International Environmental Law — Foundations: Sources, Landmark Conferences, Sustainable Development and Core Principles

Pollution drifts across borders, species migrate, oceans and the atmosphere belong to no single State, and climate change is the collective product of every country's emissions. International environmental law is the body of rules that governs how States behave towards the shared environment and towards one another in environmental matters. This note explains its sources, traces its growth through the great conferences from Stockholm (1972) to Rio+20 (2012) and the Sustainable Development Goals, and sets out its core principles: common but differentiated responsibilities, the no-harm rule, State responsibility, environmental impact assessment and precaution.

1. Introduction to International Environmental Law

International environmental law is the part of public international law that deals with the protection of the environment and the sustainable use of natural resources. It is a young field. Before 1972 it consisted of a handful of treaties on fisheries, birds and wildlife and one famous arbitration, Trail Smelter (1938 and 1941). Since then it has grown into a dense web of more than a thousand multilateral and bilateral agreements, supported by international institutions, customary rules and judicial decisions.

Its distinctive features are these. It deals with shared resources (rivers, migratory species) and global commons (the high seas, the atmosphere, outer space, Antarctica). It relies heavily on framework treaties supplemented by protocols. It makes extensive use of soft law. It is closely tied to science, since obligations are defined by reference to scientific assessments such as those of the Intergovernmental Panel on Climate Change. And it is pervaded by the tension between environment and development, and between the historical responsibility of developed countries and the development needs of developing ones.

2. Sources of International Environmental Law

Article 38(1) lists the formal sources. In the environmental field, two further categories matter a great deal: soft law (declarations, resolutions and guidelines) and the decisions of treaty bodies, such as the Conferences of the Parties, which develop treaty obligations in detail.

2.1 Treaties and environmental law

Treaties are the principal source. The Vienna Convention on the Law of Treaties, 1969 governs their conclusion, interpretation and termination. A treaty binds only its parties (pacta tertiis nec nocent nec prosunt), and must be performed in good faith (pacta sunt servanda, Article 26).

Environmental treaties have developed their own techniques:

• Framework convention and protocol. A framework treaty sets out general objectives and institutions; later protocols add specific obligations. The Vienna Convention (1985) and Montreal Protocol (1987) on the ozone layer, and the UNFCCC (1992) with the Kyoto Protocol (1997) and the Paris Agreement (2015), follow this model.
• Conference of the Parties (COP). A standing body of the parties meets regularly, adopts decisions, and develops the treaty without formal amendment.
• Annexes and appendices. Technical lists of species, substances or wastes can be revised quickly.
• Adjustment procedures. Under the Montreal Protocol, control measures can be tightened by a qualified majority and bind all parties.
• Non-compliance procedures. Facilitative compliance committees rather than adversarial dispute settlement.
• Financial mechanisms. The Global Environment Facility and dedicated funds support implementation by developing countries.

2.2 Customary international environmental law

Custom is 'a general practice accepted as law'. It requires State practice that is general and consistent, and opinio juris, the belief that the practice is legally obligatory (North Sea Continental Shelf cases, ICJ, 1969). Custom binds all States, including those not party to any treaty. The rules generally accepted as customary in the environmental field are: the duty to prevent significant transboundary harm (the no-harm rule); the duty to carry out an environmental impact assessment where there is a risk of significant transboundary harm; and the duties to co-operate, notify and consult. The status of the precautionary principle and of sustainable development as binding custom remains debated.

2.3 General principles of environmental law

General principles fill gaps where treaty and custom are silent. Some are drawn from domestic legal systems and apply in the environmental field: good faith, abuse of rights, equity, and the maxim sic utere tuo ut alienum non laedas (use your own property so as not to injure that of another), which is the root of the no-harm rule. Others are principles specific to environmental law, set out in the Stockholm and Rio Declarations: prevention, precaution, polluter pays, sustainable development, intergenerational equity and common but differentiated responsibilities. Their legal status varies from established rule to emerging principle.

2.4 Soft law in environmental law

Soft law refers to instruments that are not legally binding but have normative force: the Stockholm and Rio Declarations, Agenda 21, the Forest Principles, UN General Assembly resolutions such as the World Charter for Nature (1982) and Resolution 76/300 (2022) on the right to a healthy environment, and COP decisions.

