Marine Plastics and Microplastics under International Law: Systemic Gaps and the Path to a Global Treaty

The contemporary global ocean is fundamentally altered by synthetic polymers. Once celebrated for its durability, lightweight properties, and low production cost, plastic has transformed into one of the most pervasive and persistent threats to the marine biosphere. The scale of the crisis is immense: millions of metric tons of plastic enter the oceans annually, accumulating in marine ecosystems from the polar ice sheets to the depths of the Mariana Trench. While macro-plastics present severe, visible ecological hazards—such as the entanglement of marine mammals, ingestion by seabirds, and the destruction of coral reefs—the emergence of microplastics has introduced a more insidious layer of complexity to global environmental governance.

Microplastics, defined as plastic particles less than five millimeters in size, originate from two primary pathways. Primary microplastics are intentionally manufactured for industrial and consumer use, such as pre-production pellets (nurdles) and microbeads in personal care products. Secondary microplastics result from the weathering, fragmentation, and photo-degradation of larger plastic debris, as well as the operational wear of vehicle tires and the washing of synthetic textiles. These microparticles do not merely persist in the water column; they act as chemical vectors. Due to their hydrophobic surfaces, microplastics adsorb persistent organic pollutants from the surrounding seawater, including polychlorinated biphenyls and organochlorine pesticides, while simultaneously leaching toxic chemical additives such as phthalates and bisphenols. These toxins bioaccumulate and biomagnify across marine trophic levels, posing systemic risks to marine life and potentially human consumers.

Despite the severity of this ecological crisis, the international legal framework governing the marine environment remains fragmented, reactive, and structurally ill-equipped to address the systemic nature of plastic pollution. When the United Nations Convention on the Law of the Sea (UNCLOS) was drafted in the 1970s and adopted in 1982, the global scale of plastic production and its long-term environmental persistence were not fully appreciated. Consequently, the regime established under UNCLOS and its subsequent instruments treats marine pollution through source-specific, sectorally segregated categories. This structural design fails to capture the holistic lifecycle of plastic products, leaving major regulatory gaps between production, consumption, trade, and waste management.

This article examines the current state of international law as it pertains to marine plastics and microplastics. It analyzes the systemic gaps within existing global and regional conventions, evaluates the potential application of general principles of international environmental law, and critically assesses the ongoing negotiations for a legally binding global treaty on plastic pollution. Through this inquiry, the article highlights the urgent need to transition from downstream pollution control to upstream lifecycle regulation under a unified international legal regime.

The Fragmentation of the Current Legal Architecture

To understand the limitations of the current legal regime, one must first map the patchwork of international instruments that touch upon marine plastic pollution. These instruments can be broadly divided into those addressing vessel-source pollution, those regulating waste dumping at sea, and those targeting transboundary movements of hazardous waste. Historically, these instruments have operated in silos, each addressing a narrow aspect of the plastic crisis without confronting the systemic drivers of plastic production.

The International Convention for the Prevention of Pollution from Ships (MARPOL), specifically Annex V, represents one of the earliest and most direct attempts to prohibit the disposal of plastics into the sea from vessels. Under Annex V, which entered into force in 1988, the discharge of all plastics, including synthetic ropes, synthetic fishing nets, and plastic garbage bags, is strictly prohibited. While MARPOL Annex V has been widely ratified and has successfully reduced operational plastic waste from commercial shipping, its efficacy is severely constrained by compliance and enforcement issues. Flag state jurisdiction remains the primary mechanism for enforcement on the high seas. Many flag states, particularly open registries, lack the administrative capacity, resources, or political will to monitor and prosecute violations on the high seas. Furthermore, the regulation does not effectively address the accidental loss of containerized plastic cargo, such as plastic pellets. These incidents often escape liability under standard maritime casualty frameworks because raw plastic pellets are not classified as hazardous cargo under the International Maritime Dangerous Goods (IMDG) Code.

Similarly, the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (the London Convention) and its 1996 Protocol restrict the deliberate disposal of waste at sea. The London Protocol adopts a reverse list approach, prohibiting the dumping of all wastes unless they are explicitly listed in Annex 1. Plastics are not on this allowed list, meaning their dumping is categorically prohibited. However, the London Protocol only applies to deliberate disposal from vessels, aircraft, platforms, or other man-made structures at sea. It does not cover land-based discharges, operational discharges from vessels, or plastics entering the oceans via riverine pathways, which collectively account for the vast majority of marine plastic debris.

