International Environmental Law Explained
- Edmarverson A. Santos
- 1 minute ago
- 27 min read
Introduction
International environmental law is the body of public international law governing environmental protection and the use of natural resources where international obligations arise. Its scope extends beyond pollution crossing a territorial boundary. It also encompasses global problems such as climate change and biodiversity loss, protection of the marine environment and areas beyond national jurisdiction, and treaty obligations concerning activities conducted within States themselves. Binding rules derive principally from treaties and customary international law, alongside other applicable rules of international law. Declarations, institutional decisions, technical standards, and other non-binding instruments have also influenced the field, but their legal effect depends on their source, wording, and institutional context.
The law developed in part through traditional questions of territorial sovereignty, shared resources, and cross-border injury. The Trail Smelter arbitration remains an influential early authority on transboundary air pollution, although its reasoning was closely connected to the facts and evidentiary record before the tribunal (Trail Smelter, 1941). Principle 21 of the 1972 Stockholm Declaration combined the sovereign right of States to exploit their natural resources with responsibility to ensure that activities within their jurisdiction or control do not cause environmental damage to other States or to areas beyond national jurisdiction. Principle 2 of the 1992 Rio Declaration retained that basic formulation, while other Rio principles addressed precaution, environmental impact assessment, cooperation, participation, sustainable development, and differentiated responsibilities (United Nations, 1972; United Nations, 1992). Both declarations are non-binding instruments, even though particular propositions expressed in them may independently be embodied in treaties or reflect customary international law.
Prevention now occupies a central position in the legal structure of the field because environmental damage may be irreversible, cumulative, geographically dispersed, or difficult to remedy once it has occurred. In its Nuclear Weapons advisory opinion, the International Court of Justice stated that the general obligation of States to ensure that activities within their jurisdiction and control respect the environment of other States or areas beyond national control forms part of international environmental law (ICJ, 1996). In Pulp Mills, the Court described the principle of prevention as a customary rule rooted in due diligence and recognized a requirement under general international law to undertake an environmental impact assessment where a proposed industrial activity presents a risk of significant adverse transboundary impact (ICJ, 2010). The Court subsequently clarified that the underlying principle is not confined to industrial activities (ICJ, 2015).
The Court’s 2025 advisory opinion on Obligations of States in respect of Climate Change further clarified the contemporary relationship between environmental treaties and general international law. It identified the duty to prevent significant environmental harm and the duty to cooperate for the protection of the environment as directly relevant rules of customary international law and held that the duty of prevention applies to the climate system (ICJ, 2025). The Court also rejected the proposition that the climate treaties generally displace other applicable rules through lex specialis. Environmental obligations may consequently arise simultaneously from specialized treaty regimes and from general international law, depending on the subject and the States concerned.
Environmental principles do not, however, possess uniform legal status or operate identically. In its 2025 advisory opinion, the ICJ treated sustainable development, common but differentiated responsibilities and respective capabilities, equity, intergenerational equity, and the precautionary approach or principle as guiding principles for the interpretation and application of the most directly relevant legal rules. It declined, by contrast, to regard the polluter-pays principle as part of the applicable law for the purposes of that opinion (ICJ, 2025). The duty to prevent significant environmental harm is framed through due diligence rather than as a general rule of strict liability, while the content of precaution and other environmental principles may depend on the treaty, customary rule, regional system, or institutional setting in which they operate. Sound analysis of international environmental law consequently requires identification of the source, addressees, scope, conditions of application, and legal consequences of each claimed rule.
1. What International Environmental Law Governs
International environmental law governs environmental protection and natural-resource questions that engage international obligations. Some of these problems are physically transboundary, such as air pollution, contamination of international watercourses, and pollution entering a shared marine environment. Others concern global systems or resources, including climate change, biodiversity loss, ozone depletion, and environmental degradation in areas beyond national jurisdiction. Separate treaty regimes regulate hazardous wastes and chemicals, endangered species, desertification, marine pollution, and the conservation or sustainable use of particular natural resources (UNFCCC, 1992; CBD, 1992; Basel Convention, 1989; UNCLOS, 1982).