Soft law is especially important in environmental matters for four reasons. It allows States to agree on principles when they are not ready for binding obligations. It can be adopted and revised quickly. It often hardens into law, either by being incorporated into later treaties (Stockholm Principle 21 became Article 3 of the Convention on Biological Diversity) or by contributing to the formation of custom. And domestic courts rely on it: in Vellore Citizens' Welfare Forum (1996) the Supreme Court drew on the Brundtland Report and the Rio Declaration to declare sustainable development part of Indian law.

3. The Stockholm Conference, 1972

The United Nations Conference on the Human Environment met at Stockholm from 5 to 16 June 1972, convened by the UN General Assembly at the initiative of Sweden. It was the first global conference on the environment, and marks the birth of modern international environmental law. Delegates from 113 States attended. The opening day, 5 June, is observed every year as World Environment Day.

The Conference produced four outcomes: the Stockholm Declaration of 26 principles; an Action Plan for the Human Environment of 109 recommendations; a resolution on institutional and financial arrangements that led to the creation of UNEP; and an Environment Fund. India's Prime Minister, Indira Gandhi, linked environmental degradation to poverty and underdevelopment, and this theme, that environment and development must be pursued together, has shaped the subject ever since.

3.1 The Stockholm Declaration, 1972

The Declaration opens with a proclamation that 'man is both creature and moulder of his environment' and that the protection and improvement of the human environment is a major issue affecting the well-being of peoples and economic development. Its most important principles are:

• Principle 1: man's fundamental right to freedom, equality and adequate conditions of life, in an environment of a quality that permits a life of dignity and well-being, and his solemn responsibility to protect and improve the environment for present and future generations. This is the origin of both the human right to environment and intergenerational equity.
• Principle 2: natural resources must be safeguarded for present and future generations through careful planning or management.
• Principle 6: the discharge of toxic substances must be halted so that damage is not caused beyond the capacity of the environment to render them harmless (the assimilative capacity idea).
• Principles 8 to 12: development is essential, and environmental policies should enhance, not adversely affect, the development potential of developing countries.
• Principle 21: sovereignty over resources and responsibility not to cause transboundary harm.
• Principle 22: States shall co-operate to develop international law regarding liability and compensation for victims of pollution and environmental damage.

3.2 Principle 21 of the Stockholm Declaration

Principle 21 is the single most important provision of international environmental law. It has two halves that balance each other. The first affirms permanent sovereignty over natural resources, a principle developing countries had championed in the UN General Assembly. The second imposes a responsibility not to cause environmental damage beyond national jurisdiction, restating the rule in Trail Smelter.

Three features of the second half deserve attention. It covers activities within a State's 'jurisdiction or control', so it reaches activities on ships, aircraft and occupied territory. It protects not only other States but also 'areas beyond the limits of national jurisdiction', such as the high seas and the atmosphere. And it has been restated in Principle 2 of the Rio Declaration (with the words 'and developmental' added to 'environmental policies'), in Article 3 of the Convention on Biological Diversity, and in the preamble to the UNFCCC. The ICJ confirmed its customary status in the Nuclear Weapons Advisory Opinion (1996).

3.3 The United Nations Environment Programme (UNEP)

UNEP was established by UN General Assembly Resolution 2997 (XXVII) of 15 December 1972, following the Stockholm Conference. It is headquartered in Nairobi, the first UN body based in the developing world. It is a programme, not a specialised agency, and its role is to be the leading global environmental authority that sets the global environmental agenda, promotes coherent implementation of the environmental dimension of sustainable development, and serves as an authoritative advocate for the global environment.

Its main functions are keeping the environment under review (including the Global Environment Outlook reports), catalysing the negotiation of treaties (UNEP facilitated the Vienna Convention, the Montreal Protocol, the Basel Convention, the CBD and others), hosting the secretariats of several conventions, and supporting developing countries. After Rio+20, its governing body became the UN Environment Assembly (first session 2014), with universal membership of all UN member States.

4. The Brundtland Commission and Report

4.1 The World Commission on Environment and Development

The UN General Assembly established the World Commission on Environment and Development (WCED) in 1983 (Resolution 38/161). It was chaired by Gro Harlem Brundtland, former Prime Minister of Norway, and is therefore known as the Brundtland Commission. Its mandate was to propose long-term environmental strategies for achieving sustainable development by the year 2000 and beyond.

