The development of international law on climate change

Two landmark opinions have been issued by international courts this summer, articulating states’ duties in respect of climate change, one from the InterAmerican Court of Human Rights (IACtHR) (the IACtHR Opinion) the other from the International Court of Justice (ICJ) (the ICJ Opinion) (together, the Opinions).

These follow and, in the case of the ICJ Opinion, confirm last year’s significant decision of the International Tribunal for the Law of the Sea (ITLOS) on the UNCLOS in May 2024. See our summary of the ITLOS opinion here. 

Although the Opinions consider international law and so the duties of and between nation states and although neither advisory opinion is binding, they will have persuasive authority before national courts considering issues such as planning and licensing applications or national net zero strategies. The Opinions will also carry some political and diplomatic weight, particularly in the upcoming climate negotiations in Brazil as part of COP30, encouraging greater ambition on states’ nationally determined contributions (NDCs), and emboldening states calling for greater funding for and cooperation on adaptation. The Opinions will also strengthen plaintiffs’ and judges’ reliance on the Intergovernmental Panel on Climate Change’s (IPCC’s) reports on the science of climate change and causal theories linking greenhouse gas (GHG) emissions to climate impacts. 

What are the implications for business?

The Opinions refer to state obligations, and so do not create any direct obligations for corporates, but they do have implications for business. Both Opinions explicitly refer to states’ obligations to regulate private actors within their jurisdictions, with the IACtHR setting out specific steps that states should take to comply with international law in this regard, and the ICJ indicating that states’ due diligence obligations extend to regulating corporates within their jurisdictions, with a particular focus on fossil fuels. From the ICJ Opinion:

"a State may be responsible 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."

Below we focus on each court’s findings related to businesses, and the implications for companies globally.

The IACtHR Opinion: setting standards for regulation of businesses

In July 2025, the Inter-American Court of Human Rights affirmed the right to a stable climate as a human right. The IACtHR’s advisory function is to respond to questions posed by the nation state members of the Organization of American States (OAS) regarding the interpretation of the American Convention on Human Rights, an international treaty that contemplates the rights and liberties that must be respected by States Parties.

In its July Opinion the Court went so far as to identify concrete obligations for states to regulate the private sector which, it found, plays an ‘essential role’ in tackling climate change, noting that corporations “have obligations and responsibilities with respect to climate change, and [their] impacts […] on human rights”.

In so finding, the IACtHR noted in particular various ‘soft law’ instruments such as the Organization for Economic Cooperation and Development (OECD) Guidelines for Multinational Enterprises on Responsible Business Conduct and the Inter-American Commission’s thematic report on Business and Human Rights. The Court cited with approval the Guiding Principles on Business and Human Rights (also called the “Ruggie principles”) which set out businesses obligations to ‘protect, respect and remedy’ human rights’ abuses. The Court also referred to the Working Group on the issue of human rights and transnational corporations and other business  enterprises, “Information Note on Climate Change and the Guiding Principles on Business and Human Rights,” June 2023.

The Court is convinced that business enterprises are called on to play an essential role in addressing the climate emergency. The Court recalls that States should adopt legislative and other measures to prevent human rights violations committed by public and private enterprises and, when these occur, investigate them, punish them, and guarantee redress for their consequences. Ultimately, this is an obligation that should be complied by the business sector and must be regulated by States, which are the subjects of international law over which the Court exercises jurisdiction.

The IACtHR Opinion at paragraph 347 sets out specific measures that State Parties must take, including putting in place legislation requiring companies to perform due diligence on climate impacts along the entire value chain, to report on GHG emissions, regulation requiring businesses to cut emissions and adopt mitigation targets, and regulation aimed at stamping out greenwashing and corporate lobbying.

The IACtHR also specified that some companies bear greater responsibility for climate change impacts, and demand more supervision and monitoring, particularly focusing on hard-to-abate or fossil fuel intensive industries, particularly fossil fuel, cement manufacture and agro-industrial companies (see paragraph 353). 

The State’s obligation per the IACtHR includes monitoring, investigating, prosecuting and sanctioning business enterprises and other private persons with legal consequences ranging from injunctive relief, to ‘real compensation for the harm caused to the global climate system’. 

