Frédéric G. Sourgens, James McCulloch Chair in Energy Law, Tulane University Law School
On July 23, 2023, the International Court of Justice (the ‘Court’ or ‘ICJ’) issued its much-anticipated opinion on the Obligations of States in Respect of Climate Change. As the concurring and separate opinions make clear, the Court’s Advisory Opinion (the ‘AO’ or ‘Opinion’) is an effort by the Court to spur States significantly to increase their climate policies while also leaving substantive energy policymaking – and the choice of technologies and implementation of climate measures to States themselves. In this, the Opinion largely navigated this diplomatic tightrope act successfully. As outlined below, the key takeaways from the decision are (a) States are under a legal obligation to engage in climate policies that cumulatively realistically aim to stabilize global temperature increases at or below 1.5oC above pre-industrial levels, (b) the obligation to cooperate requires States to act for not just a national, but a global good; and (c) the withdrawal by States from relevant climate treaties does not change either of these two obligations. The Court finally was express that the failure by States to do their part in this collective effort will expose the States in question to the full gamut of remedies for breach of an international legal obligation available at international law even as it admits that causation in any climate claim will present thorny issues of proof.
International Law Background
The ICJ is the chief juridical organ of the United Nations. It is established under the UN Charter. It is the successor court to the Permanent Court of International Justice, which in turn had been constituted under the constitutive documents creating the League of Nations. The Court has two dockets. On one docket, the Court decides international legal disputes between States over which it has jurisdiction and of which it is seized. Here, the judgment of the Court creates international legal obligations for the parties to the proceedings. The second docket allows the Court to issue advisory opinions in limited circumstances, for instance when the U.N. General Assembly has asked it to do so. Opinions on this advisory docket clarify the law and provide an authoritative explanation of international legal obligations. They do not dispose of active disputes and thus do not bind the parties as a judgment in a contentious case would.
The Court in Obligations of States in Respect of Climate Change acted in its advisory powers. It did so in response to a request for an Advisory Opinion by the UN General Assembly. (AO, ¶¶ 88-92) This request was conceived of an managed by low lying island States like Vanuatu. The questions the UN General Assembly asked concerned the source of obligations with regard to climate change, the scope of these obligations, and the remedial consequences of any breach of these obligations. (AO, ¶¶ 88, 98) The Court answered each of these questions. In doing so, it clarified the core applicable international legal principles. Yet, it did not make any findings that a specific State violated international law, nor did not make a finding that specific conduct would violate international law. The Opinion in short is truly advisory and does not dictate States to adopt specific energy policies to comply with their international legal obligations.
Relevant Sources of International Law
The Court had to answer a threshold question: what part of international law governs climate change policy? There Court could choose between two possibilities. First, climate change is a phenomenon just like any other and, as such, all of international law governs how States should respond to it. Second, climate change sets a specific set of highly specialized factual challenges that States chose to deal with exclusively in a specialized regime of international climate law, distinct and apart from general international law in the form of international climate treaties (i.e., the United Nations Framework Convention on Climate Change (UNFCCC), its Kyoto Protocol, and the Paris Agreement). This second possibility would mean that climate treaties derogated from general international law by means of the Latin doctrine that the specific norms derogate from general ones or lex specialis legi generali derogat.
The Court concluded that international climate treaties did not create an independent regime of international law. Consequently, the entirety of international law is applicable to relevant State conduct affecting climate change. (AO, ¶¶ 162-71, 314) This conclusion facially broadened State exposure. It further reduced the temptation to escape international legal obligations by withdrawing from climate treaties. (See AO, ¶ 221, discussing the Kyoto Protocol in a likely veiled reference to Canada.)i It also allowed the Court to conclude “the obligation to prevent significant harm to the climate system and other parts of the environment […] applies to all States, including those that are not parties to one or more of the climate change treaties” (AO, ¶ 409) – a thinly veiled reference to the United States.
