Mapping the Maritime Boundaries: The ICJ and the Three-Stage Delimitation Methodology

The division of ocean space has long been a source of diplomatic friction and legal complexity. As coastal nations seek to secure sovereign rights over living and non-living marine resources, the precise determination of maritime boundaries becomes paramount. Under the United Nations Convention on the Law of the Sea, which is widely referred to by the abbreviation UNCLOS, coastal states are entitled to an exclusive economic zone, or EEZ, extending up to 200 nautical miles from their baselines, as well as a continental shelf that can stretch even further. However, because many states lie opposite or adjacent to one another within these distances, their claims frequently overlap.

For decades, international tribunals struggled to establish a predictable, uniform, and objective framework for resolving these overlaps. The breakthrough came with the development of the three-stage approach by the International Court of Justice, also known as the ICJ. This structured methodology, first systematically articulated in the landmark 2009 case concerning maritime delimitation in the Black Sea between Romania and Ukraine, has since become the gold standard of international maritime boundary law. It provides a transparent, predictable, and mathematically grounded framework that balances the need for legal certainty with the flexibility required to achieve an equitable result.

The Codification Gap: Articles 74 and 83 of UNCLOS

To understand the revolutionary impact of the three-stage approach, one must first examine the legal vacuum it was designed to fill. During the negotiation of UNCLOS in the 1970s and early 1980s, the issue of maritime delimitation was one of the most contentious topics. Two opposing camps emerged during the diplomatic drafts.

The first camp championed the equidistance principle, arguing that boundaries should be drawn along a line where every point is equidistant from the nearest points of the baselines of the states concerned. This group valued mathematical objectivity and legal predictability above all else. The second camp advocated for equitable principles, contending that strict geometry could lead to highly unfair outcomes due to irregular coastal configurations, such as deep concavities or the presence of small islands. This group argued that boundaries must be adjusted to ensure an equitable result, even if it meant deviating significantly from an equidistant line.

The resulting compromise is reflected in Articles 74 and 83 of UNCLOS, which govern the delimitation of the EEZ and the continental shelf, respectively. The text of these articles is identical and highly abstract, stating that the delimitation of the exclusive economic zone and continental shelf between states with opposite or adjacent coasts shall be effected by agreement on the basis of international law, as referred to in Article 38 of the Statute of the International Court of Justice, in order to achieve an equitable solution.

By failing to specify any concrete method for achieving this equitable solution, the convention left a massive task of interpretation to international courts and tribunals. It did not define what constituted an equitable solution, nor did it outline the steps a judge should take to reach one. Consequently, early jurisprudence was marked by a case-by-case pragmatism that critics argued bordered on judicial subjectivity.

The Evolution Towards a Structured Methodology

Before the standardization of the three-stage approach, the ICJ and other international tribunals relied on a variety of methods. In the famous North Sea Continental Shelf cases of 1969, which predated UNCLOS, the ICJ rejected the compulsory use of the equidistance method, emphasizing instead the concept of natural prolongation and the application of equitable principles. This decision led to an era of maritime delimitation characterized by what some legal scholars described as a lack of predictability, as tribunals attempted to balance a myriad of geographic, historical, and economic factors without a clear hierarchy.

Over the subsequent decades, the jurisprudence slowly shifted back toward a more structured relationship between geometry and equity. In the 1985 Libya/Malta case, the court recognized the utility of beginning with a median line before adjusting it in light of other circumstances. Similarly, in the 1993 Jan Mayen case between Denmark and Norway, the court integrated the concepts of equidistance and special circumstances into a more cohesive process.

The culmination of this long jurisprudential evolution occurred in the 2009 judgment in Maritime Delimitation in the Black Sea, a case involving Romania and Ukraine. In this decision, the ICJ formally synthesized its past rulings into a clear, sequential three-stage methodology. The court declared that this structured process was designed to ensure objectivity and predictability, thereby reassuring states that maritime borders would not be decided by arbitrary judicial discretion.