International obligations may also regulate conduct occurring wholly within national territory. The Convention on Biological Diversity, for example, requires parties to adopt measures concerning conservation and sustainable use within areas under their jurisdiction, while the climate regime regulates national emissions and policies as part of a collective response to changes in the global climate system. UNCLOS establishes a general obligation to protect and preserve the marine environment and requires States to prevent, reduce, and control pollution from multiple sources (UNCLOS, 1982, arts. 192–194). Since 17 January 2026, the BBNJ Agreement has provided its parties with a binding framework for the conservation and sustainable use of marine biological diversity in areas beyond national jurisdiction (BBNJ Agreement, 2023).
These regimes remain part of public international law. Rules governing treaty interpretation, State responsibility, jurisdiction, international organizations, and dispute settlement continue to apply to environmental obligations unless the relevant legal regime establishes a valid special rule. Environmental treaties are not self-contained systems detached from general international law. Their distinctive regulatory features arise largely from the characteristics of environmental risks: ecological processes cross political boundaries, scientific knowledge develops over time, private conduct may generate consequences extending beyond the immediate actor, and effective regulation often requires sustained cooperation among numerous States rather than a purely bilateral relationship (Dupuy and Viñuales, 2018).
States remain the principal makers and addressees of international environmental obligations. They negotiate treaties, contribute to the formation of customary international law, enact implementing measures, regulate activities within their jurisdiction, and participate in institutions created by multilateral environmental agreements. International organizations, treaty secretariats, Conferences of the Parties, compliance bodies, and scientific institutions may exercise important functions, but their legal authority derives from different sources and varies substantially between regimes.
Non-State actors also participate extensively in environmental governance without acquiring the same legal status as States. NGOs, scientific organizations, businesses, Indigenous Peoples, and local communities may contribute to negotiations, scientific assessment, consultation, monitoring, implementation, and compliance processes. Rio Principle 10 links environmental governance with access to information, public participation, and access to judicial and administrative proceedings, while Principle 22 recognizes the role of Indigenous people and their communities and other local communities in environmental management and development (Rio Declaration, 1992, Principles 10 and 22). Specific treaties provide additional participatory rights or institutional roles. Such participation does not by itself make non-State actors parties to international environmental agreements or subject them directly to obligations imposed on States.
2. How Environmental Protection Entered International Law
International environmental regulation emerged incrementally rather than through the creation of a comprehensive environmental code. Early agreements usually addressed particular species, resources, or sources of pollution. The 1911 Convention between the United States, Great Britain, Russia and Japan for the Preservation and Protection of Fur Seals restricted pelagic sealing in parts of the North Pacific. The 1946 International Convention for the Regulation of Whaling established an international regime for whale conservation and whaling regulation, while the 1954 International Convention for the Prevention of Pollution of the Sea by Oil targeted a specific source of marine pollution. These agreements reflected a sectoral approach to resource management and environmental injury.
The Trail Smelter arbitration became a major reference point because it confronted industrial pollution originating in one State and producing injury in another. Fumes from a smelter in Canada caused damage in the United States, and the tribunal addressed both compensation for past injury and arrangements intended to prevent future damage. Its 1941 award articulated a prohibition on using or permitting the use of territory in a manner causing serious injury by fumes in another State where the consequences were sufficiently serious and established by clear and convincing evidence (Trail Smelter, 1938; 1941). The case is historically significant, but its specific language and evidentiary standard arose from a particular bilateral dispute and should not be treated as a complete formulation of the contemporary customary duty of prevention.
The 1972 United Nations Conference on the Human Environment at Stockholm marked a broader shift in the legal treatment of environmental protection. Principle 21 of the Stockholm Declaration affirmed the sovereign right of States to exploit their own resources pursuant to their environmental policies while also recognizing their responsibility to ensure that activities within their jurisdiction or control do not damage the environment of other States or areas beyond national jurisdiction (Stockholm Declaration, 1972, Principle 21). Environmental protection was thus linked to sovereignty rather than placed outside it. The conference also strengthened the institutional position of environmental issues within the United Nations system, followed by the establishment of the United Nations Environment Programme later that year (UN General Assembly Resolution 2997 (XXVII), 1972).
The 1992 United Nations Conference on Environment and Development in Rio de Janeiro developed a more extensive framework for addressing environmental protection and development together. Principle 2 retained the core Stockholm formulation, while other provisions addressed the right to development, integration of environmental protection into development, common but differentiated responsibilities, public participation, precaution, environmental impact assessment, and cooperation (Rio Declaration, 1992, Principles 2–4, 7, 10, 15, 17 and 27). The UNFCCC and the Convention on Biological Diversity were opened for signature at Rio, demonstrating the parallel use of non-binding declarations and binding treaty regimes to address environmental problems of global scale.