Another relevant instrument is the Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and Their Disposal. Recognizing that the global trade in plastic waste often leads to mismanagement in importing nations, the Parties to the Basel Convention adopted the Plastic Waste Amendments in 2019. These amendments, which took effect in 2021, established a three-tiered classification system for plastic waste shipments. Non-hazardous plastic wastes that are clean, uncontaminated, and destined for immediate recycling are placed on the green list and are exempt from the convention's Prior Informed Consent (PIC) procedure. Conversely, contaminated, mixed, or hazardous plastic wastes require the prior written consent of both the importing and transit states before shipment can occur.

While the Basel Amendments represent a significant step toward regulating the international trade in plastic waste, they primarily address the transboundary movement of waste rather than the generation of plastic itself. They do not impose limits on plastic production, nor do they establish uniform standards for domestic waste management infrastructure. Consequently, while the Basel Convention may prevent the dumping of low-grade plastic wastes on developing nations, it does not prevent those same nations from generating vast quantities of plastic waste that ultimately leak into the marine environment due to insufficient domestic waste collection systems.

Treaty / Instrument	Primary Jurisdiction	Regulated Phase	Key Limitations for Plastics
UNCLOS (Part XII)	Zonal / Global	Sea-based and land-based discharge	Non-binding standards for land-based sources; weak enforcement mechanisms.
MARPOL (Annex V)	Flag State / Port State	Shipborne operational waste	Does not cover accidental cargo loss (e.g., nurdles); flag state enforcement issues.
London Protocol	Coastal / Flag State	Deliberate dumping at sea	Excludes operational discharges and land-based runoff.
Basel Convention	Import / Export States	Transboundary waste trade	Focuses on end-of-life trade; no controls on virgin plastic production.

What are the limits of UNCLOS in addressing microplastics?

As the constitution of the oceans, the United Nations Convention on the Law of the Sea (UNCLOS) establishes the overarching legal framework for all activities in the marine environment. Part XII of UNCLOS is dedicated specifically to the protection and preservation of the marine environment, with Article 192 articulating a fundamental, unqualified obligation: "States have the obligation to protect and preserve the marine environment." This general obligation is supplemented by Article 194, which requires states to take all measures necessary to prevent, reduce, and control pollution of the marine environment from any source.

Despite these robust declarations, the application of UNCLOS to the specific problem of microplastics reveals deep-seated structural and conceptual limitations. The first limitation lies in the treaty’s reliance on jurisdictional zones. UNCLOS divides the ocean into distinct zones—internal waters, territorial seas, exclusive economic zones, and the high seas—each with varying degrees of coastal, port, and flag state jurisdiction. This spatial fragmentation is fundamentally at odds with the physical behavior of microplastics. Once released, microparticles are transported by ocean currents across administrative boundaries, accumulating in oceanic gyres and deep-sea trenches far from their point of origin. This transboundary migration makes it virtually impossible to attribute the presence of specific microplastics in a particular maritime zone to the failures of a single state, thereby frustrating traditional mechanisms of state responsibility and jurisdiction.

Furthermore, Part XII of UNCLOS relies on a source-by-source approach to pollution regulation. Article 207 addresses pollution from land-based sources, which is widely acknowledged to be the primary pathway for microplastics, including synthetic textile fibers from wastewater and tire wear particles from road runoff. However, Article 207 is historically one of the weakest provisions in the convention. It merely obliges states to adopt laws and regulations to prevent, reduce, and control land-based pollution, taking into account internationally agreed rules, standards, and recommended practices. Unlike vessel-source pollution under Article 211, which refers to binding international rules established through competent international organizations like the IMO, Article 207 does not bind states to specific, rigorous global standards. The phrase "taking into account" grants states a wide margin of appreciation, resulting in highly uneven national implementation.

The problem is compounded by the definition of pollution under Article 1(1)(4) of UNCLOS, which defines pollution as the introduction by man, directly or indirectly, of substances or energy into the marine environment which results or is likely to result in deleterious effects such as harm to living resources and marine life, hazards to human health, and hindrance to marine activities. While scientific consensus now supports the premise that microplastics cause such deleterious effects, proving a direct causal link between a specific state’s regulatory omission and a specific harm in the marine environment remains an insurmountable evidentiary hurdle. Microplastics are ubiquitous, non-point source pollutants. They undergo physical and chemical weathering, making it impossible to identify the original producer or the state of origin once the particles have disintegrated and mixed in the ocean.

