{"id":26539,"date":"2025-08-11T09:24:35","date_gmt":"2025-08-11T14:24:35","guid":{"rendered":"https:\/\/blogs.law.columbia.edu\/climatechange\/?p=26539"},"modified":"2025-08-12T11:57:41","modified_gmt":"2025-08-12T16:57:41","slug":"harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change","status":"publish","type":"post","link":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/","title":{"rendered":"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change"},"content":{"rendered":"
<\/div>

The International Court of Justice (ICJ)\u2019s release of its <\/span>Advisory Opinion on the <\/span>Obligations of States with Respect to Climate Change<\/span><\/i><\/a> marks a watershed moment, not just because of what the court says about climate obligations, but also because of how\u00a0it says it. In responding to the legal question posed to it, the ICJ does not reinvent the law so much as weave together its many threads. Rather than treating treaty, custom, and general principles of law as enclosed, the ICJ reads them together \u2013 sometimes cumulatively, sometimes cross\u2011referentially, always purposively. That approach is not conjured from thin air. It consolidates a lineage already visible in recent jurisprudence of the International Tribunal for the Law of the Sea (ITLOS), the Inter-American Court of Human Rights (IACtHR) and the European Court of Human Rights (ECtHR), and it speaks to what many domestic courts <\/span>have been doing more for than a decade<\/span><\/a> in climate cases. The result is a careful, source\u2011sensitive account of international obligations that deepens their legal texture and clarifies the consequences of their breach.\u00a0<\/span><\/p>\n

This post analyzes the advisory opinion\u2019s treatment of sources, arguing that it reflects a deeper shift in international law\u2019s orientation that will reverberate far beyond climate litigation. It complements another<\/span> piece<\/span><\/a>, written jointly with Jorge Vi\u00f1uales, which discusses the advisory opinion\u2019s contributions to climate law and governance more broadly.\u00a0<\/span><\/p>\n

Treaty and custom in tandem \u2014 and the interpretive role of principles<\/b><\/h2>\n

From the outset, the ICJ rejected attempts to corral climate change law into a self-contained regime. As Phoebe Okowa <\/span>recalls<\/span><\/a>, a handful of large emitters (e.g. the <\/span>United States<\/span><\/a>, <\/span>Japan<\/span><\/a>, <\/span>Saudi Arabia<\/span><\/a>, <\/span>Kuwait<\/span><\/a>, <\/span>Australia<\/span><\/a>) had urged a\u00a0\u201cclimate treaties only\u201d\u00a0view \u2013 that The United Nations Framework Convention on Climate Change (UNFCCC), Kyoto Protocol, and Paris Agreement form a\u00a0<\/span>lex specialis,\u00a0<\/span><\/i>displacing broader international law. The ICJ firmly disagreed. It held that the climate accords\u00a0neither exclude nor exhaust\u00a0States\u2019 obligations under general international law. In other words, the existence of specialized climate treaties does not immunize States from\u00a0parallel customary duties or other treaty commitments. This view echoes the ICJ\u2019s classic approach in<\/span> Nicaragua<\/span><\/i>: treaty norms and custom can coexist and independently bind States (<\/span>Military and Paramilitary Activities (Merits)<\/span><\/i><\/a>, paras. 92-107).\u00a0<\/span><\/p>\n

Interpretively, the Court was explicit that the climate treaties must be interpreted in accordance with Articles 31\u201333 of the <\/span>Vienna Convention on the Law of Treaties<\/span><\/a> (VCLT), which themselves reflect customary international law. This includes the duty of good faith, the directive to read terms in their context and in light of the treaty\u2019s object and purpose, and the operation of systemic integration under Article 31(3)(c). The Court also stressed that subsequent agreements and subsequent practice may arise from decisions of the Conference of the Parties to the UNFCCC (COP), the <\/span>Conference of the Parties serving as the meeting of the Parties to the Paris Agreement (<\/span>CMA) and related bodies, and must be taken into account (para. 177). This approach allows the ICJ to read the UNFCCC, Kyoto Protocol, and Paris Agreement together, while resisting any automatic <\/span>lex posterior <\/span><\/i>displacement and insisting instead on compatibility and harmonious interpretation across instruments.\u00a0<\/span><\/p>\n