Neither the Stockholm Declaration nor the Rio Declaration is a treaty. Their legal significance derives from their influence on treaty drafting, State practice, institutional processes, and judicial reasoning, while the status of individual propositions must be assessed separately. The broader development from Trail Smelter to Stockholm and Rio also reveals a change in regulatory emphasis. International law increasingly addressed environmental risk before injury occurred through assessment, notification, consultation, cooperation, precautionary measures, and continuing regulation rather than relying primarily on compensation after damage had been completed.
3. Sources and Legal Status of Environmental Rules
The legal force of an environmental proposition depends first on its source. Multilateral environmental agreements provide much of the detailed regulation in the field, but treaty obligations bind parties according to the terms of the relevant agreement. The UNFCCC, Paris Agreement, Convention on Biological Diversity, Basel Convention, Montreal Protocol, UNCLOS, and numerous regional instruments establish different rights and duties for different groups of States. Their provisions are interpreted under the applicable law of treaties, including the rules reflected in Articles 31 to 33 of the Vienna Convention on the Law of Treaties (VCLT, 1969).
Customary international law has a different basis. Article 38(1)(b) of the Statute of the International Court of Justice refers to international custom as evidence of a general practice accepted as law, conventionally analyzed through State practice and opinio juris (ICJ Statute, art. 38(1)(b); ILC, 2018). A rule does not become customary merely because it appears repeatedly in treaties or declarations. Judicial decisions can provide authoritative evidence concerning the existence and content of customary obligations, but they remain subsidiary means for the determination of rules of law rather than an independent source equivalent to international legislation.
Environmental governance also relies heavily on non-binding instruments. The Stockholm and Rio Declarations, General Assembly resolutions, guidelines, plans of action, technical standards, and institutional decisions may influence treaty negotiations, interpretation, national legislation, and subsequent State practice. Their legal relevance can be substantial even where they do not themselves create binding obligations. The legal status of any environmental principle must be established through the source claimed for it, including applicable treaty provisions, evidence of State practice and opinio juris, or another recognized basis in international law.
Decisions adopted by Conferences of the Parties and comparable treaty bodies require equally careful treatment. Their legal effect depends on the powers granted by the constitutive treaty, the applicable decision-making procedure, and the terms of the particular decision. In its 2025 advisory opinion on climate change, the International Court of Justice explained that decisions adopted under the climate treaties may create binding obligations where the treaty provides for that effect. The Court also recognized that such decisions may constitute subsequent agreements concerning treaty interpretation under Article 31(3)(a) of the Vienna Convention when they express substantive agreement among the parties on the meaning of the treaty (ICJ, 2025, para. 184).
That reasoning does not give every COP decision across international environmental law the same legal character. A treaty-body decision may amount to a binding measure, an interpretive agreement, a procedural act, technical guidance, or a political decision depending on the governing instrument and the circumstances of its adoption. Contemporary environmental regimes develop through an interaction among original treaty provisions, amendments, institutional decisions, interpretive practice, customary rules, and non-binding standards. Distinguishing among these forms is necessary before assigning legal force to any particular environmental requirement.
4. No-Harm, Prevention, and Transboundary Pollution
The no-harm principle and the duty to prevent significant environmental harm place legal limits on the exercise of territorial sovereignty. Stockholm Principle 21 and Rio Principle 2 recognize sovereign rights over natural resources while coupling those rights with responsibility concerning activities within a State’s jurisdiction or control that affect the environment of other States or areas beyond national jurisdiction. In its advisory opinion on the Legality of the Threat or Use of Nuclear Weapons, the International Court of Justice stated that the general obligation of States to ensure that activities within their jurisdiction and control respect the environment of other States or areas beyond national control forms part of international law relating to the environment (ICJ, 1996, para. 29).
Trail Smelter provided an influential early formulation in the narrower context of transboundary air pollution. Its continuing relevance lies in the recognition that territorial sovereignty does not confer an unrestricted entitlement to externalize serious environmental injury across an international boundary. Later international jurisprudence developed the preventive obligation beyond the circumstances of industrial fumes and a bilateral claims process. In Pulp Mills on the River Uruguay, the ICJ described the principle of prevention as a customary rule having its origins in the due diligence required of a State within its territory (ICJ, 2010, para. 101).
