Background

The UN General Assembly adopted resolution 77/276 on 29 March 2023 which set out the questions on which the Court was asked to advise. Recital 1 of the resolution recognised that “climate change is an unprecedented challenge of civilisational proportions and that the well-being of present and future generations of humankind depends on our immediate and urgent response to it”.

The publication of the ICJAO was preceded by written and oral proceedings. The registry of the Court received 91 written statements in 2024 with respect to the advisory opinion proceedings. It then held two weeks of oral hearings in December 2024.

The ICJAO is divided into a number of sections. This summary will focus on the sections concerning the obligations of States and the State Responsibility for breaches of those obligations.

Obligations

The ICJ was asked to advise generally on the obligations of States in respect of Climate Change. Whilst the Court recognised that many international treaties potentially touched upon climate change, it determined it ought to focus on the most important treaties and obligations in customary international law.

Obligations deriving from international treaties

The three most important treaties were the United Nations Framework Convention on Climate Change (“UNFCCC”), which is a framework convention that entered into force in 1994, the Kyoto Protocol (1997), and the Paris Agreement (2016). The Court held that the three agreements complement each other. Whilst the UNFCCC establishes the ultimate objective and basic principles, the Kyoto Protocol and Paris Agreement flesh out more detailed obligations (para. 120). It is important to recall that international treaties are only binding on the parties to them.

Of significance, was that the ICJAO rejected the argument that the Paris Agreement was a lex specialis, which displaced other international law. The Court held the Paris Agreement and other climate change treaties should be interpreted consistently with the general international law system (paras. 168-171).

(a) The UNFCCC

As a preliminary point, it is worth noting that the Court considered that the UNFCCC ought to be interpreted in light of its objective set out in Article 2. This provides that the aim of the UNFCCC is to achieve, “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system”, and to ensure that “[s]uch a level should be achieved within a time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner” (para. 197). It is evident that this Article informs the Court’s analysis of the other treaty provisions within the UNFCCC.

Further, for those not familiar with the UNFCCC, it is worth noting that it imposes different duties on developed and developing State Parties. 

The main obligations analysed by the Court are those of mitigation and adaption, the substantive terms of which are both found in Article 4 UNFCCC.

The Court explained that there are two ways that the UNFCCC seeks to achieve mitigation. First, by limiting anthropogenic greenhouse gas (“GHG”) emissions by sources, and second, by preserving and enhancing sinks and reservoirs of GHGs (‘sinks’ involve removing GHG from the atmosphere whilst ‘reservoirs’ involve the storage of GHGs).

The Court also stressed the importance of the adaption obligations. It notes that Article 4(4) provides that certain developed parties listed in Annex II “shall” assist developing country parties that are particularly vulnerable to the adverse effects of climate change in meeting the costs of adaptation to those adverse effects (para. 211).

Throughout its analysis, the Court stresses that the obligations relating to mitigation and adaptation in the UNFCC are legally binding obligations. The failure to comply with which is an internationally wrongful act that has consequences (see below).

(b) Kyoto Protocol

The Kyoto Protocol came into force in 1997. The aim of the Protocol was to stabilise greenhouse gas emissions. It worked by agreeing periods of individual targets over a commitment period. The first period ran 2008 to 2012 and the second from 2013 to 2020. No additional commitment period has been set past 2020. Despite this, the Court considered that the Protocol was still in force and its provisions could serve as interpretative aids for the identification of obligations under the climate change treaty framework.

(c) Paris Agreement

The Paris Agreement came into force in 2016. The Court found that its “object and purpose”, reflected in Article 2(1), was to “strengthen the global response to the threat of climate change” by, inter alia, holding the increase in the global average temperature to well below 2 degrees above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 degrees above pre-industrial levels.

The ICJ further found that subsequent meetings of the parties to the Paris Agreement indicate that 1.5 degrees has become the scientifically based consensus target under the Paris Agreement, and that this is the agreed primary temperature goal for limiting the global average temperature increase under the Paris Agreement (para 224).