The First Stage: Constructing the Provisional Equidistance Line

The first step in the ICJ three-stage approach is entirely geometric and objective. The court constructs a provisional delimitation line based on the geography of the coasts of the parties. This line is established using the equidistance method for adjacent coasts, or the median line method for states that lie directly opposite one another.

Before drawing this line, the court must perform two crucial preparatory tasks: identifying the relevant coasts of the parties and determining the relevant maritime area. The relevant coasts are those whose geographical projection overlaps with the claims of the opposing state. The length of these coasts is measured carefully, as it will play a critical role in the final stage of the process.

Once the relevant coasts are identified, the court selects basepoints on these coastlines. These basepoints are the physical geographic features from which the provisional line is calculated. To ensure absolute objectivity, the court must select basepoints that represent the actual physical geography of the coast, rather than artificial or temporary features. By using specialized computer software and geodetic data, the court then plots a line where every point is exactly equidistant from the nearest basepoints on the coasts of both states.

The significance of this first stage cannot be overstated. By starting with a mathematically precise line, the court establishes a neutral starting point that is free from subjective value judgments. It treats both states equally at the outset, regardless of their political, economic, or military stature. This geometric line acts as the baseline from which all subsequent modifications are measured.

The Second Stage: Adjusting the Line for Relevant Circumstances

While the first stage is governed by strict geometry, the second stage is guided by equity. The court examines whether there are any relevant circumstances that require the provisional equidistance line to be adjusted or shifted in order to achieve an equitable result. This is where the specific, often irregular, realities of geography are taken into account.

The burden of proof during this stage lies on the party claiming that the provisional line is inequitable. The court must evaluate a wide range of factors, which international jurisprudence categorizes as relevant circumstances. The most common and legally significant of these circumstances include:

• Coastal Configuration and Concavity: If a coastline is deeply concave, a strict equidistance line can have a cut-off effect, squeezing one state out of its natural maritime projection. In such cases, the court may adjust the angle of the line to prevent this inequity, as seen in the historical disputes of the North Sea.
• The Presence and Influence of Islands: Islands are often the most contentious features in maritime delimitation. Under international law, not all maritime features are treated equally. The court must determine whether an island should be given full effect, partial effect, or no effect at all. For instance, a tiny, uninhabited island located far from the mainland might distort the provisional equidistance line in a highly disproportionate manner. To avoid this, the court may choose to ignore the island when constructing the line, or enclave the island within its own small territorial sea, preventing it from shifting the overall EEZ or continental shelf boundary.
• Disparity in Coastal Lengths: A significant difference in the lengths of the respective coastlines of the states can be a major relevant circumstance. If one state has a coastline that is many times longer than that of its neighbor, a strict provisional median line might allocate them nearly equal maritime space, which could be deemed inequitable. The court may adjust the provisional line to reflect this geographical reality, although it repeatedly emphasizes that this is not a matter of direct mathematical proportion.
• Security and Navigational Interests: In rare instances, the court may consider the security interests of the parties or the preservation of vital shipping lanes. However, these factors are usually secondary to geographical considerations and are only used to fine-tune the boundary rather than dictate its overall path.

During this second stage, the ICJ carefully filters out factors that it deems legally irrelevant. For example, the court has consistently ruled that relative economic wealth, historical fishing patterns, and the location of offshore oil and gas concessions are generally not relevant circumstances. By restricting the list of relevant circumstances primarily to geographical factors, the court maintains the objectivity of the process and avoids making decisions based on transient political or economic situations.

The Third Stage: The Disproportionality Test

The third and final stage of the methodology serves as an ex-post facto check, or a sanity test, to ensure that the adjusted boundary line does not lead to an unconscionable or highly inequitable result. This stage is known as the disproportionality test.