The ICJ spelled out in detail how treaty and custom interact, placing these sources in a mutually reinforcing relationship. Treaties are to be read within the \u201centire legal system prevailing at the time of interpretation,\u201d including relevant customary rules; treaty provisions can also shed light on the content of custom or even accelerate its development (para. 311). At the same time, customary norms\u00a0continuously complement\u00a0the relevant treaties; they bind even States not party to a specific treaty and fill treaty gaps (para. 315; see also <\/span>Julian Arato and Justina Uriburu<\/span><\/a>). The ICJ even recognized COP decisions as potentially contributing to the formation of custom; however, it cautioned that each decision must be assessed for the requisite State practice and <\/span>opinio juris <\/span><\/i>(para. 288). This careful, case-by-case method mirrors the <\/span>International Law Commission (ILC)\u2019s guidance<\/span><\/a> that no single formula creates or defines custom; rather, one must examine practice and belief in normative obligation.\u00a0<\/span><\/p>\n

Substantively, the ICJ singled out two customary norms as \u201cmost directly relevant\u201d to climate change:\u00a0(1) the duty to prevent significant environmental harm to the environment\u00a0(through \u201cdue diligence\u201d to avoid foreseeable damage), and (2) the duty to co-operate\u00a0in good faith for environmental protection. Both are framed as obligations owed by <\/span>all<\/span><\/i> States and both operate beyond the formal membership of particular treaties. Additionally, the ICJ explicitly considered \u201cthe human rights recognized under customary international law\u201d as part of the applicable law (para. 145), with the customary law on State responsibility providing the overarching framework for determining breaches and legal consequences (para. 420; see also <\/span>Federica Paddeu and Miles Jackson<\/span><\/a>\u2019s analysis).\u00a0<\/span><\/p>\n

For the duty to prevent significant environmental harm, the ICJ referred to its previous confirmations of the norm\u2019s customary status and agreement among participants about the same (para. 132). It rejected a narrow, direct\u2011transboundary\u2011harm template and applied the no\u2011harm rule to a global, cumulative process, while reaffirming that \u201cuse all the means at [a State\u2019s] disposal\u201d remains the operative standard of conduct (para. 281). The duty to co\u2011operate is treated both as a rule of custom and as an interpretive guide for other rules; a conclusion the ICJ reached after surveying a wide range of sources demonstrating State practice and\u00a0<\/span>opinio juris<\/span><\/i> (para. 140). Interpretively, the ICJ stressed that the principle of good faith is not a mere VCLT adornment but applies equally to custom: it structures how States perform co\u2011operation, including the continuous development and implementation of collective climate policy based on an equitable distribution of burdens. Read together, the duty to co\u2011operate and good faith require more than the exchange of finance or technology; they require a sustained, reviewable practice of equitable effort aligned with treaty temperature goals and with customary prevention obligations (paras. 303-307). That is why the ICJ seemed at pains to say that co\u2011operation is not optional and is not satisfied simply by participation in treaty processes (paras. 308-315; see also the <\/span>joint declaration of Judges Charlesworth, Brant, Cleveland, and Aurescu<\/span><\/a>).<\/span><\/p>\n

The ICJ\u2019s treatment of human rights and custom merits separate emphasis. The Court placed human rights at the heart of the legal analysis, recognized the human right to a clean, healthy and sustainable environment as essential \u201cunder international law\u201d (paras. 387-393) and used that recognition, together with interdependence, to specify the content of States\u2019 climate obligations (para. 457).<\/span> Judge Aurescu<\/span><\/a> argued \u2013 persuasively in my view \u2013 that the evidentiary record would have supported an explicit customary characterization; <\/span>Judge Bhandari<\/span><\/a> and <\/span>Judge Tladi<\/span><\/a> asserted the ICJ recognized as much. Whether or not one reads the advisory opinion as going that final step, the result is a customary law of prevention and cooperation whose content is informed by human rights and by best available science. That is a powerful convergence for courts and policymakers alike (as further analyzed by <\/span>David Boyd<\/span><\/a> and<\/span> Corina Heri<\/span><\/a> in their respective contributions).\u00a0<\/span><\/p>\n