Still, general international law in the Court’s estimation did not significantly expand the substantive scope of obligations regarding climate change. The Court in the remainder of the Opinion laid out that States created a reasonably robust set of international legal obligations in the three climate treaties in question. Consequently, with one exception, the other areas of international law reflected obligations already contained in the climate regime rather than expanding upon those obligations (the one exception being the UN Convention on the Law of Sea – a Convention that the U.S. has not ratified. The U.S. ironically did not ratify citing concerns that the Convention might unduly impinge its sovereignty. The Opinion does little to assuage that concern.) (AO, ¶ 347)
The Relevant Substantive Legal Obligations to Mitigate Climate Change
The Court built its Opinion around the twin duties of stringent due diligence and cooperation. The Opinion logically opened with the stringent due diligence obligation to mitigate climate change. (AO, ¶ 138) The obligation to cooperate then applies to this primary duty. (See AO, ¶ 141 (discussing the link between cooperation and prevention))ii
The pivot of the Court’s analysis of climate responsibilities and remedies at international law is that “a State that does not exercise due diligence in the performance of its primary obligation to prevent significant harm to the environment, including to the climate system, commits an internationally wrongful act entailing its responsibility.” (AO, ¶ 409) The Court took pains to explain that “what constitutes a wrongful act is not the [greenhouse gas] emissions in and of themselves but actions or omissions causing significant harm to the climate system in breach of a State’s international obligations.” (AO, ¶ 429) What matters is not the specific level of a State’s emissions. Rather, as the Court noted, what matters is whether a State acted with due diligence. Due diligence, in turn, is an international legal obligation of conduct as opposed to of result. (See AO, ¶ 347) This distinction matters, as “in the case of an obligation of conduct, a State acts wrongfully if it fails to use all means at its disposal to bring about the objective envisaged under the obligations, but will not act wrongfully if it takes all measures at its disposal with a view to fulfilling the obligation even if the desired objective is ultimately not achieved.” (AO, ¶ 208) By contrast, “[i]n the case of an obligation of result, a State acts wrongfully if it fails to bring about the result required under the obligation.” (Id.) The Court expressly rejected the applicability of the strict-liability ‘polluter pays’ principle. (AO, ¶ 160) Consequently, States do not need to achieve specific greenhouse gas emission outcomes but rather must adopted their climate, energy, transportation, and industrial policies diligently.
The ‘stringent’ nature of due diligence does not alter how due diligence works. Rather, the Court tied stringency factually to the IPCC’s conclusions regarding the severity of climate harms and the high likelihood of their occurrence (i.e., high risk). (AO, ¶¶ 137-39) Consequently, the Court’s Opinion did not create a new doctrine of stringent due diligence separate and apart from due diligence in international law. Rather, it used stringent due diligence to reflect the severity of the climate crisis, which it describes as ‘existential.’ (AO, ¶ 73)
Due diligence requires States to “put in place a natural system, including legislation, administrative procedures and an enforcement mechanism necessary to regulate the activities in question and … exercise adequate vigiliance to make such a system function efficiently, with a view to achieving the intended objective.” (AO, ¶ 281 (quoting Climate Change, Advisory Opinion, International Tribunal for the Law of the Sea, ITLOS Reports 2024, ¶ 235)). Due diligence begins with the acquisition and analysis of scientific and technological information. (AO, ¶¶ 283-86) While the IPCC authoritatively publishes ever more precise scientific analysis regarding climate change (AO, ¶ 284), technology is more dynamic. The Court importantly noted that “the availability of technological means to prevent or mitigate relevant harm influences what can reasonably be expected of a State” where “the cost of the relevant technologies may be a crucial factor in determining what can reasonably be expected of a State, depending on its capabilities.” (AO, ¶ 286) Due diligence establishes the appropriate level of risk tolerance by reference to international rules and standards (including Conferences of the Parties COPs under climate treaties.) (AO, ¶¶ 287-89) States then must use such measures as are available given their current means to implement the relevant standard to the scientific facts they have established. (AO, ¶¶ 290-92) Consequently, the different factual circumstances of each State are relevant factors to a State’s due diligence obligation, thus incorporating the principle of common but differentiated responsibility from international environmental and climate law into the due diligence standard set out by the Court. (Id.) Due diligence further requires States to take into account plausible indications of potential risks, i.e., act with precaution. (AO, ¶¶ 293-94) Due diligence imposes a dynamic and continuing obligation and “requires States parties, inter alia, to make ongoing efforts to formulate and elaborate rules, standards and recommended practices and procedures.” (AO, ¶ 351)
This due diligence obligation is not limited to State conduct as such. Rather, the Court made clear that it can be held responsible for a lack of diligence where it fails to supervise companies under its jurisdiction. The Court noted that “a State may be responsibility where, for example, it has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction.” (AO, ¶ 428) The Court here was unhelpfully vague. Yet, in the immediately preceding paragraph, the Court notes what “the granting of fossil fuel exploration licenses or the provision of fossil fuel subsidies […] may constitute an internationally wrongful act which is attributable to the State.” (AO, ¶ 427) It thus leaves open the possibility that Scope 3 emissions of oil and gas companies and other vertically integrated firms will be attributable in part to their respective home State. It is likely that domestic litigation in jurisdictions like the Netherlands will further seize upon this point.
