory, the ICJ took a substantial step toward mandatory cooperation. The Corfu Channel judgment gave states like Albania a choice: either eliminate serious dangers themselves (e.g., by removing submerged mines) or cooperate with other states (e.g., by consenting to foreign minesweeping). In either scenario, states could not act without regard to foreign interests. They must take actions reasonably calculated to prevent serious harm to other states, including by affirmatively warning other states of dangers. As the ICJ recognized in Corfu Channel, these minimal requirements to suppress serious threats and provide notice to other states are necessary to respect “elementary considerations of humanity” and honor other states’ equal rights.

Cooperating for Environmental Conservation

States cannot satisfy their due diligence obligations in many contexts without robust cooperation. Consider again the Trail Smelter dispute. In theory, Canada could have shut down the Trail smelter, but this would have delivered a crushing blow to the British Columbian mining industry. Economic and political considerations dictated that Canadian regulators should pursue an intermediate solution, setting emissions standards at levels that would maximize the smelter’s capacity for zinc and lead production without inflicting serious harm across the border in violation of international law. But how much would the smelter have to reduce its emissions to achieve this objective? To answer this question, Canadian regulators would have to conduct a rigorous environmental impact assessment, taking into account wind patterns, terrain, flora, fauna, human settlement, and other factors on the American side of the border. Collecting this information would require the United States’ cooperation. Canada could not realistically satisfy its obligation to regulate harmful transboundary pollution from the smelter without consulting and exchanging information with the United States.

Due diligence also constrained how Canada could handle US objections to its governance of the Trail smelter. As noted above, Canada accepted responsibility for the smelter’s adverse environmental impacts in the 1920s and early 1930s. Nonetheless, Canada continued to resist the United States’ requests to make even more aggressive cuts to the smelter’s pollution. The dispute between the two states centered partly on scientific uncertainty about the smelter’s environmental impact under various atmospheric conditions. However, Canada and the United States also disagreed about what level of harm would be serious enough to trigger the duty to prevent significant harm. Given the indeterminacy of “significant harm” and the triggering conditions of the duty, due diligence required Canada to strive in good faith to pursue further consultation, negotiation, or third-party dispute resolution, such as an independent fact-finding commission, mediation, arbitration, or adjudication. By cooperating with the United States to resolve their factual and legal disagreements in the Court of Arbitration, Canada satisfied the due diligence standard.

The Trail smelter controversy thus illustrates how, in practice, the duty to prevent transboundary harm may generate cooperation that satisfies the due diligence requirements of the prevailing regime of mandatory cooperation. To satisfy the due diligence standard, states must make a reasonable, good-faith effort to investigate risks of significant transboundary harm. They must notify, exchange information, and consult, negotiate, and accept third-party dispute resolution with other states. Only by taking these kinds of steps can they demonstrate due regard for other states’ equal sovereignty and avoid an impermissible abuse of right.

Codifying Mandatory Cooperation

Within the past half-century, mandatory cooperation has become deeply embedded in international environmental law. In June 2022, the international community celebrated the fiftieth anniversary of the first global summit on environmental conservation: the UN Conference on the Human Environment, held in Stockholm, Sweden, June 5–16, 1972. A crowning achievement of the Stockholm Conference was its landmark declaration enshrining “common principles to inspire and guide the peoples of the world in the preservation and enhancement of the human environment.”Footnote ²³ The Stockholm Declaration recognizes the need for “extensive cooperation among nations and action by international organizations in the common interest.”Footnote ²⁴ Although the Stockholm Declaration does not spell out precisely which forms of international cooperation are essential, it does affirm that states have a “responsibility to ensure that activities within their jurisdiction or control do not cause damage to the environment of other States or of areas beyond the limits of national jurisdiction.”Footnote ²⁵

After the Stockholm Conference, the international community elaborated duties of cooperation for environmental conservation in a series of treaties, declarations, and other instruments. That same year, the UN General Assembly recognized that international cooperation for the environment, as contemplated in the Stockholm Declaration, demanded that states exchange technical data related to transboundary pollution “in the best spirit of co-operation and good-neighborliness.”Footnote ²⁶ In 1979, the UN Economic Commission for Europe’s (UNECE) Long Range Transboundary Air Pollution Agreement (LRTAP) committed member-states to combat air pollution by exchanging information, consulting, and pursuing collaborative research to identify and eliminate significant threats.Footnote ²⁷ Under LRTAP’s cooperative framework, member states have subsequently established additional protocols to combat some of the most deadly atmospheric pollutants, including sulfur, nitrogen, and volatile organic compounds.Footnote ²⁸ UNECE’s 1992 Convention on Transboundary Effects of Industrial Accidents also obligates states to identify risks of transboundary harm from industrial accidents, share relevant information, and consult with one another to develop effective prevention and mitigation strategies.Footnote ²⁹ When accidents occur, states-parties must notify other interested states, assess transboundary harm, and solicit assistance to address the harm.Footnote ³⁰