The mechanics of this test are strictly quantitative. The court calculates two ratios:

1. The ratio of the lengths of the relevant coasts of the two states, measured in accordance with their general direction.
2. The ratio of the maritime areas allocated to each state by the adjusted delimitation line.

Once these two ratios are calculated, the court compares them. The goal is not to achieve a perfect mathematical match between the ratio of coastal lengths and the ratio of allocated water space. International law does not recognize a right to a proportional share of maritime space based solely on coastal length. Instead, the test is designed to verify that there is no gross disproportion between the two ratios.

If the comparison reveals a massive, shocking imbalance that cannot be justified by geography, the court must go back and adjust the line further. However, in almost all cases where the three-stage approach has been applied, the carefully constructed provisional line, as adjusted in the second stage, has successfully passed the disproportionality test. This success demonstrates that the first two stages are highly effective at producing an equitable outcome.

The Concept of the Single Maritime Boundary

An important dimension of modern maritime delimitation is the concept of the single maritime boundary. Historically, the continental shelf, which relates to the seabed and subsoil, and the EEZ, which relates to the superjacent water column, were governed by separate legal regimes. Theoretically, a state could have had one boundary for fishing rights in the water column and a different boundary for oil exploration on the seabed.

Recognizing the administrative, political, and practical chaos that dual boundaries would cause, states almost always ask the ICJ to draw a single maritime boundary for all resources. The three-stage approach is perfectly suited for this purpose. When applying the methodology to a single maritime boundary, the court must ensure that the chosen line is equitable for both the seabed resources and the water column, a requirement that further highlights the necessity of the flexible second stage.

Consolidation and International Alignment

Since its codification in 2009, the three-stage approach has been consistently applied and refined by the ICJ, as well as by other international tribunals. The alignment between different international bodies has created a remarkably unified body of international maritime law.

The ICJ reaffirmed and applied the methodology in several landmark cases, including the Territorial and Maritime Dispute between Nicaragua and Colombia in 2012, the Maritime Delimitation in the Caribbean Sea and the Pacific Ocean between Costa Rica and Nicaragua in 2018, and the Maritime Delimitation in the Indian Ocean between Somalia and Kenya in 2021. In each of these cases, the court utilized the three-stage structure to resolve complex geographical disputes involving offshore islands, concave coastlines, and disputed maritime projections.

Furthermore, the International Tribunal for the Law of the Sea, known by the abbreviation ITLOS, has fully embraced the three-stage approach. In its historic 2012 judgment concerning the Delimitation of the Maritime Boundary in the Bay of Bengal between Bangladesh and Myanmar, ITLOS adopted the ICJ methodology word for word, applying it to the continental shelf beyond 200 nautical miles. This judicial harmony has also extended to ad-hoc arbitral tribunals, such as the tribunal that resolved the maritime boundary dispute between Croatia and Slovenia, and the one that decided the dispute between Ghana and Côte d'Ivoire in 2017.

This widespread institutional adoption has elevated the three-stage approach to the status of customary international law. States preparing for maritime negotiations or litigation now know exactly what legal standards will be applied to their dispute. This predictability has also had a major diplomatic benefit: by making the legal outcome of a dispute highly foreseeable, it encourages states to reach negotiated settlements rather than engage in costly and protracted international litigation.

The Balance of Predictability and Equity

The three-stage approach of the ICJ represents one of the greatest successes of international judicial law-making. By creating a structured process that transitions smoothly from objective geometry in the first stage, to equitable adjustment in the second stage, and finally to a mathematical verification in the third stage, the court has successfully resolved the historic tension between the equidistance and equitable principles factions that paralyzed the UNCLOS negotiations.

This methodology has transformed maritime delimitation from an unpredictable exercise in judicial discretion into a rigorous, transparent, and highly structured legal process. In doing so, the ICJ has not only resolved complex bilateral disputes but has also contributed significantly to the rule of law and the maintenance of peace and security across the oceans of the world.