Some of the separate opinions and declarations spotlight areas where the ICJ could have applied the two-pronged assessment more explicitly or rigorously (e.g. fixed baselines\/ outer limits of maritime zones, <\/span>Judge Aurescu<\/span><\/a> paras. 1-13; continuity of statehood, <\/span>Judge Tomka<\/span><\/a> paras. 1-11; and, as noted above, the right to a clean, healthy and sustainable environment). But these observations do not so much undercut the opinion as indicate what remains at the frontier of international climate law scholarship and practice.<\/span><\/p>\n

General Principles and Global Equity<\/b><\/h2>\n

Equally intriguing is the ICJ\u2019s engagement with general principles of law. The ICJ never once pronounced the formula \u201cgeneral principles of law\u201d in the opinion, yet it plainly worked with such principles throughout. In identifying \u201cother principles\u201d that are \u201cpart of the applicable law\u201d \u2013 sustainable development, common but differentiated responsibilities and respective capabilities (CBDR\u2011RC), equity, intergenerational equity and the precautionary approach or principle \u2013 the ICJ incorporated them into the very <\/span>corpus juris <\/span><\/i>governing climate change, alongside treaty and custom.<\/span><\/p>\n

In doing so, the Court silently endorsed the classification proposed by the ILC in its <\/span>near-finalized work on the topic<\/span><\/a>: general principles of law may be \u201cderived from national legal systems\u201d or \u201cformed within the international legal system\u201d (ILC draft conclusion\u202f3). Most of the principles on the ICJ\u2019s list appear to have been viewed through the lens of the <\/span>latter category<\/span><\/a>. The advisory opinion traces their origins primarily to treaties, other international instruments, and judicial reasoning at the international level rather than in recognition in domestic legal systems. Yet the ICJ\u2019s approach is flexible enough to accommodate hybrid origins. Equity, for example, is so deeply embedded in many domestic legal traditions that the ICJ may have regarded a fresh comparative analysis as unnecessary. At the same time, it has long circulated autonomously in international jurisprudence (see e.g. <\/span>Justice Margaret White<\/span><\/a>). A similar duality of origin may explain why the \u201cprecautionary approach or principle\u201d features in the list as \u201claw,\u201d despite its ambiguous phrasing (which <\/span>Judge Yusuf<\/span><\/a> and <\/span>Judge Charlesworth<\/span><\/a> criticized).\u00a0<\/span><\/p>\n

The rigor of the ICJ\u2019s methodology is illustrated by its treatment of the \u201cpolluter pays\u201d principle \u2013 the only candidate principle expressly rejected. Noting the principle\u2019s absence from climate treaties and the sector\u2011specific character of the State practice on which proponents relied, the Court concluded that the polluter pays principle did not apply \u201cfor the purposes of this Advisory Opinion\u201d (para. 160). The ICJ\u2019s reference to national <\/span>and<\/span><\/i> international practice signals that both categories of general principles are subject to a shared evidentiary threshold of \u201crecognition\u201d (ILC draft conclusion 2). An open-ended question is whether the evidence examined by the Court was sufficient to make a determination about the polluter pays principle\u2019s status as a general principle of law. According to <\/span>Judge Bhandari<\/span><\/a> it was not; he argued that the Court overlooked the principle\u2019s \u201cnormative and jurisprudential grounding in international environmental law\u201d and in doing so, \u201cmisse[d] an opportunity to strengthen the accountability architecture essential for addressing climate change<\/span>\u201d<\/span> (para. 2). <\/span>Judge Nolte<\/span><\/a>, in turn, underscored that the Court did not close the door to \u201ca likely future development of the law\u201d with respect to the polluter pays principle (para. 17).<\/span><\/p>\n