The Paris Agreement refers to the principle of “common but differentiated responsibility and respective capacities, in the light of different national circumstances” (Article 2(2)). Whilst the UNFCCC refers to this principle, the last clause is not contained in that Convention. The Court considered this significant and found that it reflected the fact that the status of a State as a developed or developing State is not static but depends on the current circumstances of the State concerned (para. 226). The significance of which is that State Parties that were considered ‘developing’ historically, may now be considered ‘developed’ and so attract more onerous obligations. 

The Court also set out the obligations of mitigation, adaption and cooperation that arise under the Paris Agreement.

It is in the analysis of mitigation that one sees the significance of the temperature goal being recognised as 1.5 degrees as opposed to 2 degrees. The court finds that the obligations in Article 4 to “aim to reach a global peaking of greenhouse gas emissions as soon as possible…and to undertake rapid reductions thereafter”, so as to achieve net zero emissions by balancing anthropogenic GHG emissions and their removal by sinks by the second half of the century, should be informed by the ultimate goal to limit temperature rising to 1.5 degrees (para. 230-231).

To achieve this mitigation goal, the Paris Agreement sets out the scheme of nationally determined contributions (NDCs). NDCs must, under Article 4(3), represent a party’s “highest possible ambition”. The ICJ explained that in light of the customary obligation to prevent significant harm to the environment and the obligation of due diligence (see below) that this means that parties must 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 1.5°C 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 system (para. 245).

In terms of the obligations to adapt to climate change, the Court notes that the Paris Agreement does not set out any specific actions that must be taken for adaption. As a result, it provides relatively little analysis of the adaption obligations.

A summary of the ICJ’s findings under the treaty frameworks is found in paras. 268-270.

Obligations deriving from customary international law

Customary international law is international law that does not arise from treaty, but from state practice and opinion juris. In contrast to obligations deriving from treaties, obligations in customary international law are binding on all States.

There are a number of rules of customary international law that the Court considered relevant. Just two are discussed below.

(a) The duty to prevent significant environmental damage

The Court has previously found that there is a customary duty on States to prevent significant environmental harm (see Pulp Mills on the River Uruguay (Argentina v. Uruguay), Judgment, I.C.J. Reports 2010 (I)). In the ICJAO, the Court found that this duty applies in the context of climate change (para. 136).

Although some parties had strongly opposed this, the Court found that the mere fact that climate change was caused cumulatively and was the result of varied and multifaceted forms of conduct, was not a bar to the duty arising in the context of climate change. This was even though it may be difficult to identify a specific share of responsibility of any particular State. The Court stressed that the duty arises as a result of the “general risk of significant harm to which States contribute, in markedly different ways, through the activities undertaken within their jurisdiction or control” (para. 279).

The duty to prevent significant environmental harm requires States to act with due diligence. It is an obligation of conduct that requires States to take, to the best of their ability, appropriate, and if necessary, precautionary measures, which take account of scientific and technological information, as well as relevant rules of international standards, and which depend on each States’ respective capabilities (para. 136).

As far as climate change is concerned, the Court found that “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” (para. 282).

Further, the ICJ explained that due diligence requires certain procedural steps, which may include the undertaking of environmental impact assessments in relation to projects within their jurisdiction (para. 298).

(b) The duty of cooperation

A theme throughout the ICJAO was the ICJ’s emphasis on the importance of cooperation, which is also a duty under customary international law. The importance of this duty is encapsulated neatly in paragraph 308, where the ICJ states “Climate change is a common concern. Co-operation is not a matter of choice for States but a pressing need and a legal obligation.”

Relationship between obligations arising under treaties and the obligations under customary international law

Several parties had argued that the Paris Agreement displaced the rules of customary international law. This argument was rejected by the ICJ which noted that for this to occur, the Paris Agreement would need to conflict with customary international law, which it did not.