Mandatory cooperation also features prominently in treaties addressing oceanic pollution, including the 1982 UN Convention on the Law of the Sea (UNCLOS).Footnote ³¹ In addition to requiring states-parties to prohibit and penalize pollution of the marine environment, UNCLOS demands that they cooperate to prevent significant oceanic pollution and eliminate or reduce its adverse effects on the marine environment.Footnote ³² When states-parties become aware of imminent dangers to the marine environment, they must immediately notify other states that are likely to be impacted.Footnote ³³ In addition, states-parties are obligated to cooperate in “promoting studies, undertaking programmes of scientific research and encouraging the exchange of information and data acquired about pollution of the marine environment” to assess better “the nature and extent of pollution, exposure to it, and its pathways, risks and remedies.”Footnote ³⁴ They must conduct environmental impact assessments for pollution of the marine environment,Footnote ³⁵ and they must cooperate to formulate and elaborate international rules, standards, and best practices for protecting the marine environment.Footnote ³⁶ Similar cooperative obligations appear in other treaties that address marine pollution.Footnote ³⁷ Based on these developments in treaty law and consistent state practice, the International Tribunal for the Law of the Sea (ITLOS) has characterized mandatory cooperation as “a fundamental principle in the prevention of pollution of the marine environment under … general international law.”Footnote ³⁸

Another important codification of mandatory cooperation is the Convention on Environmental Impact Assessment in a Transboundary Context, the product of a UN-sponsored conference in Espoo, Finland (Espoo Convention).Footnote ³⁹ The Espoo Convention requires states-parties to provide notice of activities that are likely to cause a significant adverse transboundary impact.Footnote ⁴⁰ They must also conduct environmental impact assessments for such activities, consult with other interested states regarding preventing or mitigating transboundary harms, and provide mutual assistance in eliminating or reducing such harms.Footnote ⁴¹ When states-parties decide whether to proceed with an activity that may have transboundary impacts, they must take “due account” of the likely effect on other states.Footnote ⁴² Final decisions must be communicated to other states in writing, explaining the reasons and considerations on which decisions are based.Footnote ⁴³ If other states object, states-parties must pursue further consultations to consider alternative courses of action.Footnote ⁴⁴ The Espoo Convention thus offers a detailed roadmap for international cooperation concerning transboundary pollution.Footnote ⁴⁵

In 2008, the UN International Law Commission (ILC) incorporated central features of this burgeoning treaty practice into its Draft Articles on the Prevention of Transboundary Harm from Hazardous Activities (Draft Articles).Footnote ⁴⁶ Consistent with Trail Smelter and Corfu Channel, the Draft Articles provide that states must “take all appropriate measures to prevent significant transboundary harm or at any event to minimize the risk thereof,” and they characterize this obligation as “one of due diligence.”Footnote ⁴⁷ In addition, the Draft Articles demand that states cooperate by providing notice of possible transboundary harm, transmitting relevant technical information about risks, and consulting with one another to “seek solutions based on an equitable balance of interests.”Footnote ⁴⁸ If consultations do not lead to an agreed solution, the state of origin may decide how to proceed, but it must account for the interests of other states.Footnote ⁴⁹ As the ILC has recognized, this approach to transboundary harm draws inspiration from the Lake Lanoux Case discussed in Chapter 2. Like Lake Lanoux, the Draft Articles respect territorial jurisdiction by denying potentially affected states veto power over activities outside their territory. But the Draft Articles also guard against the abuse of territorial sovereignty by requiring states to exercise due diligence to stop activities within their jurisdiction from generating significant transboundary harm.Footnote ⁵⁰ When disputes arise concerning these requirements, the Draft Articles contemplate that states may convene an impartial fact-finding commission or they may settle their disputes through other “peaceful means … chosen by mutual agreement,” such as negotiation, mediation, conciliation, arbitration, or adjudication.Footnote ⁵¹

Cooperating through Courts and Tribunals

In recent years, international courts and tribunals have applied mandatory cooperation to transboundary pollution in a variety of settings. The most important and influential case is the ICJ’s 2010 judgment in a dispute concerning Uruguay’s construction of pulp mills on the River Uruguay.Footnote ⁵² Decades earlier, Argentina and Uruguay had concluded a bilateral treaty establishing detailed rules to govern the river’s use and environmental conservation, and they had established the Administrative Commission of the River Uruguay (CARU) to facilitate conciliation and judicial settlement of disputes.Footnote ⁵³ They also agreed to prepare environmental impact assessments for projects that might impact the river’s water and work through CARU to notify the other party, exchange relevant information, and consult and negotiate over any unresolved concerns.Footnote ⁵⁴ Uruguay took significant steps to comply with these requirements before proceeding with plans to build two pulp mills: it prepared preliminary environmental impact assessments, delivered these assessments to CARU for review, and engaged in consultations with Argentina. Uruguay began constructing the mills, however, before Argentina had a reasonable opportunity to study the possible environmental impacts and before differences between the two states had been resolved.