The advisory opinion\u2019s exposition of equity illustrates both the promise and the ambiguity of general principles\u2019 normative yield. Recalling\u00a0<\/span>Continental Shelf (Tunisia\/Libya)<\/span><\/i><\/a>, the opinion reproduces the assertion that \u201cthe legal concept of equity is a general principle directly applicable as law,\u201d but \u2013 crucially \u2013 omits the adverb \u201c<\/span>Moreover<\/span><\/i>\u201d that once separated that proposition from the next sentence on equity\u2019s interpretive role (para. 152). By eliding that connective word, the Court fused two propositions that were originally distinct: equity can generate substantive norms, <\/span>and<\/span><\/i> it can guide the choice between competing readings of positive law. This fusion enabled the Court, in an earlier paragraph, to declare that CBDR\u2011RC, while derived from equity, \u201cdoes not establish new obligations\u201d and instead \u201cguides the interpretation\u201d of existing law (para. 151).\u202fJudge\u202fXue\u2019s <\/span>separate opinion<\/span><\/a> refuses that narrowing move: once a principle has been acknowledged as applicable law, she argues, it must possess \u201cits own substantive content\u201d (para. 3).\u202fEquity therefore cannot be confined to a merely hermeneutic function when the issue at stake is distributive justice in climate action. This tension between the majority and Judge\u202fXue foreshadows future litigation. By characterizing CBDR\u2011RC and the other listed principles as interpretive canons <\/span>for now<\/span><\/i>, the Court leaves open the possibility that, in concrete disputes, those same principles may give rise to freestanding obligations \u2013 particularly where equity demands differentiation based on historical responsibility or capacity.\u202f<\/span><\/p>\n

While the ambiguity about the listed principles\u2019generative function is real, this should not overshadow the significance of the ICJ\u2019s conclusion that they are<\/span> applicable as \u201cguiding principles for the interpretation and application\u201d of the relevant legal rules (para. 161). Given the plethora of rules the ICJ considered \u201cdirectly relevant,\u201d the implications of this recognition are far-reaching. CBDR-RC, for example, must be applied systematically \u201cbeyond its express articulation in different treaties\u201d (para. 151) to ensure that the relevant law takes due account of \u201cthe historical responsibility of certain States\u201d and \u201cdifferent current capabilities\u201d (para. 148; see further <\/span>Judge Sebutinde<\/span><\/a>, paras 9-12; <\/span>Judge Yusuf<\/span><\/a>, paras 20-29; <\/span>Judge Xue, <\/span><\/a>paras 51-78; and <\/span>Judges Bhandari and Cleveland<\/span><\/a>, paras 24-27). Similarly, the Court\u2019s recognition of intergenerational equity is remarkable, even if the extent of its law-generating capacity remains unsettled: a concept whose legal status was long debated (see e.g. <\/span>Katalyn Sulyok<\/span><\/a>) is now affirmed as a manifestation of equity that must inform the application of both treaties and custom, ensuring \u201cdue regard for the interests of future generations\u201d across the board. <\/span>\u00a0<\/span>The precautionary principle also received significant validation: while the Court called it both an \u201capproach\u201d and a \u201cprinciple\u201d, it nevertheless deemed it determinative of the standard of due diligence required under custom and treaties (e.g. para. 178; see also <\/span>Judge Charlesworth<\/span><\/a>\u2019s elaboration on how the prevention and precaution principles interact with due diligence).<\/span><\/p>\n

Taken together, these moves reposition general principles at the heart of global climate governance. By confirming their status as law and by insisting that they infuse the interpretation of all \u201cdirectly relevant\u201d rules, the Court has equipped litigants and policymakers with a vocabulary of equity that transcends the confines of any single treaty regime. The section thus lays doctrinal groundwork for a more differentiated, future\u2011oriented, and justice\u2011centred application of international climate obligations. At the same time, it gives the ILC\u2019s work on general principles increased salience by revealing the need for a conceptual map and methodology to enhance transparency in future invocations of these principles. Moreover, the ICJ\u2019s near-silence on principles derived from national legal systems leaves room for comparative research to render this source more inclusive (see also <\/span>Imogen Saunders<\/span><\/a>).\u00a0<\/span><\/p>\n