Prevention is not a general rule of strict liability for environmental damage. The customary duty is an obligation of conduct requiring States to exercise the diligence appropriate to the risk rather than guaranteeing that no significant harm will ever occur. Pulp Mills connected due diligence with the adoption of appropriate rules and measures, vigilance in their enforcement, administrative control, and monitoring of public and private operators (ICJ, 2010, para. 197). A finding of environmental damage does not, by itself, establish breach of the preventive obligation. The legal inquiry concerns whether the State took the measures required by the applicable standard of due diligence.
The content of that standard varies with the circumstances. In its 2025 advisory opinion on climate change, the ICJ confirmed that due diligence is context-dependent and capable of evolving with scientific and technological knowledge. The Court identified considerations including the probability and seriousness of harm, available scientific information, relevant international rules and standards, the capabilities and national circumstances of the State, and measures reasonably available to it (ICJ, 2025, paras. 280–294). A higher level of risk demands greater vigilance. Differences in capacity may influence what can reasonably be required, but limited capability does not create a general exemption from the obligation.
Transboundary pollution illustrates the practical importance of preventive regulation. Airborne pollutants can cross several national borders, industrial projects may affect shared rivers and ecosystems, hazardous wastes may move through international trade, and marine pollution can spread beyond the jurisdiction in which it originates. Treaty regimes translate general preventive logic into more specific obligations. UNCLOS, for example, requires States to prevent, reduce, and control pollution of the marine environment and to ensure that activities under their jurisdiction or control do not cause pollution damage to other States and their environment or spread beyond areas where they exercise sovereign rights (UNCLOS, 1982, art. 194).
International jurisprudence has progressively clarified these obligations without reducing them to a single universal formula. In Gabčíkovo-Nagymaros, the ICJ recognized the relevance of newly developed environmental norms and standards to the continuing performance of a long-standing treaty and referred to the need to reconcile economic development with environmental protection (ICJ, 1997). Pulp Mills provided a more precise articulation of customary prevention and due diligence, while the 2015 Costa Rica–Nicaragua judgments clarified the relationship between significant transboundary risk, environmental impact assessment, and substantive environmental harm (ICJ, 2010; ICJ, 2015). In 2025, the Court applied the customary duty to prevent significant environmental harm to the climate system despite the diffuse and cumulative character of greenhouse gas emissions, confirming that the global nature of the risk does not remove it from the scope of the customary preventive obligation (ICJ, 2025).
5. Precaution and Scientific Uncertainty
Prevention and precaution address different aspects of environmental risk. The duty of prevention requires States to exercise due diligence against significant environmental harm, while precaution informs preventive conduct when uncertainty remains about the probability, magnitude, or consequences of potentially serious damage. The distinction is not absolute. Precaution can operate within the broader preventive obligation by affecting what due diligence requires when scientific knowledge is incomplete.
Principle 15 of the Rio Declaration contains the most widely cited general formulation. It provides that, where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used to postpone cost-effective measures to prevent environmental degradation, while calling for the precautionary approach to be applied according to States’ capabilities (Rio Declaration, 1992, Principle 15). The formulation permits regulatory action before scientific certainty has been reached, but it neither requires prohibition of every uncertain activity nor prescribes a uniform response to every environmental risk.
The legal effect of precaution depends heavily on its source. Article 3(3) of the UNFCCC, for example, directs parties to take precautionary measures to anticipate, prevent, or minimize the causes of climate change and mitigate its adverse effects. Precautionary formulations also appear in treaty regimes concerning biosafety, persistent organic pollutants, fisheries, and marine environmental protection. Differences in wording, thresholds, institutional setting, and the measures prescribed mean that treaty-specific precautionary obligations cannot be converted automatically into a single universal rule.
International courts and tribunals have approached the wider doctrine with care. In Pulp Mills on the River Uruguay, Argentina argued that the precautionary principle altered the allocation of the burden of proof. The International Court of Justice accepted that a precautionary approach might be relevant to the interpretation and application of the 1975 Statute of the River Uruguay, but held that this did not reverse the burden of proof (ICJ, 2010, para. 164). The judgment did not recognize an autonomous precautionary rule with a uniform customary content.