4.2 Our Common Future (1987)

The Commission's report, Our Common Future, was published in 1987. It argued that environmental degradation and poverty are linked, that the environment and development cannot be treated separately, and that the existing pattern of growth was unsustainable. Its lasting contribution is the definition of sustainable development.

The Report also proposed a set of legal principles for environmental protection and sustainable development, prepared by an expert group on environmental law, including a fundamental human right to an adequate environment, intergenerational equity, prior assessment of activities, and co-operation over shared resources. Its most direct consequence was the decision to convene the Rio Conference.

4.3 The concept of sustainable development

Sustainable development has four legal elements: intergenerational equity, sustainable use of natural resources, equitable use or intragenerational equity, and integration of environmental considerations into economic and development decisions. At Johannesburg (2002) it was expressed as three interdependent pillars: economic development, social development and environmental protection. The ICJ described it in Gabčíkovo-Nagymaros (1997) as the concept that expresses the need to reconcile economic development with protection of the environment. Its detailed treatment, including its reception in Indian law, is in Topics 7 and 15.

5. The Rio Conference (Earth Summit), 1992

The United Nations Conference on Environment and Development met at Rio de Janeiro from 3 to 14 June 1992, twenty years after Stockholm. It was then the largest gathering of world leaders ever held. Where Stockholm had focused on the human environment, Rio made sustainable development the organising idea.

Rio produced five major outcomes: the Rio Declaration on Environment and Development; Agenda 21; the Forest Principles; and two binding conventions opened for signature, the United Nations Framework Convention on Climate Change and the Convention on Biological Diversity. It also led to the Commission on Sustainable Development (1992) and to the negotiation of the UN Convention to Combat Desertification (1994).

5.1 The Rio Declaration, 1992

The Rio Declaration contains 27 principles. It is soft law, but many of its principles have been incorporated into treaties, recognised as custom or, in India, declared part of domestic law. The principles most often examined are:

1. Principle 1: human beings are at the centre of concerns for sustainable development and are entitled to a healthy and productive life in harmony with nature.
2. Principle 2: sovereign right over resources and responsibility not to cause transboundary damage (restating Stockholm Principle 21).
3. Principle 3: the right to development must be fulfilled so as to equitably meet developmental and environmental needs of present and future generations.
4. Principle 4: environmental protection shall constitute an integral part of the development process (integration).
5. Principle 7: common but differentiated responsibilities.
6. Principle 10: access to information, public participation and access to justice.
7. Principle 13: development of national law regarding liability and compensation for victims.
8. Principle 15: the precautionary approach.
9. Principle 16: internalisation of environmental costs and the polluter pays approach.
10. Principle 17: environmental impact assessment as a national instrument.
11. Principles 18 and 19: notification of emergencies and prior notification of activities with significant transboundary effects.

5.2 Agenda 21

Agenda 21 is a comprehensive, non-binding programme of action for sustainable development into the twenty-first century, in 40 chapters arranged in four sections: social and economic dimensions (poverty, consumption patterns, health, human settlements); conservation and management of resources (atmosphere, land, forests, desertification, mountains, agriculture, biodiversity, oceans, freshwater, toxic chemicals and wastes); strengthening the role of major groups (women, children and youth, indigenous peoples, non-governmental organisations, local authorities, workers, business, scientists, farmers); and means of implementation (finance, technology transfer, education, capacity-building, institutions and legal instruments). Chapter 28 encouraged 'Local Agenda 21' initiatives by local authorities.

5.3 The Forest Principles, 1992

Attempts to negotiate a binding forest convention at Rio failed because developing countries with large forests resisted international control of a national resource. The compromise was a non-binding 'Non-Legally Binding Authoritative Statement of Principles for a Global Consensus on the Management, Conservation and Sustainable Development of All Types of Forests', the first global consensus on forests. It affirms the sovereign right of States to use their forests, calls for sustainable management to meet the social, economic, ecological, cultural and spiritual needs of present and future generations, and recognises the role of indigenous people and forest dwellers. It has since been followed by the UN Forum on Forests and the non-legally binding UN Forest Instrument (2007).

6. After Rio: Johannesburg, Rio+20 and the SDGs

6.1 The Johannesburg Summit, 2002

The World Summit on Sustainable Development was held at Johannesburg from 26 August to 4 September 2002, ten years after Rio. It adopted the Johannesburg Declaration on Sustainable Development and the Johannesburg Plan of Implementation. Its main contribution was conceptual and practical rather than legal: it described sustainable development as resting on three interdependent and mutually reinforcing pillars, economic development, social development and environmental protection, at the local, national, regional and global levels. It also launched voluntary 'Type II' partnerships between governments, business and civil society. It produced no new treaty.