Emancipatory Potential Unlocked<\/b><\/h2>\n

Why does the ICJ\u2019s holistic approach to sources matter? Because it transforms what could have been a modest restatement of obligations into a bold affirmation of international law\u2019s capacity to drive equitable climate action at a global scale. By confirming that States must act not only under treaties but\u00a0also under general international law to avoid and repair climate harm, the ICJ essentially tells the world that\u00a0our shared norms and principles demand climate justice (though <\/span>Judge Yusuf<\/span><\/a>, <\/span>Judge Sebutinde<\/span><\/a> and <\/span>Judge Charlesworth<\/span><\/a> insisted, from different angles, that the ICJ should have gone further in linking the law to the realities of climate change). This approach frees the entire field of climate law from the constraints of particular agreements and roots it in something larger \u2013 the idea of\u00a0legal duties owed to present and future generations. This idea is further consolidated by the ICJ\u2019s finding that climate obligations are <\/span>erga omnes<\/span><\/i> under custom and<\/span> erga omnes partes <\/span><\/i>under the treaties (paras. 439-443). Accordingly, any State can invoke responsibility if these duties have been breached, as per the rule codified in Article 48 of the ILC\u2019s <\/span>Articles on the Responsibility of States for Internationally Wrongful Acts<\/span><\/a> (para. 442; see also <\/span>Luciano Pezzano<\/span><\/a>). The grounding of <\/span>all <\/span><\/i>applicable obligations into the general law of State responsibility is precisely where the advisory opinion\u2019s cross-source method delivers bite.<\/span><\/p>\n

For vulnerable States and communities long frustrated by the slow pace of negotiations, this result is vindication. The ICJ\u2019s opinion itself is not binding, but as a clarification of binding law from the UN\u2019s principal judicial organ, it carries the highest degree of legal weight and political legitimacy (see e.g., <\/span>Soenke Kreft and Maren Solmecke<\/span><\/a>). In addition to reshaping international relations, it is bound to have deep <\/span>implications for\u00a0climate litigation<\/span><\/a> worldwide. Activists and advocates now have authoritative language to bolster cases in domestic courts; for example, national judges seeing that the world\u2019s highest court considers\u00a0failure to regulate greenhouse gas emissions\u00a0a breach of international law may feel empowered (and indeed obligated) to interpret domestic duties in light of that standard. We can anticipate litigants invoking the ICJ opinion to argue that government inaction violates legal obligations \u2013 the due diligence duty to prevent harm, the right to a clean, healthy and sustainable environment, etc. Where governments, courts, or other organs of the State ignore the opinion, they will do so at their own peril. Breaches trigger legal consequences which, if left unaddressed, will accumulate and intensify over time. As noted <\/span>elsewhere<\/span><\/a>, the new baseline that emerges through this framing marks \u201ca fundamental reset\u201d of how we understand international law\u2019s application to climate change.<\/span><\/p>\n

It is worth stressing that none of this emerges <\/span>ex nihilo<\/span><\/i>. The ICJ\u2019s integrated reading reflects and consolidates an interpretive technique that a growing number of international courts have already deployed to align discrete treaty regimes with overarching climate objectives. ITLOS, in its 2024\u202f<\/span>Request for an Advisory Opinion submitted by the Commission of Small\u202fIsland\u202fStates on Climate Change and International Law<\/span><\/i> (Case\u202fNo.\u202f31)<\/span><\/a>, treated the Paris Agreement not as a self\u2011contained<\/span> lex\u202fspecialis <\/span><\/i>but as one of many reference points for interpreting States\u2019 due diligence obligations under the United Nations Convention on the Law of the Sea. The IACtHR had taken a parallel step seven years earlier in its <\/span>Advisory Opinion OC-23-17<\/span><\/a>, reading the American Convention on Human Rights together with environmental treaties and custom to recognise an autonomous right to a healthy environment. In turn, the ECtHR\u2019s Grand Chamber in\u202f<\/span>Verein\u202fKlimaSeniorinnen\u202fSchweiz and Others v. Switzerland<\/span><\/i><\/a> grounded its dynamic interpretation of Articles\u202f2 and\u202f8 of the European Convention on Human Rights in the goals and equity principles of the UNFCCC and the Paris Agreement, thereby weaving climate law and human rights doctrine into a single fabric. Most recently, the IACtHR in its <\/span>Advisory Opinion OC-32\/25<\/span><\/a> distilled a<\/span> jus cogens<\/span><\/i> obligation not to cause irreversible damage to the climate and the environment from general principles of law \u2013 including the precautionary principle, the polluter pays principle, and intergenerational equity \u2013 and fundamental human rights. Domestic courts \u2013 such as the German Federal Constitutional Court in <\/span>Neubauer<\/span><\/i><\/a> \u2013 have, in parallel, brought intergenerational equity and carbon\u2011budgeting into human rights doctrine (see further <\/span>here <\/span><\/a>and <\/span>here<\/span><\/a>).\u00a0<\/span><\/p>\n