Subsequent environmental jurisprudence has linked precaution more closely to due diligence. In its 2011 advisory opinion on activities in the Area, the Seabed Disputes Chamber treated the precautionary approach as an integral element of the due-diligence obligations applicable under the deep-seabed regime and noted a trend toward its incorporation into customary international law (ITLOS, 2011, paras. 131–135). In the 2024 climate advisory opinion, ITLOS held that States Parties to UNCLOS must apply the precautionary approach when exercising due diligence in relation to marine pollution from anthropogenic greenhouse-gas emissions, including where scientific evidence concerning the probability or severity of harm remains insufficient (ITLOS, 2024, para. 242).
The ICJ developed the relationship further in its 2025 advisory opinion on climate change. It treated precaution as one of the principles guiding interpretation and application of the most directly relevant rules of international law and, more specifically, held that the precautionary approach or principle, where applicable, informs the measures required by due diligence under the customary duty to prevent significant environmental harm (ICJ, 2025, paras. 161, 293–294). A State cannot disregard plausible indications of a serious environmental risk merely because scientific certainty has not yet been reached.
Precaution consequently affects the timing and intensity of environmental regulation rather than supplying a universal command to prohibit uncertain activities. Scientific uncertainty may influence monitoring requirements, information gathering, risk assessment, regulatory safeguards, and the point at which preventive action becomes necessary. The governing rule, the seriousness of the threatened harm, available scientific knowledge, State capabilities, and the measures contemplated by the applicable legal regime determine what precaution is demanded in a particular case.
6. Environmental Impact Assessment and Cooperation
Environmental impact assessment gives procedural effect to prevention by requiring potentially significant environmental consequences to be examined before an activity proceeds. In Pulp Mills, the ICJ concluded that the practice of environmental impact assessment had gained sufficient acceptance among States to constitute a requirement under general international law where a proposed industrial activity presented a risk of significant adverse transboundary impact, particularly on a shared resource (ICJ, 2010, para. 204). Assessment enables a State to identify foreseeable risks before deciding whether and under what conditions an activity may be authorized.
The customary requirement was clarified in the joined Costa Rica–Nicaragua cases. The Court held that a State must first determine whether a proposed activity carries a risk of significant transboundary harm. An environmental impact assessment is required when such a risk exists, and the obligation is not confined to industrial projects of the kind involved in Pulp Mills (ICJ, 2015, para. 104). Costa Rica breached this obligation by failing to conduct an assessment before constructing Route 1856 along the San Juan River.
General international law does not prescribe one detailed methodology for every environmental impact assessment. Pulp Mills left the specific content of an EIA to be determined through domestic legislation or the authorization process, taking account of the nature and magnitude of the proposed activity, its likely environmental effects, and the requirements of due diligence (ICJ, 2010, para. 205). The assessment must be undertaken before implementation. Where the circumstances warrant continuing supervision, monitoring environmental effects after operations begin remains part of the preventive process.
Environmental assessment also connects due diligence with interstate cooperation. Where an assessment confirms a risk of significant transboundary harm, notification and good-faith consultation with the potentially affected State may be required to determine appropriate preventive or mitigating measures (ICJ, 2015, para. 104). These procedures permit States to exchange information, evaluate possible consequences, and consider measures capable of reducing the risk. The customary obligation identified by the Court is one of notification and consultation; it was not characterized as a general requirement to obtain the affected State’s consent.
Treaties may impose substantially more elaborate procedures. The 1991 Espoo Convention requires its parties to assess specified activities likely to cause significant adverse transboundary effects and establishes detailed rules governing notification, environmental documentation, public participation, and interstate consultation (Espoo Convention, 1991, arts. 2–5). These treaty obligations cannot simply be projected onto non-parties as though every procedural detail represented customary international law.
The law of the sea contains its own assessment requirement. Article 206 of UNCLOS applies where States have reasonable grounds for believing that planned activities under their jurisdiction or control may cause substantial pollution of, or significant and harmful changes to, the marine environment. In its 2024 advisory opinion, ITLOS confirmed that Article 206 applies to planned activities capable of producing such effects through anthropogenic greenhouse-gas emissions and emphasized the relevance of cumulative environmental effects (ITLOS, 2024). The comparison illustrates a recurring feature of environmental law: a general preventive obligation may coexist with treaty rules that prescribe different thresholds, procedures, and institutional consequences.
