The International Court of Justice has released its advisory opinion on climate obligations, and it is already reshaping how scholars and policymakers understand global climate governance. For years, climate law was often described as fragmented because it is built on several treaties adopted at different moments. The Court’s opinion challenges that perception. It explains how the main treaties fit together, it narrows the freedom States claim when drafting their national climate plans, and it presents climate protection as a responsibility owed to everyone.
Linking the Climate Treaties into One Framework
A key contribution of the opinion lies in how it interprets the relationship between the United Nations Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol, and the Paris Agreement. These instruments were often treated as separate political compromises. The Court rejected that view. It applied Articles 26 and 30 of the Vienna Convention on the Law of Treaties (VCLT), which require States to interpret treaties in good faith and to ensure consistency when several treaties address the same subject matter.
According to the Court, the UNFCCC, Kyoto Protocol, and Paris Agreement form a continuous legal framework. Earlier obligations do not disappear simply because a new agreement was adopted. Scholars have long argued that climate treaties were designed to work cumulatively rather than to replace one another, and the Court’s reasoning reflects this view (Bodansky, The Art and Craft of International Environmental Law, 2010). As a result, the aim set out in the UNFCCC to stabilise greenhouse gas concentrations still matters, and the Paris Agreement must be interpreted in light of these earlier commitments.
Furthermore, this interpretation strengthens the legal character of the climate regime. It reduces the space for States to argue that the Paris Agreement operates as a flexible political platform with few legal constraints. Instead, the opinion confirms that climate treaty obligations must be read together, which makes the underlying duties more consistent and continuous.
NDCs and the End of Unlimited Discretion
Another important element of the opinion concerns Nationally Determined Contributions under Article 4 of the Paris Agreement. Governments have often described NDCs as expressions of national preference with significant policy freedom. The Court concluded that this freedom is not unlimited. Every NDC must reflect a State’s highest possible ambition, align with the Agreement’s temperature goal (1.5 °C), and be informed by science, including the findings of the Intergovernmental Panel on Climate Change (IPCC).
Academic commentary has long noted that NDCs still carry legal weight even though they are nationally determined (Rajamani & Brunnée 2017). The Court’s reasoning supports this interpretation. It stressed that the obligation to pursue domestic mitigation measures requires genuine effort. It also stated that each update of an NDC must show progression. This means that a State cannot submit a plan that falls outside what scientific assessments identify as necessary for achieving the temperature goal.
This reading of the Paris Agreement has practical implications. Domestic courts increasingly review national climate plans, and the advisory opinion provides a stronger legal basis for examining whether national contributions match scientific requirements. It also emphasises that the principle of Common but Differentiated Responsibilities and Respective Capabilities remains part of the legal framework. This principle recognises that while all States share responsibility for addressing climate change, those with greater financial and technological capacity are expected to contribute more to mitigation. In addition, it reflects the idea that countries with a longer history of emissions should take the lead in reducing them.
Climate Protection as an Obligation Owed to All
The advisory opinion also addresses the nature of the duty to protect the climate system. The Court characterised this duty as an ‘’erga omnes’’ obligation. This means that States owe it to the international community as a whole. Any State may raise concerns about another State’s conduct, even if it has not suffered a direct injury.
This approach builds on earlier decisions where the Court recognised that some obligations protect shared interests of the international community. In environmental law, the foundations of this approach can be seen in the Pulp Mills case, where the Court reaffirmed that States must act with due diligence when there is a risk of significant environmental harm, and in the Corfu Channel case, which established that a State may not allow activities within its territory that cause serious harm to others.
Scholars have argued that climate stability has become a global common interest, and the advisory opinion reflects this development (Trivedi & Jolly 2023). By recognising that climate protection is an ‘’erga omnes’’ obligation, the Court broadened the possibilities for international accountability. It also confirmed that States remain responsible for regulating activities of private actors, since the duty to prevent significant harm includes controlling emissions within their jurisdiction.
Looking Ahead
The advisory opinion does not introduce a new enforcement system, but it clarifies the legal expectations that States face. It shows that the main climate treaties form a unified framework, limits the discretion States can claim when drafting their national climate plans, and confirms that climate protection is a responsibility owed to the international community. This helps bring legal structure to debates that often appear political.
These developments come at the same time as COP 30 in Belém, where negotiators struggled to reach agreement on finance, trade measures, and pathways away from fossil fuels. The final Mutirão decision did not include a roadmap for transitioning away from fossil fuels or commitments on subsidy reform, although countries did agree to triple adaptation finance by 2035 and launched a two-year work programme on climate finance (International Institute for Sustainable Development 2025). These mixed results show why the advisory opinion matters. When political negotiations stall, the Court’s interpretation of treaty obligations provides a stable reference point for assessing State conduct.
References:
· Bodansky, D. (2010). The Art and Craft of International Environmental Law. Harvard University Press.https://www.academia.edu/10086962/The_Art_and_Craft_of_International_Environmental_Law
· Intergovernmental Panel on Climate Change (IPCC). (2021). Sixth Assessment Report.
https://www.ipcc.ch/assessment-report/ar6/
· International Court of Justice. (1949). Corfu Channel (United Kingdom v. Albania), Judgment of 9 April 1949.
https://www.icj-cij.org/case/1
· International Court of Justice. (2010). Pulp Mills on the River Uruguay (Argentina v. Uruguay), Judgment of 20 April 2010.
https://www.icj-cij.org/case/135
· International Institute for Sustainable Development. (2025, November 22). COP 30 Outcome: What it means and what’s next. IISD. https://www.iisd.org/articles/insight/cop-30-outcome-what-it-means-and-whats-next
· Paris Agreement, opened for signature 22 April 2016, entered into force 4 November 2016.
Full text (UNFCCC): https://unfccc.int/sites/default/files/english_paris_agreement.pdf
· Rajamani, Lavanya & Brunnée, Jutta. (2017). The Legality of Downgrading Nationally Determined Contributions under the Paris Agreement: Lessons from the US Disengagement. Journal of Environmental Law, 29(3), 537–551. https://doi.org/10.1093/jel/eqx024
· Trivedi, A., & Jolly, S. (2023). Construing Climate Change Adaptation as Global Public Good Under International Law: Problems and Prospects. Liverpool Law Review, 44, 37–62. https://doi.org/10.1007/s10991-022-09317-3
· United Nations. (1969). Vienna Convention on the Law of Treaties, adopted 23 May 1969, entered into force 27 January 1980.
Full text (UN): https://legal.un.org/ilc/texts/instruments/english/conventions/1_1_1969.pdf