The Court further explained that as customary law binds all States, and is dependent upon State practice, it is possible that a State that is not a party to the UNFCCC, Paris Agreement or Kyoto Protocol, may find it more difficult to show how it is complying with its customary duties than a State which is party to the agreements or at least acts consistently with those agreements (paras. 314-315).

Legal consequences arising from States’ acts and omissions that cause significant harm to the climate and other parts of the environment

The Court found that the rules for responsibility for breaches of obligations under the climate change treaties were found in the well-established rules on State responsibility. However, the Court also accepted that the nature of climate change meant that several aspects regarding the rules of State Responsibility required a more detailed articulation.

First, the court noted the issues regarding attribution. In international law, attribution concerns how acts are attributed to the State. In the climate change context this is relevant insofar as private actors within a State’s jurisdiction are the actors actually emitting GHG emissions, as opposed to the organ of the State.

In this regard, the Court found that the “failure of a State to take appropriate action to protect the climate system from GHG emissions — including through fossil fuel production, fossil consumption, the granting of fossil fuel exploration licences or the provision of fossil subsidies — may constitute an internationally wrongful act which is attributable to that State. The Court also emphasizes that the internationally wrongful act in question is not the emission of GHGs per se, but the breach of conventional and customary obligations identified earlier in the ICJAO pertaining to the protection of the climate system from significant harm resulting from anthropogenic emissions of such gases. (para. 427)

In relation to whether the emissions of GHGs by private actors could be attributed to a State, the Court found that the obligation of due diligence meant that the question of attribution did not arise. Rather, the issue would be whether the state in question regulated the actions of private actors in line with their due diligence obligations (para. 428).

Whilst the Court also noted concerns of participating States that the cumulative nature of climate change meant States’ contribution to climate change could not be determined, the Court rejected this argument as a rationale for why State responsibility could not be invoked. The Court found that it is scientifically possible to determine each States’ total contribution to global emissions, taking into account both historical and current emissions.

The second aspect of State responsibility discussed by the Court concerned the rules on causation. The Court noted that to invoke international responsibility, causation is not actually required. Rather, all that needs to be shown is that there was an international wrongful act and that it was attributed to a State (para. 433). However, causation does arise where reparations are sought and hence it was necessary to address.

Whilst the Court again noted the arguments that causation was too difficult to show in the climate change context, the ICJ ultimately held that the Court’s existing jurisprudence, which requires a “sufficiently direct and certain causal nexus” between an alleged wrongful action or omission and the alleged damage, was flexible enough to address the challenges arising in the context of climate change (para. 436).

The third issue to be addressed by the Court under the heading of State Responsibility was whether any or all of the obligations were ‘erga omnes’ or ‘erga omnes partes’. Erga omnes obligations mean they are owed to all other States and so any State can bring proceedings to enforce the obligation. Erga omnes partes obligations are obligations owed by all State Parties to all other State Parties.

In this regard, the ICJ observed that the rules of international law relating to the global common good, such as the climate system, may produce erga omnes obligations and that consequently the obligation to prevent significant transboundary harm under customary international law was erga omnes (para. 440). Further, the obligations of States Parties under the UNFCCC and the Paris Agreement were erga omnes partes (para. 441).

The court goes on to discuss the responsibility of a wrongful state. Of the most significance is the finding that reparations may be due in some circumstances (paras. 449-450). This opens the door for injured States to potentially bring claims for climate reparations against developed states that they consider to have breached their obligations under the climate treaties or customary international law.

Concluding remarks

The ICJAO is the third advisory opinion in a string of climate related advisory opinions that have been or are due to be published. Arguably, the ICJAO is the most authoritative of the four advisory opinions due. It has gone further than many expected in opening the door for reparations and recognising the responsibility of States in relation to private actors.