Finally, UNCLOS lacks the regulatory machinery to address the upstream lifecycle of plastics. The convention is designed to manage activities occurring at sea or direct discharges into the sea. It does not possess the mandate to regulate industrial manufacturing standards, chemical additives, or product designs that occur entirely within the domestic jurisdictions of states. For instance, the microplastics that enter the marine environment from the washing of synthetic clothing or the wear of vehicle tires are the product of consumer goods design and industrial manufacturing. Because UNCLOS cannot regulate the chemical composition of polymers or dictate product design standards at the manufacturing stage, its capacity to prevent the generation of microplastics is fundamentally restricted. It is a downstream instrument trying to manage an upstream flood.

Regional Seas Conventions and the Geography of Governance

Due to the limits of global regimes, regional agreements have emerged as crucial testing grounds for marine environmental governance. The Regional Seas Programme, launched by the United Nations Environment Programme (UNEP) in 1974, has facilitated the adoption of several regional treaties that address marine litter with varying degrees of specificity and legal force.

In the North-East Atlantic, the OSPAR Convention has been at the forefront of regional efforts. The OSPAR Commission has developed a Marine Litter Action Plan that targets both macro-plastics and microplastics through measures such as recommending the prohibition of microbeads in cosmetics, establishing voluntary programs for fishers to return marine litter to ports, and implementing ecological quality objectives. Similarly, the Helsinki Commission (HELCOM) has implemented a Regional Action Plan on Marine Litter for the Baltic Sea, focusing on improving municipal waste systems and raising consumer awareness about microfiber pollution.

In the Mediterranean, the Barcelona Convention framework includes the Protocol for the Protection of the Mediterranean Sea against Pollution from Land-Based Sources and Activities. This legally binding protocol required the adoption of a Regional Plan on Marine Litter Management in 2013, making the Mediterranean the first region to adopt legally binding measures to combat marine litter. The plan outlines specific targets, including a ban on single-use plastic bags, the establishment of extended producer responsibility schemes, and the integration of marine litter management into national coastal zone management plans.

However, the regional approach suffers from severe geographical and substantive unevenness. The regions with the highest rates of plastic leakage, such as Southeast Asia, South Asia, and West Africa, often have the weakest regional legal frameworks. The Coordinating Body on the Seas of East Asia (COBSEA) and the Abidjan Convention, while having action plans on marine litter, lack the stringent, legally binding enforcement mechanisms and financial resources found in the OSPAR or Mediterranean regimes. This regulatory disparity leads to transboundary leakage, as plastic waste generated in states with weak regulatory regimes is transported by ocean currents into the maritime zones of neighboring states with more stringent standards. This imbalance underscores the need for a global, legally binding instrument that establishes baseline standards for all states, preventing regulatory arbitrage and ensuring equitable capacity-building support.

How does international law regulate land-based plastic pollution?

To appreciate the legal gaps in controlling land-based plastic pollution, which accounts for approximately eighty percent of all marine debris, it is necessary to examine how international law interacts with domestic waste management and industrial production. Unlike maritime activities, which are subject to highly developed global treaties under the auspices of the International Maritime Organization (IMO), land-based pollution has traditionally been treated as a matter falling almost exclusively within domestic jurisdiction.

The primary global instrument addressing land-based marine pollution is the Global Programme of Action for the Protection of the Marine Environment from Land-based Activities (GPA), adopted in 1995 under the coordination of UNEP. The GPA is a non-binding, soft-law instrument that provides a conceptual framework for states to develop national and regional action plans. It identifies marine litter as a priority source category and encourages states to implement waste minimization strategies, improve waste management infrastructure, and promote recycling.

While the GPA has been instrumental in raising awareness and fostering regional initiatives, its non-binding nature means it lacks any enforcement mechanisms, compliance monitoring, or mandatory funding structures. States are free to implement its guidelines to the extent that their financial and technical capacities allow. Consequently, the GPA has been unable to stem the exponential increase in land-based plastic leakage, particularly in rapidly urbanizing developing nations that lack the capital-intensive waste management systems required to process municipal solid waste.