The Court married due diligence with the obligation to cooperate. (AO, ¶¶ 140-42) Customary international law recognizes a duty to cooperate and this duty is further codified in relevant climate treaties. (AO, ¶ 261) The Court emphasized that “international cooperation is indispensable in the field of climate change.” (AO, ¶ 215) Climate treaties like the UNFCCC codify that cooperation means “to provide financial, assistance, technology transfer and other forms of support to developing country parties, especially those countries that are particularly vulnerable to the adverse effects of climate change, to assist them in meeting their commitments under the UNFCCC.” (AO, ¶ 217) Consequently, cooperation is a means by which States help to close the gap between the means available to some States and the actions they need to pursue to effectively mitigate emissions. The Court advised that “States are free to select the means of co-operating, as long as such means are consistent with the obligations of good faith and due diligence.” (AO, ¶ 261) The reference to good faith suggests that States must honestly and reasonably offer such assistance as they are able to give to meet the cumulative global due diligence objectives.
The Court then determined the ultimate standard State due diligence must seek to meet in the Paris Agreement and later COP decisions. It noted that the Paris Agreement set a well-below-2oC temperature goal. (AO, ¶ 223) The Court then consulted COP decisions as subsequent treaty practice. On the basis of this practice, it moved the global temperature goal to 1.5oC. (AO, ¶ 224) The Court interpreted the central mechanism of the Paris Agreement in its Article 4, the preparation of nationally determined contributions (NDCs) by each State to the reduction of greenhouse gas emissions, as obligating States cumulatively to take all efforts to meet this goal – in the words of the Court, “in the exercise of their discretion, parties are obliged to exercise due diligence and ensure that their NDCs fulfil their obligations under the Paris Agreement and thus, when taken together, are capable of achieving the temperature goal of limiting global warming to 1.5oC above pre-industrial levels, as well as the overall objective of the ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate.’” (AO, ¶ 245)
The Court continued to consult other international environmental treaties, customary international law, the law of the sea, and international human rights law. Each of these areas of law confirm the same conclusions it has already reached – that is, States must act with stringent due diligence and in good faith cooperation with each other jointly and cumulatively to stabilize temperatures at 1.5oC above pre-industrial levels.
I note for what it is worth that the Court’s interpretation of the Paris Agreement is in significant tension with the U.S. position advanced at the time of its signing. The presence of substantive legal obligations that make the formulation and implementation of nationally determined contributions non-discretionary arguably mean that the Paris Agreement could not have been concluded by the U.S. without Senate ratification. The Obama administration argued that such ratification was not needed due to the purely procedural nature of the obligations undertaken by the U.S. in the Paris Agreement.iii The ICJ now interpreted the Paris Agreement in a manner that is inconsistent with this representation. While this does not have current repercussions given the U.S.’ withdrawal from the Paris Agreement, it may create difficulties for any future Presidential decision to rejoin the Paris Agreement.
It is further important to read the Opinion carefully. A casual reading of the decision may suggest that States have a duty of result – to achieve 1.5oC. The Court was purposeful in its exposition to urge States to be maximally ambitious in their efforts to mitigate climate change. Still, it also purposefully shied away from any strict liability regime or other regime imposing an obligation of results on States. States must go the full extent of their available means. The Court does not go so far as to prescribe how a State should determine what means are ‘available’ to mitigate emissions. States therefore have significant policy discretion in establishing what means are available to support climate measures. This returns to the State the margin of appreciation that the Court’s interpretation of Article 4 of the Paris Agreement appeared to negate. If state revenue is available either to mitigate climate change or to provide shelter and clean water to vulnerable populations, the Opinion emphatically does not require States to privilege climate action over other critical policy priorities that similarly implicate the realization of human rights. States may not justify mitigation action that falls short of a facially fair contribution to greenhouse gas mitigation by reference to a preference for economic growth. They may however justify the same measure by reference to fiscal limitations, as well as to other pressing governmental needs that are on a similar level of urgency with climate mitigation.