Fearing significant pollution to the Uruguay River, Argentina initiated proceedings in the ICJ. Argentina asked the ICJ for a judgment that Uruguay had violated the 1975 Statute by proceeding with construction of the two pulp mills without sufficient bilateral cooperation to ensure the river’s environmental conservation. The Court agreed

Citing Corfu Channel, the Court linked these procedural duties to “the principle of prevention,” which, “as a customary rule, has its origins in the due diligence that is required of a State in its territory.”Footnote ⁵⁶ Uruguay’s failure to provide timely notice before authorizing construction was significant, in the Court’s opinion, because “the obligation to notify is intended to create the conditions for successful co-operation between the parties, enabling them to assess the plan’s impact on the river on the basis of the fullest possible information and, if necessary, to negotiate the adjustments needed to avoid the potential damage that it might cause.”Footnote ⁵⁷ Until this consultation and negotiation had concluded, Uruguay was obligated to withhold its authorization for construction; otherwise, “the negotiations between the parties would no longer have any purpose.”Footnote ⁵⁸ Ultimately, however, because Argentina had not presented any conclusive evidence showing that Uruguay’s pulp mills threatened substantial harm to Argentina, the Court declined Argentina’s request to grant compensatory damages and order the closing of the pulp mills.Footnote ⁵⁹ Instead, the Court concluded its judgment by simply affirming that Uruguay had violated its obligations of mandatory cooperation and reminding the parties of their continuing “duty to co-operate with each other” to prevent future pollution.Footnote ⁶⁰ Thus, Pulp Mills cleared the way for Uruguay to continue constructing and operating the disputed pulp mills while encouraging continued cooperation with Argentina through CARU.Footnote ⁶¹

Although Pulp Mills involved legal duties enshrined in a bilateral treaty, international courts and tribunals have recognized that the obligation to cooperate in addressing transboundary pollution applies to the entire international community as a general requirement of customary international law. For example, in the Case Concerning Land Reclamation by Singapore in and Around the Straits of Johor (Land Reclamation),Footnote ⁶² an ITLOS tribunal characterized the duty to cooperate in preventing transboundary environmental harm as a “fundamental principle” not only under UNCLOS but also under “general international law.”Footnote ⁶³ At issue in the case was Malaysia’s challenge to Singapore’s efforts to expand its territory by reclaiming land from the Straits of Johor.Footnote ⁶⁴ Malaysia argued that the project would degrade the marine environment in the straits by adversely impacting the usual patterns of coastal erosion, sedimentation, and water flow through the straits. Malaysia also challenged Singapore’s failure to provide notice, exchange information, and consult and negotiate with Malaysian authorities as required under UNCLOS. While the proceedings were pending, the parties managed to resolve their differences by agreeing to jointly sponsor an environmental impact assessment by an independent body.Footnote ⁶⁵ Singapore also agreed to share relevant information, consult with Malaysian authorities, consider their views, and negotiate toward a mutually acceptable resolution.Footnote ⁶⁶ Accordingly, by the time the tribunal was ready to rule on Malaysia’s claims, the parties had already reached a mutually acceptable path forward consistent with their duties of mandatory cooperation under customary international law. All that remained was for the ITLOS tribunal to incorporate the parties’ agreement into its October 2003 order granting provisional measures.Footnote ⁶⁷

In other proceedings, the ICJ has agreed that under customary international law, the requirements of mandatory cooperation apply to all states in disputes concerning significant transboundary harm. In Certain Activities Carried Out by Nicaragua in the Border Area (San Juan River), Costa Rica and Nicaragua each claimed that the other had violated international law by failing to conduct an environmental impact assessment for works undertaken along the San Juan River.Footnote ⁶⁸ Costa Rica faulted Nicaragua for not undertaking a sufficiently robust environmental impact assessment before commencing a dredging project in the San Juan that might cause harm to the Colorado River and vulnerable wetlands within Costa Rica’s borders.Footnote ⁶⁹ Nicaragua counterclaimed that Costa Rica had failed to conduct an environmental impact assessment for a road it had constructed alongside the river.Footnote ⁷⁰ Quoting Pulp Mills, the Court agreed with the parties that the obligation to undertake an environmental impact assessment for activities that risk transboundary harm “may now be considered a requirement under general international law” that applies even without a binding treaty.Footnote ⁷¹ Nicaragua satisfied this obligation by undertaking a preliminary assessment before beginning its dredging project, the Court concluded,Footnote ⁷² but Costa Rica violated international law by failing to undertake an environmental impact assessment before building the road.Footnote ⁷³

In sum, Pulp Mills, Land Reclamation, and San Juan River illustrate how mandatory cooperation can facilitate peaceful dispute resolution in accordance with equitable principles. When states conduct environmental impact assessments, notify one another of ecological risks, and share other relevant information, they can better understand the harmful impacts of transboundary pollution and develop more effective prevention and mitigation strategies. Through negotiation and third-party dispute resolution, they can reach solutions that equitably accommodate their respective rights and interests.