6.2 The Rio+20 Conference, 2012: The Future We Want

The United Nations Conference on Sustainable Development (Rio+20) met at Rio de Janeiro from 20 to 22 June 2012. Its outcome document, 'The Future We Want', was endorsed by the UN General Assembly. It reaffirmed the Rio principles, including common but differentiated responsibilities; recognised the green economy in the context of sustainable development and poverty eradication as an important tool; replaced the Commission on Sustainable Development with a High-level Political Forum on Sustainable Development; strengthened UNEP, leading to universal membership of its governing body; and, most importantly, launched the process to develop Sustainable Development Goals.

6.3 The Sustainable Development Goals

The UN General Assembly adopted 'Transforming our World: the 2030 Agenda for Sustainable Development' by Resolution 70/1 on 25 September 2015. It contains 17 goals and 169 targets to be achieved by 2030, succeeding the eight Millennium Development Goals (2000 to 2015). Unlike the MDGs, the SDGs apply to all countries, not only developing ones, and integrate the economic, social and environmental dimensions.

Seven goals are directly environmental: Goal 6 (clean water and sanitation), Goal 7 (affordable and clean energy), Goal 11 (sustainable cities and communities), Goal 12 (responsible consumption and production), Goal 13 (climate action), Goal 14 (life below water) and Goal 15 (life on land). In India, NITI Aayog co-ordinates implementation and publishes the SDG India Index ranking States and Union Territories.

6.4 SDGs and environmental law

The SDGs are not legally binding, but they matter to environmental law in four ways. They supply measurable targets and indicators against which environmental policy can be assessed. They drive national legislation and policy: India's waste management rules, the renewable energy programme and the Jal Jeevan Mission for household tap water can be read as contributions to Goals 12, 7 and 6. They reinforce the principle of integration, since progress on one goal depends on others. And courts and tribunals increasingly refer to them in interpreting environmental obligations, much as they referred to the Rio Declaration. Goal 16 (peace, justice and strong institutions) links the agenda to the environmental rule of law and access to justice.

7. Common but Differentiated Responsibilities (CBDR and CBDR-RC)

The principle has two elements. The common element is that all States share responsibility for protecting the global environment. The differentiated element is that their responsibilities differ, because of their different historical contributions to the problem and their different capacities to address it.

The UNFCCC (Article 3(1)) adds a qualification: Parties should protect the climate system 'on the basis of equity and in accordance with their common but differentiated responsibilities and respective capabilities'. This is the CBDR-RC formulation. It shifts some emphasis from historical responsibility to present capacity.

Differentiation has taken different forms over time. The Montreal Protocol gave developing countries a ten-year grace period and a Multilateral Fund. The Kyoto Protocol imposed binding emission targets only on developed countries listed in Annex B. The Paris Agreement abandoned that rigid division: all Parties submit nationally determined contributions, and CBDR-RC is to be applied 'in the light of different national circumstances' (Article 2(2)), a model of self-differentiation. India has consistently invoked CBDR-RC, together with equity and the right to development, in climate negotiations.

8. The No-Harm Principle and Transboundary Environmental Harm

The no-harm principle requires a State to prevent activities within its jurisdiction or control from causing significant harm to the environment of other States or of areas beyond national jurisdiction. It is the oldest and best established rule of international environmental law.

The rule was developed in a series of decisions. In Corfu Channel (ICJ, 1949) the Court stated every State's obligation not to allow knowingly its territory to be used for acts contrary to the rights of other States. In Lac Lanoux (1957) the tribunal held that an upstream State must take the interests of a downstream State into account and consult in good faith. In the Legality of the Threat or Use of Nuclear Weapons Advisory Opinion (1996, paragraph 29) the ICJ declared that the general obligation of States to ensure that activities within their jurisdiction and control respect the environment of other States or of areas beyond national control is now part of the corpus of international law relating to the environment. In Pulp Mills (2010) and Certain Activities / Construction of a Road (Costa Rica v. Nicaragua, 2015) the Court explained that the obligation is one of due diligence, requiring a State to use all the means at its disposal to prevent significant harm.