While the precise jurisprudential interactions merit more detailed analysis, seeing the ICJ\u2019s reasoning as part of a continuum helps explain both its historical significance and its normative power. By aligning the core of its analysis with this broader transnational trend, the ICJ confirmed that systemic integration is the beating heart of international climate law.\u202fWhat was once viewed as progressive experimentation (as analyzed <\/span>here <\/span><\/a>with S\u00e9bastien Jodoin) has become orthodox international law.\u202fThat very institutionalization explains why the advisory opinion feels both spectacular and evolutionary: spectacular, because the ICJ has historically hewed to incrementalism; evolutionary, because its method crystallizes a decades\u2011long jurisprudential arc that has already normalized cross\u2011regime reasoning in environmental and human rights litigation.<\/span><\/p>\n

Conclusion\u00a0<\/b><\/h2>\n

The ICJ\u2019s advisory opinion on climate change may come to be remembered as the moment international law explicitly rose to the climate challenge. Yet, what the opinion offers is not a new edifice but a sturdier legal architecture. By advancing an \u201call of the above\u201d approach to international law\u2019s sources; by treating these sources as interlocking parts of a living legal system; and by recognizing<\/span> erga omnes<\/span><\/i> and <\/span>erga omnes partes<\/span><\/i> duties with concrete consequences for responsibility, the Court has given States, courts and litigants a legally rigorous, source\u2011sensitive map. This map clearly shows how each source can discipline the others: principles channel discretion; custom supplies baselines where treaty text is thin; treaty institutions specify and update standards; human rights ground both interpretation and the content of obligations, including as customary law. That method is not merely elegant. It is action\u2011forcing, because it ties the work of implementation (from enhanced Nationally Determined Contributions to finance and technology transfer) to good\u2011faith co\u2011operation, due diligence and rights\u2011based constraints, and because it makes clear that breaches sound in responsibility with the full suite of consequences.<\/span><\/p>\n

That is why the Opinion will travel: it offers a vocabulary courts and other decision\u2011makers are already speaking \u2013 and an invitation to use it with greater confidence.<\/span><\/p>\n

 <\/p>\n

Margaretha Wewerinke-Singh served as lead counsel for Vanuatu in these proceedings, together with Julian Aguon at Blue Ocean Law, but writes in an academic capacity.<\/span><\/i><\/p>\n","protected":false},"excerpt":{"rendered":"