Catalyzing Environmental Cooperation

At the opening session of the 1972 Stockholm Conference, Secretary-General Maurice Strong declared that the Conference’s aim was “to reconcile man’s legitimate, immediate ambitions with the rights of others, with respect for all life supporting systems, and with the rights of generations yet unborn.”Footnote ⁷⁴ Strong explained that the Stockholm Declaration “holds that all nations must accept responsibility for the consequences of their own actions on environments outside their borders,” and he asserted that “it is essential that this fundamental principle be accepted here if we are to establish a minimum basis for effective international cooperation following this conference.”Footnote ⁷⁵ As Strong correctly perceived, and as this chapter has shown, there is a vital link between state responsibility to prevent transboundary pollution, on the one hand, and international cooperation for environmental protection, on the other. States cannot satisfy their due diligence obligations with respect to transboundary pollution unless they also embrace international cooperation. Put another way, cooperation is necessary for international law to govern transboundary pollution. Once states recognized in Stockholm that they must prevent transboundary pollution, it followed that they would also have to share information, consult, negotiate, and accept third-party dispute resolution to ensure their efforts were effective. Cooperation thus became mandatory under international environmental law.Footnote ⁷⁶

These developments have transformed global governance. As this chapter has shown, states have enshrined the requirements of mandatory cooperation in international environmental treaties; the ILC has endorsed mandatory cooperation in its Draft Articles on Transboundary Harm; and international courts and tribunals have applied the requirements of mandatory cooperation as customary international law. There is no longer any serious doubt that international law obligates states to cooperate with one another to ensure that activities within their jurisdiction do not introduce harmful pollution abroad. But the ramifications of mandatory cooperation for environmental protection run deeper still. Mandatory cooperation has accelerated the progressive development of international environmental law by pressing states to share information, consult, and negotiate with one another. Since Stockholm, states have concluded a host of agreements to combat transboundary pollution, including air pollution,Footnote ⁷⁷ water pollution,Footnote ⁷⁸ hazardous wastes,Footnote ⁷⁹ and nuclear safety.Footnote ⁸⁰ The rapid proliferation of environmental treaties and institutions following the Stockholm Conference reflects the new reality that states are expected to work together as committed partners to address transboundary pollution. Mandatory cooperation has become both a cornerstone of international environmental law and a powerful catalyst for the law’s progressive development.

Much of the development of international environmental law consists of “soft law” instruments rather than legally binding treaties.Footnote ⁸¹ Nonbinding declarations, such as the Stockholm Declaration, the 1992 Rio Declaration on Environment and Development,Footnote ⁸² and the 2002 Johannesburg Declaration on Sustainable Development,Footnote ⁸³ articulate broad principles and suggest best practices for responsible environmental stewardship. Critics sometimes dismiss such nonbinding declarations as phantom laws that compromise international law’s integrity as a normative system.Footnote ⁸⁴ However, reliance on soft commitments need not be viewed as a “pathological phenomenon” that enfeebles international legal order.Footnote ⁸⁵ In the environmental arena, soft law instruments usually reflect serious efforts to bolster states’ political commitments and practical capacities to prevent transboundary environmental harm. When these efforts are successful, they can reduce the need for binding treaties. To the extent states relate to each other as neighbors, they may find ways to resolve their differences through friendly cooperation based on informal norms and practices rather than resort to legal obligations and enforcement.Footnote ⁸⁶ Even if treaties might be preferable in a perfect world, soft law commitments are sometimes the most robust form of normative consensus that states can achieve through diligent cooperation. Mandatory cooperation thus helps to explain and contextualize soft law’s prevalence in international environmental governance, showing why it may be worth celebrating, or at least having, even as the international community continues to pursue more intensive commitments to prevent or minimize transboundary pollution.

Yet despite evident progress, the dangers of transboundary pollution have never been greater. In the next chapter, we consider how mandatory cooperation applies to the supreme existential danger of our time: atmospheric pollution and its devastating contribution to global warming. As we will see, in principle, states must exercise due diligence to prevent carbon emissions from exacerbating global warming. This obligation flows, in part, from the general duty to prevent transboundary harm. Additionally, states now accept that climate change and its adverse effects qualify as a common concern of humanity that is entrusted to the joint stewardship of the international community as a whole. Accordingly, joining the fight against climate change is no longer an option states can take up or refuse; it is mandatory under international law.