The Opinion further does not once mention that there is an obligation to reduce global reliance on fossil fuels. The Joint Declaration of Judges Bhandari and Cleveland draws attention to the fact that the Opinion does not do so and chides it for the omission. The omission therefore must be seen as purposeful. States have a free choice of means how to reduce net emissions and the accumulation of greenhouse gases in the atmosphere and how to meet the warming goal of 1.5oC. Consequently, circular carbon are just as feasible a policy approach as other forms of decarbonization.
Viewed together, this strongly suggests that the Court did not impose a specific policy approach on States. It rather left States with a choice of means how they wish to proceed with the decarbonization of their energy systems. It also does not impose ruinous obligations on States to achieve climate mitigation at all costs. Rather, it is noteworthy that the Court makes reference to sustainable development as one of the guiding principles for climate mitigation. It thus notes “‘the need to reconcile economic development with protection of the environment.’” (AO, ¶ 147 (quoting Gabčikovo-Nagymaros Project, ¶ 140)). The Opinion therefore guides pragmatic decisionmaking in noting the urgency of meeting the 1.5oC temperature goal. It does not constrain how decisionmakers are to meet it. Or, in the words of the Court, “parties do not enjoy unfettered discretion” but rather have “a due diligence obligation to do [their] utmost to ensure that the [policies they] put forward represent [their] highest possible ambition.” (AO, ¶ 270).
The Relevant Substantive Legal Obligations to Adapt to Climate Change
The Court also discussed the duties of adaptation to climate change. Adaptation, just like mitigation, is governed by the twin duties of due diligence (AO, ¶ 258) and cooperation. (AO, ¶ 260) The Opinion highlights the importance of protecting both States and individuals from the consequences of climate change. Yet, it is fair to say that adaptation was less prominent in the decision than mitigation. The Court nevertheless articulated the broad collective support obligations owed also in the adaptation context.
Legal Consequences of Breach of Climate Obligations
The Court began the second part of its Opinion on consequences of breach with the observation that “responsibility for breaches of obligations under the climate change treaties, and in relation to loss and damage associated with the adverse effect of climate change, is to be determined by applying the well-established rules on State responsibility under customary international law.” (AO, ¶ 420) The Court confirmed that climate claims in principle can draw on all international legal remedies (duty of performance, cessation and non-repetition, reparation in the form of restitution, compensation, and satisfaction). (AO, ¶¶ 446-455)
The Court further addressed the questions of attribution and causation. In the attribution context, the Court wrestles with the physical nature of climate change as a reaction to cumulative emissions by multiple States. It acknowledges the problems of proof this cumulative nature will cause. (AO, ¶ 430) It nevertheless indicated that it would not be disinclined to accept a pro rata emission allocation to link particular emission excesses to specific claims. (AO, ¶ 431)
The Court then tipped its hand with regard to the difficulty of proving causation of harm in the climate context when it “observ[ed] that causation of damage is not a requirement for the determination of responsibility as such.” (AO, ¶ 433) Here, the Court found itself in a doctrinal tangle of its own making. Thus, the Court submitted that “[f]or a finding of State responsibility, what is required is an internationally wrongful act and its attribution to the State, whether the act causes harm or not.” (AO, ¶ 433) As noted by Jutta Brunnée, the seminal Pulp Mills and Costa Rica/ Nicaragua cases were precisely criticized in the literature “with many commentators arguing that a failure to exercise due diligence in harm prevention, including a procedural failure, should give rise to a violation of the harm prevention rule, whether transboundary harm had been caused or not.” (Jutta Brunnée, Procedure and Substance in International Environmental Law 14 (2020)) It is thus true that causation of harm is not always necessary to determine international legal responsibility. The statement is however precisely contested with regard to environmental due diligence in international law. In this context, the Court in the past has insisted that a failure of due diligence on its own “in the absence of transboundary environmental harm, […] breache[s] a procedural obligation, but not the (substantive) harm prevention rule as such.” (Id. at 13-14). Even in the Opinion itself, diligence is consistently linked to harm. The causation element therefore is fairly central to the Court’s own liability regime.