This blog has aimed to provide a summary of just some of the main findings of the ICJAO. The ICJAO touches upon a number of other matters that arise in environmental contexts, not all of which can be addressed here (such as international human rights law, the right to a healthy environment, intergenerational equity and the precautionary principle).

The implications for the UK will take time to ascertain given the length and breadth of the ICJAO.

On the one hand, it is unclear that the ICJAO will necessarily result in more domestic litigation concerning climate change. Whilst the interpretation of the obligations under the Paris Agreement and UNFCCC are important, it does not change the fact that neither treaty is incorporated in to domestic law and it is not for the domestic courts to determine whether the UK has violated its obligations under an international treaty (R (Plan B Earth) v Prime Minister (“Plan B”)  [2021] EWHC 3469 (Admin) at para. 25).

Further, in the past few years, Claimants have challenged the Government’s measures under the Climate Change Act 2008 for reducing GHG emissions, as well as the measures concerning mitigating and adapting to climate change (e.g. Plan B). In this case, it was claimed that the measures undertaken by the government were under-ambitious to meet the goals set out in the Paris Agreement and implemented by the Climate Change Act 2008 (as amended). The Court rejected the challenge. In doing so, the Court referred to the way that the statutory duties are framed as not being duties to achieve specific outcomes and that the Climate Change Act 2008 gives the Climate Change Committee the oversight role in holding the Government to account and that the Court’s role was accordingly much more limited (see, Plan B paras. 32-35). It is unclear whether the ICJAO would change this analysis.

On the other hand, there is scope for the ICJAO to inform the approach to cases going forward. The ICJAO did emphasise that the obligations of conduct in the Paris Agreement to adopt NDCs must actually be capable of meeting the 1.5 degrees target and that the obligation of due diligence is stringent. This analysis may give claimants more scope to challenge the UK’s policies concerning climate change on the basis that there is a substantive target behind the measures.

In addition, customary international law, whilst not automatically part of the common law, is a source of the common law on which courts in England and Wales can draw as appropriate (The Law Debenture Trust Corporation plc (Appellant) v Ukraine [2023] UKSC 11, para. 204). The appropriateness depending on whether there are any constitutional principles, statute or common law indicating that effect should not be given to the rule of international law (paras. 204-205). As such, the ICJAO analysis on the international customary duty to not prevent significant environmental damage may provide a source of development of the common law going forward.

Moreover, in cases where the UK’s compliance with the Paris Agreement is justiciable (e.g. R (FoE) v Secretary of State for International Trade [2023] EWCA Civ 14), it is possible that the ICJAO may inform the analysis undertaken by courts. In this case, for example, the Court of Appeal held that article 2(1)(c) of the Paris Agreement “does not create an obligation on the UK to demonstrate that its overseas funding was consistent with a pathway towards limiting global warming to well below 2 [degrees] and pursing efforts to 1.5 [degrees]” (para. 55(vi)). However, it is interesting to note para. 427 of the ICJAO, which provided that “failure of a State to take appropriate action to protect the climate system from GHG emissions — including through fossil fuel production, fossil 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.” (emphasis added).

Moreover, at a ‘soft law’ level, the Climate Change Committee is likely to be even more emboldened in its scrutiny of the Government’s policies under the Climate Change Act 2008. In a similar vein, if there is a drive by injured States to seek reparations, this may put pressure on the Government to take more robust action on climate change to minimise its liability at the international level.

Emma Rowland is a pupil barrister at Francis Taylor Building.

Richard Honey KC appeared in R (Plan B Earth) v Prime Minister [2021] EWHC 3469 (Admin) and R (FoE) v Secretary of State for International Trade [2023] EWCA Civ 14. Ned Westaway appeared in R (Plan B Earth) v Prime Minister [2021] EWHC 3469 (Admin). Conor Fegan appeared in R (FoE) v Secretary of State for International Trade [2023] EWCA Civ 14. They have not been involved in the drafting of this post.