7. Environmental Law Principles of Balance and Equity
Environmental protection frequently raises questions about development, access to natural resources, regulatory costs, historical contributions to environmental degradation, and differences in State capacity. International environmental law addresses these distributive concerns through principles whose functions and legal status are not uniform. Sustainable development, common but differentiated responsibilities and respective capabilities, intergenerational equity, and polluter-pays influence different aspects of legal interpretation and regulatory design.
Sustainable development is primarily associated with the integration of environmental protection and economic and social development. Rio Principle 3 refers to the developmental and environmental needs of present and future generations, while Principle 4 provides that environmental protection must constitute an integral part of the development process (Rio Declaration, 1992, Principles 3–4). In Gabčíkovo-Nagymaros, the ICJ referred to the need to reconcile economic development with environmental protection through the concept of sustainable development (ICJ, 1997, para. 140). The concept did not operate as an autonomous rule resolving the dispute independently of the treaty obligations before the Court.
The 2025 climate advisory opinion gave sustainable development a more explicit interpretive function. The Court considered its continuous and widespread recognition and treated it as a principle capable of guiding treaty interpretation and the application of customary environmental obligations, including prevention and cooperation (ICJ, 2025, para. 147). Its function remains connected to existing legal rules rather than supplying a fixed formula for deciding every conflict between economic activity and environmental protection.
Common but differentiated responsibilities and respective capabilities concerns the allocation of environmental burdens among States facing a common problem under unequal circumstances. Rio Principle 7 recognizes common responsibilities while acknowledging differentiation, and Article 3(1) of the UNFCCC incorporates equity and CBDR-RC into the climate regime. Article 2(2) of the Paris Agreement provides that the Agreement will be implemented to reflect equity and CBDR-RC in light of different national circumstances.
Differentiation varies between regimes. The Montreal Protocol, Kyoto Protocol, UNFCCC, and Paris Agreement employ different legal techniques to account for economic capacity, development, responsibility, and national circumstances. In its 2025 advisory opinion, the ICJ treated CBDR-RC as a guiding principle relevant to climate obligations and connected differing capabilities with the content of due diligence (ICJ, 2025, paras. 148–151, 161, 290–292). Greater capacity may support a more demanding standard of conduct, but limited capacity does not create a general exemption from the obligation to exercise due diligence.
Intergenerational equity addresses the temporal distribution of environmental benefits and burdens. Article 3(1) of the UNFCCC refers expressly to protecting the climate system for the benefit of present and future generations, while the Paris Agreement recognizes intergenerational equity in its preamble. The ICJ treated intergenerational equity in 2025 as a manifestation of equity relevant to the interpretation and application of existing obligations, requiring due regard for the interests of future generations and the long-term effects of present conduct (ICJ, 2025, paras. 156–157). Its function is interpretive rather than an unlimited source of independent legal obligations.
Polluter-pays has a different trajectory. Rio Principle 16 calls on national authorities to promote the internalization of environmental costs and the approach that the polluter should, in principle, bear the cost of pollution (Rio Declaration, 1992, Principle 16). The concept has acquired binding effects in particular treaty and domestic legal contexts, but its broader international status remains more limited. In its 2025 advisory opinion, the ICJ did not regard polluter-pays as part of the applicable law for the purposes of the climate proceedings and separately left open the possible development of forms of strict liability for hazardous or otherwise non-wrongful activities (ICJ, 2025, paras. 159–160).
The expression “environmental law principles” consequently covers legally different propositions. Some principles assist in determining the content of customary obligations; others derive binding force from particular treaties; several guide interpretation or differentiation; and some remain more influential in policy or domestic regulation than as autonomous rules of general international law. Their common environmental purpose does not establish a common legal status.
8. How Environmental Treaties Regulate Global Problems
Environmental treaties rarely operate as static agreements whose regulatory content is exhausted at the moment of adoption. Many create institutions capable of responding to new scientific knowledge, changing environmental conditions, and implementation problems. Framework conventions may establish general duties and institutional structures, while protocols, annexes, amendments, technical standards, treaty-body decisions, reporting systems, scientific assessments, financial arrangements, and compliance procedures develop the regime over time. The legal effect of each instrument still depends on the authority conferred by the underlying treaty.