The International Court of Justice (ICJ)\u2019s release of its Advisory Opinion on the Obligations of States with Respect to Climate Change marks a watershed moment, not just because of what the court says about climate obligations, but also because of how\u00a0it says it. In responding to the legal question posed to it, the ICJ does […]<\/p>\n","protected":false},"author":2336,"featured_media":26335,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[69613,5673,69207],"tags":[69255,177],"class_list":{"0":"post-26539","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-blog-series","8":"category-litigation","9":"category-cross-cutting-issues","10":"tag-advisory-opinion","11":"tag-icj","12":"czr-hentry"},"yoast_head":"\nHarmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change - Climate Law Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change - Climate Law Blog\" \/>\n<meta property=\"og:description\" content=\"The International Court of Justice (ICJ)\u2019s release of its Advisory Opinion on the Obligations of States with Respect to Climate Change marks a watershed moment, not just because of what the court says about climate obligations, but also because of how\u00a0it says it. In responding to the legal question posed to it, the ICJ does […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/\" \/>\n<meta property=\"og:site_name\" content=\"Climate Law Blog\" \/>\n<meta property=\"article:published_time\" content=\"2025-08-11T14:24:35+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2025-08-12T16:57:41+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2025\/08\/08-e1755017757180.jpg\" \/>\n\t<meta property=\"og:image:width\" content=\"1595\" \/>\n\t<meta property=\"og:image:height\" content=\"1757\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/jpeg\" \/>\n<meta name=\"author\" content=\"Margaretha Wewerinke-Singh\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:creator\" content=\"@toniatigre\" \/>\n<meta name=\"twitter:site\" content=\"@sabincenter\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Margaretha Wewerinke-Singh\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"15 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/\"},\"author\":{\"name\":\"Margaretha Wewerinke-Singh\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#\\\/schema\\\/person\\\/25d163e261c920a883b184da07c9cf7b\"},\"headline\":\"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change\",\"datePublished\":\"2025-08-11T14:24:35+00:00\",\"dateModified\":\"2025-08-12T16:57:41+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/\"},\"wordCount\":3380,\"publisher\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#organization\"},\"image\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/files\\\/2025\\\/08\\\/08-e1755017757180.jpg\",\"keywords\":[\"Advisory Opinion\",\"ICJ\"],\"articleSection\":[\"Blog Series\",\"Climate Litigation\",\"Cross-cutting Issues\"],\"inLanguage\":\"en-US\"},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/\",\"url\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/\",\"name\":\"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change - Climate Law Blog\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#website\"},\"primaryImageOfPage\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/#primaryimage\"},\"image\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/files\\\/2025\\\/08\\\/08-e1755017757180.jpg\",\"datePublished\":\"2025-08-11T14:24:35+00:00\",\"dateModified\":\"2025-08-12T16:57:41+00:00\",\"breadcrumb\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/#breadcrumb\"},\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/\"]}]},{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/#primaryimage\",\"url\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/files\\\/2025\\\/08\\\/08-e1755017757180.jpg\",\"contentUrl\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/files\\\/2025\\\/08\\\/08-e1755017757180.jpg\",\"width\":1595,\"height\":1757,\"caption\":\"=============== Jeroen Bouman \\\/ photos Heesterstraat 25 2563 RA Den Haag The Netherlands TEL +31 70 3030244 FAX +31 70 3194450 GSM +31 6 25052555 EMAIL info@jeroenbouman.com WEB www.jeroenbouman.com\"},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/2025\\\/08\\\/11\\\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\\\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#website\",\"url\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/\",\"name\":\"Climate Law Blog\",\"description\":\"\",\"publisher\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#organization\"},\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"en-US\"},{\"@type\":\"Organization\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#organization\",\"name\":\"Sabin Center for Climate Change Law\",\"url\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/\",\"logo\":{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#\\\/schema\\\/logo\\\/image\\\/\",\"url\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/files\\\/2023\\\/02\\\/21-SabinBlog_Banner-1.png\",\"contentUrl\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/files\\\/2023\\\/02\\\/21-SabinBlog_Banner-1.png\",\"width\":2752,\"height\":260,\"caption\":\"Sabin Center for Climate Change Law\"},\"image\":{\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#\\\/schema\\\/logo\\\/image\\\/\"},\"sameAs\":[\"https:\\\/\\\/x.com\\\/sabincenter\"]},{\"@type\":\"Person\",\"@id\":\"https:\\\/\\\/blogs.law.columbia.edu\\\/climatechange\\\/#\\\/schema\\\/person\\\/25d163e261c920a883b184da07c9cf7b\",\"name\":\"Margaretha Wewerinke-Singh\",\"description\":\"Margaretha Wewerinke-Singh is Associate Professor of Sustainability Law at the Faculty of Law of the University of Amsterdam.