Three points define the rule's scope. The harm must be significant, not trivial. The obligation is one of conduct (due diligence), not of result, so a State is not liable merely because harm occurs despite all reasonable care. And it applies to harm to areas beyond national jurisdiction as well as to other States, which is why the ICJ applied it to the climate system in its 2025 Advisory Opinion.

9. State Responsibility for Environmental Harm

Under the International Law Commission's Articles on Responsibility of States for Internationally Wrongful Acts (2001), every internationally wrongful act of a State entails its international responsibility (Article 1). There is a wrongful act when conduct is attributable to the State and constitutes a breach of an international obligation (Article 2). The consequences are cessation, assurances of non-repetition and full reparation in the form of restitution, compensation or satisfaction.

Environmental cases present special difficulties: harm is often caused by private operators rather than the State; causation is hard to prove; damage may be diffuse, long-term or to the environment itself rather than to property. International law has responded in several ways:

• Due diligence. The State is responsible if it fails to regulate and control private activities with reasonable care (Pulp Mills, 2010).
• Strict liability in specific regimes. The Space Liability Convention, 1972 makes a launching State absolutely liable for damage caused by its space objects on the surface of the earth.
• Civil liability conventions. Treaties on oil pollution and nuclear damage channel liability to the operator or shipowner, backed by compulsory insurance and funds.
• The ILC's work on transboundary harm: the Draft Articles on Prevention of Transboundary Harm from Hazardous Activities (2001) and the Principles on the Allocation of Loss in the case of Transboundary Harm (2006), which call for prompt and adequate compensation to victims even where the State has not breached any obligation.
• Compensation for environmental damage as such. In Certain Activities Carried Out by Nicaragua in the Border Area (Compensation), ICJ 2018, the Court for the first time awarded compensation for damage to the environment itself, including the loss of ecosystem goods and services.

10. Environmental Impact Assessment in International Law

An environmental impact assessment (EIA) is a procedure for evaluating the likely environmental effects of a proposed activity before a decision is taken to authorise it. It began in domestic law with the US National Environmental Policy Act of 1969 and has since become an obligation of international law.

• Rio Principle 17 calls for EIA as a national instrument for activities likely to have a significant adverse impact.
• The Espoo Convention on Environmental Impact Assessment in a Transboundary Context (1991) requires parties to notify and consult affected States.
• Article 14 of the CBD and Article 206 of UNCLOS require assessment of activities that may harm biodiversity or the marine environment.
• In Pulp Mills (2010) the ICJ held that it may now be considered a requirement under general international law to undertake an EIA where there is a risk that a proposed industrial activity may have a significant adverse impact in a transboundary context, and that the content of the assessment is for each State to determine in its domestic law, having regard to the nature and magnitude of the project.
• In Costa Rica v. Nicaragua (2015) the Court extended the obligation beyond industrial activities: a State must first ascertain whether there is a risk of significant transboundary harm and, if there is, conduct an EIA, notify and consult.
• The BBNJ Agreement under UNCLOS, which entered into force on 17 January 2026, requires EIAs for activities in areas beyond national jurisdiction.

In India, EIA is required by the EIA Notification, 2006 under the EPA, and the Supreme Court has described prior environmental clearance as the foundation of the regulatory scheme.

11. The Precautionary Principle in International Law

The precautionary principle addresses scientific uncertainty. It is distinct from the principle of prevention, which applies where the risk of harm is known. It first appeared in German law (Vorsorgeprinzip) and in declarations on the protection of the North Sea in the 1980s, and was incorporated into many treaties, including the Vienna Convention and Montreal Protocol (in their preambles), the UNFCCC (Article 3(3)), the Cartagena Protocol (Articles 10(6) and 11(8)) and the Stockholm Convention on Persistent Organic Pollutants.

Its status in general international law remains contested. In the EC — Hormones case (1998) the WTO Appellate Body declined to decide whether it had become customary law. In the Southern Bluefin Tuna cases (1999) the International Tribunal for the Law of the Sea ordered provisional measures on the basis that the parties should act with 'prudence and caution'. In Pulp Mills (2010) the ICJ said a precautionary approach may be relevant in interpreting a treaty but does not operate as a reversal of the burden of proof. In its 2011 Advisory Opinion on activities in the Area, the Seabed Disputes Chamber of ITLOS observed a trend towards making the precautionary approach part of customary international law.

12. Related Topics and Provisions