The ozone regime demonstrates the value of this adaptive structure. The 1985 Vienna Convention established a framework for research, information exchange, cooperation, and further legal measures. The 1987 Montreal Protocol introduced binding controls on specified ozone-depleting substances and subsequently developed through amendments, adjustments, scientific and technical assessment, a non-compliance procedure, and financial assistance. The 2016 Kigali Amendment extended the regime to the phasedown of hydrofluorocarbons, substances with high global-warming potential that had become widely used as substitutes for ozone-depleting chemicals.
Climate governance developed differently. The UNFCCC established the foundational legal and institutional framework, and the 1997 Kyoto Protocol imposed quantified emission limitation and reduction commitments on specified developed-country parties. Kyoto remains in force, although its second commitment period ended in 2020. The 2015 Paris Agreement introduced a different and more universal architecture in which all parties undertake mitigation obligations while differentiation continues through equity, national circumstances, capabilities, and CBDR-RC.
Article 4(2) of the Paris Agreement requires every party to prepare, communicate, and maintain successive nationally determined contributions and to pursue domestic mitigation measures with the aim of achieving their objectives. National determination does not confer unrestricted discretion over the content of an NDC. Article 4(3) requires successive contributions to represent progression and reflect each party’s highest possible ambition. In 2025, the ICJ held that this language imposes legal constraints on the formulation of NDCs: their level of ambition must be capable of making an adequate contribution to the temperature goal, and parties must exercise stringent due diligence in preparing and communicating them (ICJ, 2025, paras. 242–246). The Paris system combines those obligations with transparency, periodic global stocktakes, accounting rules, and a facilitative compliance mechanism (Paris Agreement, 2015, arts. 4, 13–15).
Biodiversity treaties use another combination of general and specialized regulation. The Convention on Biological Diversity is organized around conservation, sustainable use, and the fair and equitable sharing of benefits arising from genetic resources (CBD, 1992, art. 1). The Cartagena Protocol regulates specified risks associated with living modified organisms, while the Nagoya Protocol develops rules governing access to genetic resources and benefit-sharing. National strategies, reporting, scientific bodies, and treaty institutions support implementation, but their authority remains tied to the relevant constitutive instruments.
Marine environmental protection is spread across overlapping agreements. Part XII of UNCLOS establishes general duties concerning protection and preservation of the marine environment, pollution control, cooperation, monitoring, assessment, and enforcement. More specialized agreements regulate shipping, dumping, fisheries, regional seas, and particular sources of pollution. Since 17 January 2026, the BBNJ Agreement has been in force for its parties, adding binding rules on marine genetic resources, area-based management tools, environmental impact assessments, and capacity-building in relation to marine biological diversity in areas beyond national jurisdiction (BBNJ Agreement, 2023).
Hazardous substances and wastes are governed through several legally distinct but institutionally connected regimes. The Basel Convention regulates transboundary movements and disposal of hazardous and other covered wastes; the Rotterdam Convention establishes a prior informed consent procedure for certain hazardous chemicals and pesticides in international trade; the Stockholm Convention targets persistent organic pollutants; and the Minamata Convention regulates mercury across production, trade, products, industrial processes, emissions, releases, storage, and waste. Their overlap does not merge them into one treaty system.
International environmental law is consequently organized through specialized regulatory regimes rather than a single hierarchy headed by a global environmental legislature. Their institutions exercise powers granted by the relevant treaties, while general international law continues to govern questions such as interpretation, responsibility, and dispute settlement. Scientific cooperation, common principles, coordinated institutions, and overlapping subject matter connect these regimes without eliminating their separate legal identities.
9. Compliance, Responsibility, and Dispute Settlement
Environmental treaties frequently seek compliance through continuing supervision and assistance rather than relying primarily on adversarial enforcement. Reporting obligations, transparency systems, inventories, implementation plans, scientific review, technical assistance, financing, and specialized compliance committees allow difficulties to be identified while implementation is still possible. The design reflects the fact that many environmental objectives depend on repeated national action and collective performance over long periods.
The mechanisms themselves differ. Article 15 of the Paris Agreement establishes an expert-based mechanism that is facilitative, transparent, non-adversarial, and non-punitive. The Montreal Protocol operates a separate non-compliance procedure through its Implementation Committee and the Meeting of the Parties. The Basel Convention likewise uses a mechanism directed toward implementation and compliance, while the Minamata Convention establishes a facilitative Implementation and Compliance Committee that takes account of national circumstances and capabilities. These procedures can address failures without performing the same legal function as a court adjudicating State responsibility.