\",\"sameAs\":[\"https:\\\/\\\/www.linkedin.com\\\/in\\\/mtigre\",\"https:\\\/\\\/x.com\\\/toniatigre\"],\"url\":\"#molongui-disabled-link\"}]}<\/script>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change - Climate Law Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/","og_locale":"en_US","og_type":"article","og_title":"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change - Climate Law Blog","og_description":"The International Court of Justice (ICJ)\u2019s release of its Advisory Opinion on the Obligations of States with Respect to Climate Change marks a watershed moment, not just because of what the court says about climate obligations, but also because of how\u00a0it says it. In responding to the legal question posed to it, the ICJ does […]","og_url":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/","og_site_name":"Climate Law Blog","article_published_time":"2025-08-11T14:24:35+00:00","article_modified_time":"2025-08-12T16:57:41+00:00","og_image":[{"width":1595,"height":1757,"url":"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2025\/08\/08-e1755017757180.jpg","type":"image\/jpeg"}],"author":"Margaretha Wewerinke-Singh","twitter_card":"summary_large_image","twitter_creator":"@toniatigre","twitter_site":"@sabincenter","twitter_misc":{"Written by":"Margaretha Wewerinke-Singh","Est. reading time":"15 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/#article","isPartOf":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/"},"author":{"name":"Margaretha Wewerinke-Singh","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#\/schema\/person\/25d163e261c920a883b184da07c9cf7b"},"headline":"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change","datePublished":"2025-08-11T14:24:35+00:00","dateModified":"2025-08-12T16:57:41+00:00","mainEntityOfPage":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/"},"wordCount":3380,"publisher":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#organization"},"image":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/#primaryimage"},"thumbnailUrl":"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2025\/08\/08-e1755017757180.jpg","keywords":["Advisory Opinion","ICJ"],"articleSection":["Blog Series","Climate Litigation","Cross-cutting Issues"],"inLanguage":"en-US"},{"@type":"WebPage","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/","url":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/","name":"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change - Climate Law Blog","isPartOf":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#website"},"primaryImageOfPage":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/#primaryimage"},"image":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/#primaryimage"},"thumbnailUrl":"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2025\/08\/08-e1755017757180.jpg","datePublished":"2025-08-11T14:24:35+00:00","dateModified":"2025-08-12T16:57:41+00:00","breadcrumb":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/#breadcrumb"},"inLanguage":"en-US","potentialAction":[{"@type":"ReadAction","target":["https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/"]}]},{"@type":"ImageObject","inLanguage":"en-US","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/#primaryimage","url":"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2025\/08\/08-e1755017757180.jpg","contentUrl":"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2025\/08\/08-e1755017757180.jpg","width":1595,"height":1757,"caption":"=============== Jeroen Bouman \/ photos Heesterstraat 25 2563 RA Den Haag The Netherlands TEL +31 70 3030244 FAX +31 70 3194450 GSM +31 6 25052555 EMAIL info@jeroenbouman.com WEB www.jeroenbouman.com"},{"@type":"BreadcrumbList","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/2025\/08\/11\/harmonizing-sources-hardening-duties-inside-the-icjs-advisory-opinion-on-climate-change\/#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/blogs.law.columbia.edu\/climatechange\/"},{"@type":"ListItem","position":2,"name":"Harmonizing Sources, Hardening Duties \u2013 Inside the ICJ\u2019s Advisory Opinion on Climate Change"}]},{"@type":"WebSite","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#website","url":"https:\/\/blogs.law.columbia.edu\/climatechange\/","name":"Climate Law Blog","description":"","publisher":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#organization"},"potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/blogs.law.columbia.edu\/climatechange\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-US"},{"@type":"Organization","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#organization","name":"Sabin Center for Climate Change Law","url":"https:\/\/blogs.law.columbia.edu\/climatechange\/","logo":{"@type":"ImageObject","inLanguage":"en-US","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#\/schema\/logo\/image\/","url":"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2023\/02\/21-SabinBlog_Banner-1.png","contentUrl":"https:\/\/blogs.law.columbia.edu\/climatechange\/files\/2023\/02\/21-SabinBlog_Banner-1.png","width":2752,"height":260,"caption":"Sabin Center for Climate Change Law"},"image":{"@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#\/schema\/logo\/image\/"},"sameAs":["https:\/\/x.com\/sabincenter"]},{"@type":"Person","@id":"https:\/\/blogs.law.columbia.edu\/climatechange\/#\/schema\/person\/25d163e261c920a883b184da07c9cf7b","name":"Margaretha Wewerinke-Singh","description":"Margaretha Wewerinke-Singh is Associate Professor of Sustainability Law at the Faculty of Law of the University of Amsterdam.","sameAs":["https:\/\/www.linkedin.com\/in\/mtigre","https:\/\/x.com\/toniatigre"],"url":"#molongui-disabled-link"}]}},"_links":{"self":[{"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/posts\/26539","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/users\/2336"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/comments?post=26539"}],"version-history":[{"count":1,"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/posts\/26539\/revisions"}],"predecessor-version":[{"id":26542,"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/posts\/26539\/revisions\/26542"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/media\/26335"}],"wp:attachment":[{"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/media?parent=26539"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/categories?post=26539"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.law.columbia.edu\/climatechange\/wp-json\/wp\/v2\/tags?post=26539"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}