The Court next attempted to sketch a broad framework for causation in the climate context. It ultimately concluded “that while the causal link between the wrongful actions or omissions of a State and the harm arising from climate change is more tenuous than in the case of local sources of pollution, this does not mean that the causal link is impossible in the climate context; it merely means that the link must be established in each case through an in concreto assessment […]”. (AO, ¶ 438) This again suggests that climate causation is the Achilles heal of any claim seeking to asset a claim on the basis of the climate regime elaborated by the Court.
The Court goes on to declare that climate mitigation obligations are obligations erga omnes partes. This is a term of art which means that “‘[a]ny State other than an injured State is entitled to invoke the responsibility of another State.” (AO, ¶ 442, quoting International Law Commission Articles on State Responsibility, Art. 48). The key limitation on a non-injured party in pursuing such a claim is that a non-injured State is not entitled to claim reparation. (AO, ¶ 443).
It is difficult to escape the conclusion that the remedial discussion posed significant problems for the Court. Thus, the declaration of an erga omnes partes obligation highlights the importance of the underlying obligation to mitigate climate change to the mind of any international lawyer. Yet, this conclusion seems oddly symbolic. An obligation erga omnes partes still requires an injury to be actionable even by non-injured parties. This is precisely the problem in the climate mitigation context. To make a breach of due diligence actionable under the obligation to prevent harm one generally does require harm. If the causation of harm cannot be proved, a critical element needed to unlock remedial consequences is missing. One thus is in a position to raise that there has been a procedural failure to mitigate climate change – yet, this procedural failure alone is insufficient without more to found a successful claim in the international legal arena.
This again confirms that the Opinion is not so much a means to support future litigation against States that are appear insufficiently ambitious in their climate policies. Rather, the Opinion is a tool to help policymakers map out how they are to tackle climate change. They have a communal obligation of neighborliness to address climate change. They must do so within their means, cooperatively, and diligently. Yet, the decision provides little by way of actionable threat if a State falls short.
What Are the Consequences of the Opinion?
The Opinion will likely have a marked impact on positions in climate negotiations. The Court as much as noted that existing climate contributions are broadly insufficient. As such, particularly affected States have an additional argument, premised in international law, to press their case for greater ambition and faster implementation of the decarbonization of global energy systems. Similarly, States in the Global South can press their case for more meaningful cooperation on the part of industrial and post-industrial economies. It is also likely that some of the Court’s articulations will find their way into climate litigation. In both instances, the ultimate impact of the Opinion is likely to be more limited than appears on its face. In the negotiation context, the reality of limited fiscal means, prevalent energy security concerns, and the overall techno-commercial limitations of alternatives to decarbonize global energy systems faster while maintaining current energy access and economic productivity will blunt the force of legal arguments drawn from an Advisory Opinion. In the litigation context, the Advisory Opinion highlights that there is as of yet no ready means to overcome the causation issues climate change claims present.
This does not make the Opinion less momentous. The articulation that the community of States as a whole has a legal obligation to do its utmost to prevent the 1.5oC warming is meaningful. Similarly, the Court’s studious neutrality on means will help States in allowing them the needed flexibility to increase their ambition. The decision has greatly clarified the obligations of the community of States to decarbonize the global economy. Now, it is up to policymakers and market participants to turn legal obligation into lived and livable reality. As the Court noted, progress towards this goal has been slow. Still, it has been measurable. The Court’s role is to urge this progress on. It has acquitted itself admirably of this task.
For further reading, see Frederic G. Sourgens, Climate Prevention, 37 Geo. Envtl L. Rev. 165 (2025). I anticipate much of the framework outlined in the Court’s Opinion on the basis of an earlier advisory opinion by the International Tribunal for the Law of the Sea. In the article, I propose how the remaining blind spots in the Court’s Opinion could be tackled.