Beyond soft law, the regulation of land-based sources under international law is hindered by the principle of permanent sovereignty over natural resources. This principle, a cornerstone of international law, protects a state’s right to exploit its own resources and regulate its domestic industries, including the petrochemical and manufacturing sectors, without external interference. Because the production of plastic polymers is deeply intertwined with domestic economic development and energy policies, states have historically resisted international efforts to impose direct restrictions on polymer manufacturing or the chemical composition of consumer products.

Consequently, international law has historically only addressed land-based plastic pollution at the waste phase, rather than the production phase. This focus on the waste phase ignores the economic reality that virgin plastic production is heavily subsidized and artificially cheap, making recycled plastics economically uncompetitive. Without international legal mechanisms that internalize the environmental costs of plastic production and incentivize alternative materials, land-based regulatory efforts remain confined to managing an ever-growing volume of waste that overwhelms local infrastructure.

Can state responsibility be invoked for marine plastic damage?

In the absence of a comprehensive, legally binding global treaty specifically targeting plastic pollution, scholars and international lawyers have increasingly explored whether the traditional rules of state responsibility can be invoked to hold states liable for the damage caused by marine plastics and microplastics. The law of state responsibility, codified in the International Law Commission’s (ILC) Articles on Responsibility of States for Internationally Wrongful Acts, dictates that every internationally wrongful act of a State entails the international responsibility of that State.

The primary international obligation in this context is the duty of due diligence to prevent transboundary environmental harm. This principle, deeply rooted in customary international law, was famously articulated in the Trail Smelter arbitration, which established that 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. This principle was subsequently reaffirmed and expanded by the International Court of Justice (ICJ) in cases such as Pulp Mills on the River Uruguay and the Construction of a Road in Costa Rica along the San Juan River, where the Court confirmed that the duty to prevent transboundary harm is an obligation of conduct, requiring states to exercise due diligence to prevent significant transboundary damage.

Applying this framework to marine plastic pollution presents significant legal obstacles. The first challenge is establishing attribution. Under the ILC Articles, the conduct of private entities—such as petrochemical companies, plastic manufacturers, or individual consumers who discard plastic waste—is not generally attributable to the state unless the state acknowledges and adopts the conduct as its own, or if the private entities are acting under the direction or control of the state. Therefore, a state cannot be held directly responsible for the acts of private polluters. Instead, the breach must lie in the state’s own failure to exercise due diligence—meaning its failure to enact, implement, and enforce adequate regulatory frameworks to control plastic production, consumption, and waste management within its jurisdiction.

To establish a breach of the due diligence obligation, a complaining state would need to prove that the defendant state failed to take all reasonable measures within its power to prevent the leakage of plastics into the marine environment. The standard of due diligence is not an obligation of result; a state is not automatically responsible simply because some plastic waste escapes into the ocean. Rather, it is an obligation of conduct, measured against an objective standard of administrative and regulatory vigilance. In the South China Sea Arbitration, the Annex VII Tribunal interpreted Article 192 and 194 of UNCLOS as requiring states to take active, diligent steps to protect the marine environment from harm caused by both state and private actors.

However, demonstrating a lack of due diligence in the context of microplastics is extraordinarily difficult. Given the ubiquitous, non-point source nature of microplastic pollution, a state can argue that its regulatory measures—such as municipal waste collection, environmental impact assessments, and basic recycling programs—meet the standard of due diligence, even if those measures are ultimately insufficient to stop microscopic plastic particles from entering the water column through wastewater or atmospheric deposition.

The second, and perhaps more formidable, obstacle is proving causation and injury. In international litigation, a claimant state must establish a direct causal link between the breach of the obligation (the defendant state’s regulatory failure) and the specific injury suffered. Because marine plastics from thousands of different sources accumulate and mix in the global commons, it is currently scientifically impossible to trace a specific concentration of microplastics in a claimant state’s exclusive economic zone to the regulatory failures of a specific landward state. While chemical fingerprinting techniques are improving, they cannot yet attribute microscopic weathered fragments to a specific state’s territory with the degree of certainty required by international courts.