The ICJ Opinion: states’ duties extend to regulation of companies on climate

Soon after the IACtHR opinion, on 23 July 2025, the ICJ, the principal judicial organ of the United Nations, issued its Advisory Opinion on the Obligations of States in Respect of Climate Change. The ICJ’s role is to settle, in accordance with international law, legal disputes submitted to it by States and to give advisory opinions on legal questions referred to it by the UN. The Opinion was commissioned from the Court by the UN General Assembly in 2023.

In its July Opinion, the Court drew from a wide range of legal sources, including the UN Framework Convention on Climate Change, the Kyoto Protocol, and the Paris Agreement, customary international law, and international human rights law. The Court also clarified that both (1) the duty to prevent significant harm to the environment; and (2) the duty to cooperate for the protection of the environment in respect of climate change constituted customary international law duty applicable to all states, even those not party to climate treaties (or, like the US, those that have pulled out of such agreements).  Further, the Court found that international law on climate change cannot be displaced by the lex specialis of treaty law. In other words, states cannot derogate from their broader legal duties by reference to narrower treaty obligations.

…the most directly relevant applicable law consists of the Charter of the United Nations, the UNFCCC, the Kyoto Protocol, the Paris Agreement, UNCLOS, the ozone layer treaties, the Biodiversity Convention, the Desertification Convention, the customary duty to prevent significant harm to the environment and the duty to co-operate for the protection of the environment, and international human rights law…

As noted above, the Court confirmed the May 2024 ITLOS opinion that GHG emissions constitute pollution of the marine environment within the meaning of UNCLOS, that Article 192 of UNCLOS includes a positive obligation to take measures to protect and preserve the marine environment, and that Article 194(1) of UNCLOS requires states to take all necessary measures to reduce and control pollution.

The Court set out that in order to comply with their legal duties, states must exercise stringent due diligence in preparing their nationally determined contributions (NDCs) - each country's efforts to reduce national emissions and adapt to the impacts of climate change – which reflect the highest possible ambition towards the 1.5C target set out in the Paris Agreement. States have only limited discretion in this.

“because of the seriousness of the threat posed by climate change, the standard of due diligence to be applied in preparing the NDCs is stringent. This means that each party has to do its utmost to ensure that the NDCs it puts forward represent its highest possible ambition in order to realize the objectives of the Agreement.”¹

The Court found that states parties to the Paris Agreement through their NDCs must make an adequate contribution to the achievement of the temperature goal”. This finding will have an immediate impact on preparations for COP30 in Brazil where countries are due this year to submit their updated (and more ambitious) NDCs.

Impact on climate cases in national Courts

ICJ opinions can impact decision-making by national courts on state conduct, particularly via public law challenges to administrative decision-making. The ICJ opinion may thus increase legal challenges to administrative decisions around planning and licensing particularly regarding high-emissions activities such as fossil fuel production, consumption, and exploration. 

The declaration from ICJ Judges Bhandari and Cleveland, accompanying the Opinion, went further in suggesting that policies which increase fossil fuel supply might be illegal, referring particularly to the UK Supreme Court’s recent decision in Finch v Surrey County Council and proposing that governments must ensure that environmental impact assessments for fuel projects include “Scope 3” emissions (downstream emissions of end users). As a result of Finch in the UK the government launched a consultation and issued supplementary guidance for assessing the effects of downstream scope 3 emissions on climate from offshore oil and gas projects.² The judgment has already impacted licensing for a coal mine (which has since led to a claim against the UK government)³ and a challenge to offshore oil and gas fields.⁴ Other jurisdictions may, with the support of these ICJ judges’ statements, reach similar conclusions regarding Scope 3 emissions.

The Opinion will add weight to the deployment of climate science in Courts; relying on evidence from the Intergovernmental Panel on Climate Change (IPCC) the ICJ affirmed that the IPCC’s reports “constitute the best available science on the causes, nature and consequences of climate change”. This is a strong juridical endorsement of the IPCC’s work, which is still challenged routinely by climate sceptics.

The ICJ opinion found that it was possible to legally attribute climate change to emissions from certain countries. The Court firstly distinguished between “attribution” as understood in the field of climate science, meaning the evaluation of whether certain factors caused an event, in this context the extent to which climate change is “attributable” to GHG emissions; and “attribution” in the context of determining state responsibility for internationally wrongful acts – in the premises, the failure to limit climate change. 

