How is the World Court Controlling the Future of Emissions?

With the Paris Agreement aim of a 1.5-2°C reduction in carbon emissions by 2030 soon approaching, countries, businesses and industries are reassessing their impact on the environment and climate.

In a historic decision, the International Court of Justice (ICJ) has delivered a powerful Advisory Opinion that confirms states’ legal obligations under international law to prevent and mitigate climate change. 

Welcomed by the International Union for Conservation of Nature (IUCN), the ruling is being hailed as a major step forward in the global fight to limit warming to 1.5°C.

A milestone for climate law

The ICJ, also known as the World Court, responded to a request by the UN General Assembly seeking clarification on the legal duties of states regarding climate change and the consequences for failing those duties. 

Following extensive hearings with participation from 96 states and 11 international organisations, the ICJ unanimously confirmed that all states must act with a high level of due diligence to avoid causing environmental harm, particularly in preventing GHG emissions that endanger current and future generations.

IUCN’s role and reaction

The IUCN played a central role throughout the proceedings, representing both state and non-state members and offering its legal and scientific expertise via the World Commission on Environmental Law (WCEL).

“IUCN welcomes this important Advisory Opinion, which strengthens the global legal framework and reinforces the urgent imperative to limit warming to 1.5 °C,” says Dr Grethel Aguilar, IUCN Director General.

“IUCN has been actively engaged at every stage of these historical proceedings and congratulates the ICJ on this key decision that will shape the future of our planet.  

“Climate change is not only a crisis of rising temperatures; it is a crisis for both humanity and nature with profound implications for human rights.  

“By undermining ecosystems, climate change weakens the life-support systems upon which people’s rights to life, health, food, water and culture depend. 

“As the global authority on the state of nature, IUCN urges all States to strengthen their commitments, recognising that protecting nature is not just part of the solution, it is essential to delivering climate justice and safeguarding our collective future." 

Legal clarity for the environment

The ICJ’s Advisory Opinion confirms that states are bound not only by climate treaties such as the Paris Agreement but also by customary international law, human rights law, the law of the sea and the duty to prevent transboundary harm.

“The highest judicial body of the UN has spoken and it has spoken clearly: every State has an obligation under international law to act with a stringent level of due diligence to prevent global warming from exceeding the 1.5°C threshold,” explains Prof Christina Voigt, Lead Counsel and WCEL Chair. 

“This legal duty stems not only from the Paris Agreement, but also from human rights law, the law of the sea and the customary duty to prevent transboundary harm. 

“The ICJ’s clarification, which fully aligns with IUCN’s arguments, is crucial for the coherence of international law, which now offers a full canvas of parallel obligations, all pointing in the same direction: to protect the climate system on the basis of stringent due diligence."

Legal consequences for climate inaction

The Court's Opinion outlines that any breach of the identified legal obligations constitutes an internationally wrongful act. In such cases, responsible states are required to:

• Cease the wrongful conduct
• Provide guarantees against repetition
• Make full reparation, including compensation, where a clear causal link to harm can be shown.

Importantly, the Court clarified that responsibility is not limited by geographical or development status, meaning that all injured states, including small island developing states, have equal entitlement to remedy. 

The Court also affirmed the scientific possibility of identifying each state's contribution to global emissions, reinforcing accountability across jurisdictions.

“Today marks the end of a long journey. WCEL has worked tirelessly for more than two years on these historic proceedings. We were present at every step,” comments Prof Francesco Sindico, Co-Chair of the WCEL Climate Change Law Specialist Group, who coordinated the WCEL team.

“We are pleased to see that all our key points are reflected in the Court’s Opinion. We welcome the Court’s articulation of a clear legal path for States, one grounded in the Paris Agreement, other international treaties and customary international law. 

“This path demands a high level of due diligence and close attention to the Paris temperature goal.

“WCEL will now turn its focus to the potential impact of these three Advisory Opinions on climate multilateralism and nature protection.” 

Human rights and nature

The Opinion underscores a human rights-based approach to climate governance. 

IUCN has committed to carrying this forward at key events, including the IUCN World Conservation Congress in October 2025 and the UN Climate Change Conference (COP30) in Belem, Brazil.

Through its participation in the ICJ proceedings, as well as recent advisory opinions from the Inter-American Court of Human Rights and the International Tribunal for the Law of the Sea, IUCN has reinforced its position as a leading voice in international climate and environmental law.