The general law of State responsibility answers a different question. Under Article 2 of the International Law Commission’s Articles on Responsibility of States for Internationally Wrongful Acts, an internationally wrongful act exists when conduct attributable to a State constitutes a breach of an international obligation binding upon it (ILC, 2001, art. 2). Environmental damage is not an additional universal element that must always be proven. A procedural obligation, such as a duty to conduct an environmental impact assessment, may be breached without proof that the anticipated environmental injury ultimately occurred. Conversely, the existence of environmental damage does not establish automatically that a preventive obligation was violated.
Where responsibility is established, the ordinary consequences of an internationally wrongful act apply unless a relevant special rule provides otherwise. They may include cessation of continuing wrongful conduct and, where appropriate, assurances or guarantees of non-repetition, together with full reparation for injury caused by the wrongful act (ILC, 2001, arts. 30–31). Reparation may consist of restitution, compensation, satisfaction, or a combination of these forms (arts. 34–37).
Environmental injury raises distinctive remedial questions because ecological loss may extend beyond commercially measurable property damage. In its 2018 compensation judgment in Certain Activities Carried Out by Nicaragua in the Border Area, the ICJ confirmed that damage to the environment itself is compensable under international law. It awarded compensation for the impairment or loss of environmental goods and services, as well as restoration costs attributable to Nicaragua’s internationally wrongful conduct (ICJ, 2018). The judgment did not create automatic liability for environmental degradation; causation and the connection between the wrongful conduct and the compensable injury remained necessary.
Judicial settlement is decentralized. The ICJ may adjudicate an environmental dispute only where the States concerned have supplied a valid jurisdictional basis. ITLOS and arbitral tribunals constituted under UNCLOS may determine marine environmental disputes within the jurisdiction conferred by that Convention, while other tribunals derive authority from particular treaties or agreements. Regional human rights systems may also address environmental harm where the requirements of the applicable human rights treaty are satisfied. The Inter-American Court of Human Rights examined environmental obligations connected to protected rights in Advisory Opinion OC-23/17, and the European Court of Human Rights held in Verein KlimaSeniorinnen Schweiz and Others v. Switzerland that Article 8 of the European Convention encompasses effective protection against serious adverse effects of climate change, subject to the Convention’s jurisdictional and standing requirements (IACtHR, 2017; ECtHR, 2024).
No international court possesses compulsory jurisdiction over all environmental disputes and all States. International environmental law instead relies on overlapping mechanisms: domestic regulation, reporting and review, treaty institutions, financial and technical assistance, interstate cooperation, adjudication where jurisdiction exists, and the general law of State responsibility. This decentralized structure limits neither the binding character of applicable environmental obligations nor the need to identify the precise legal mechanism through which a particular obligation can be enforced.
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Conclusion
International environmental law is a legally identifiable field of public international law rather than a single comprehensive environmental code. General rules of international law operate alongside specialized regimes governing climate change, biodiversity, oceans, hazardous substances, wastes, atmospheric protection, and shared natural resources. Its institutional structure reflects environmental problems that cross borders, accumulate over time, depend on scientific knowledge, and often require sustained cooperation among numerous States.
Prevention is one of the field’s strongest organizing ideas, but it operates through several legally distinct mechanisms. Due diligence, environmental impact assessment, notification, consultation, monitoring, precaution, treaty-specific standards, and institutional compliance procedures address environmental risks at different stages and under different sources of law. When binding obligations are breached, the general law of State responsibility may govern the consequences unless the applicable regime contains relevant special rules.
The legal status of environmental principles cannot be inferred from their prominence in environmental policy. No-harm, prevention, precaution, sustainable development, CBDR-RC, intergenerational equity, polluter-pays, and environmental impact assessment differ in source, scope, addressees, and legal effect. Determining what international environmental law requires depends on identifying the applicable rule, the authority from which it derives, the actors it binds, the circumstances triggering it, and the consequences attached to non-compliance.
References
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Convention on Biological Diversity (1992) adopted 5 June 1992, entered into force 29 December 1993, 1760 UNTS 79.
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Vienna Convention on the Law of Treaties (1969) adopted 23 May 1969, entered into force 27 January 1980, 1155 UNTS 331.