Regarding the first understanding of attribution, the Court stated that scientific evidence presented to it establishes that anthropogenic GHG emissions have caused “significant harm to the climate system and other parts of the environment”; particularly, the IPCC’s 2023 Synthesis Report “includes data clearly linking the human contribution to climate change to observed increases in heatwaves, flooding and drought.” The ICJ further noted that, since under the UNFCCC States are required to communicate information on their emissions, “it is scientifically possible to determine each State’s total contribution to global emissions, taking into account both historical and current emissions.” The 2023 IPCC report, for instance, “includes data on cumulative net emissions by region, and other studies include data on current and historical emissions that can be attributed to individual States.” This finding will embolden courts to find that causation can be established between emissions and climate harms, including potentially where corporates are responsible for such emissions. This finding will embolden courts to find that causation can be established between emissions and climate harms, including potentially where corporates are responsible for such emissions. 

The ICJ also affirmed the 1.5C limit in global warming, set by the Paris Agreement, as a matter of international law. This will inevitably increase political and legal pressure on states to institute policies and regulations aligned with a 1.5C transition pathway. With the potential for these norms to be translated into domestic regulation, we can expect to see additional pressure from both regulators and investors on companies to align with a 1.5C target and adopt more rigorous transition plans – for example, this might feed into the consultation recently launched by the UK government on mandatory disclosure of climate transition plans for UK-regulated financial institutions and FTSE 100 companies.⁵

The clear link established by the ICJ opinion between climate change and human rights will encourage more litigation linking human right impacts to climate harm at the national level, already a significant trend. The Court recognised the human right to a clean, healthy, and sustainable environment and noted that climate change through sea level rise, desertification, drought and natural disaster impaired the human rights to life, health, an adequate standard of living (including access to food, water and housing), the right to privacy, family and home and the rights of women, children and indigenous peoples. Unlike the IACtHR, the ICJ did not cite the Ruggie Principles or the OECD Guidelines which seek to translate human rights obligations of state onto private businesses or set out due diligence obligations of corporations. However, the ICJ opinion strengthens the argument that climate change is a human rights issue, opening up additional powers and precedents for litigants, including against corporates. 

Regulation of private actors

Importantly, the ICJ articulated that States have a positive obligation to regulate the activities of private actors as a matter of due diligence and that breaches of international law by States may include failures to put in place legislative measures in relation to fossil fuel production, licensing or subsidies.

"failure of a State to take appropriate action to protect the climate system from GHG emissions — including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies — may constitute an internationally wrongful act which is attributable to that State"

The ICJ was not as prescriptive as the IACtHR but stated the State’s mitigation obligations in more general terms:

As far as climate change is concerned, such appropriate rules and measures include, but are not limited to, regulatory mitigation mechanisms that are designed to achieve the deep, rapid, and sustained reductions of GHG emissions that are necessary for the prevention of significant harm to the climate system.

Additional measures that may be adopted by states could include those set out in the IACtHR Opinion, including stricter emissions standards, enhanced carbon pricing, more onerous climate risk disclosure and reporting requirements, and mandatory due diligence regulation (such as the EU’s Corporate Sustainability Due Diligence regime – see here).

Investor-state disputes – the climate defence?

Finally, the ICJ Opinion may also have an impact on the prospects for corporations challenging states in investment treaty claims. Where investor claims arise from measures taken by states to reduce emissions (such as the recent challenge to the UK government’s withdrawal of planning permission for a coal mine), the ICJ Opinion could be relied on as a defence by a State that it was required to take such action as a matter of international law. It remains to be seen whether this will be a successful tactic in state-investor arbitrations. 

Conclusion

In summary, whilst not binding and aimed at nation states, the ICJ and IACtHR Opinions together represent a clear statement at the international level that climate change is scientifically proven to be anthropogenic and dangerous to humanity and that the reduction of GHG emissions is not merely a political aim but a legal obligation requiring states to take measures, including by regulating private actors. 

We can expect the Opinions to be cited in courts for years to come as plaintiffs seek to amplify the ambition of nation states, block fossil fuel development and push for stricter regulation of businesses’ emissions and transition strategies.

IACtHR Opinion

ICJ Opinion

¹Summary of the Advisory Opinion of 23 July 2025 page 13

²Environmental Impact Assessment (EIA) – Assessing effects of downstream scope 3 emissions on climate

³Investors in quashed coal mine sue government 

⁴Consent for Rosebank and Jackdaw oil and gas fields was unlawful - BBC News

⁵Climate-related transition plan requirements - GOV.UK