Moreover, under customary international law, the obligation to prevent transboundary harm is typically triggered by "significant" or "material" damage to another state’s territory or resources. While there is overwhelming scientific evidence of the ecological harm caused by microplastics, quantifying this harm in monetary terms or demonstrating specific, localized material damage to a state’s fisheries or marine ecosystems that can be isolated from other stressors, such as climate change and overfishing, remains a profound scientific and legal challenge. Consequently, while the conceptual framework of state responsibility exists, its practical application to marine plastic pollution remains largely theoretical and practically unviable under current international judicial standards.

Will the global plastic treaty establish legally binding targets?

The limitations of the existing legal framework led to a historic breakthrough in March 2022, when the United Nations Environment Assembly (UNEA) adopted Resolution 5/14, titled "End plastic pollution: Towards an international legally binding instrument." This resolution established an Intergovernmental Negotiating Committee (INC) with the mandate to draft a legally binding treaty on plastic pollution, addressing the full lifecycle of plastic, by the end of 2024.

The negotiations have become a battleground over the structural design, scope, and level of ambition of the future instrument. The central question dominating the INC sessions is whether the treaty will establish legally binding, globally harmonized targets, or if it will rely on a decentralized, pledge-and-review model similar to the Paris Agreement on climate change.

A coalition of ambitious nations, known as the High Ambition Coalition to End Plastic Pollution, which includes the European Union, Norway, Rwanda, and several small island developing states, advocates for a treaty with legally binding global targets. This coalition argues that to effectively address the crisis, the treaty must include:

• Mandatory global limits on the production of virgin plastic polymers, aimed at reducing the overall volume of plastic entering the global economy.
• Globally harmonized criteria for product design, including bans or phase-outs of high-risk, avoidable plastic products, single-use plastics, and hazardous chemical additives (such as phthalates and bisphenols) that complicate recycling and pose toxicological risks.
• Standardized, mandatory targets for recycling rates, recycled content in new products, and extended producer responsibility (EPR) schemes.

In contrast, an opposing coalition, consisting of major fossil fuel and petrochemical producing nations, advocates for a much more flexible, bottom-up approach. These states argue that a top-down, legally binding production cap would infringe upon national sovereignty, disrupt global supply chains, and harm economic development. They propose that the treaty should focus primarily on downstream waste management and clean-up, rather than upstream production controls. Under their preferred model, the treaty would rely on National Action Plans (NAPs) where each state determines its own domestic targets and strategies, with no legally binding global caps on polymer production or chemical design.

The resolution of this debate will determine the treaty’s ultimate efficacy. If the treaty adopts the decentralized, bottom-up model, it risks suffering from the same structural weaknesses as the Paris Agreement, where aggregate national pledges are insufficient to meet the global ecological necessity. Without globally binding targets on the production of virgin polymers, the supply of cheap, fossil-fuel-derived plastic will continue to grow, outstripping any improvements in waste management infrastructure.

Moreover, the inclusion of microplastics in the treaty’s scope presents unique negotiating challenges. To regulate microplastics, the treaty must address both primary microplastics (such as intentionally added microbeads in cosmetics and industrial pre-production pellets) and secondary microplastics (such as synthetic microfibers and tire wear particles). Regulating primary microplastics is relatively straightforward and can be achieved through global bans on specific product categories. However, regulating secondary microplastics requires complex, sector-specific intervention standards, such as mandating the installation of microfiber filters on domestic and industrial washing machines, or redesigning tire compounds to reduce friction-induced wear. Whether negotiating states have the political appetite to impose such detailed, intrusive regulations on diverse industrial sectors remains highly uncertain.

Conclusion

The crisis of marine plastics and microplastics represents a systemic failure of global environmental governance. The current international legal architecture—characterized by the spatial fragmentation of UNCLOS, the source-specific limitations of MARPOL and the London Protocol, and the non-binding nature of land-based regulatory instruments—is structurally incapable of addressing a pollutant that spans the global commons and originates deep within domestic industrial economies.

The ongoing negotiations for a global plastic treaty offer a historic opportunity to correct these systemic flaws. To be successful, the new instrument must transcend the traditional downstream focus of ocean law and embrace a comprehensive, lifecycle approach that regulates plastics from chemical synthesis and product design through to waste management and environmental remediation. This requires establishing legally binding global targets to cap virgin polymer production, eliminating toxic chemical additives, and creating robust financial and technical transfer mechanisms to support developing nations.

Only by bridging the gap between land-based industrial regulation and maritime environmental protection can international law successfully safeguard the marine biosphere from the enduring legacy of the plastic age.