Landmark international legal opinion means countries must act decisively on climate change

In 2019, a group of students from the University of the South Pacific in Fiji were given a brief to undertake a piece of climate advocacy. As students from one of the most climate-vulnerable regions on the earth, they understood the task as a fight against the existential threat to the future of their communities, homes and cultures. 

That assignment sparked a global legal campaign which, six years later, resulted in a landmark advisory opinion from the International Court of Justice (ICJ). 

The Request

Following the campaign led by the Pacific Island students, the ICJ was asked to advise on what countries’ legal obligations are under international law to protect the planet from climate change and what the legal consequences would be if their failure to act causes serious harm.  

The questions put to the ICJ formed part of a broader international effort seeking to clarify the legal duties of countries and businesses in addressing the climate emergency: 

• On 3 July 2025, an advisory opinion from the Inter-American Court Human Rights emphasised that the climate emergency creates obligations for countries to prevent, protect and remedy harms from the effects of climate change.  

• A 2024 decision by the International Tribunal for the Law of the Sea clarified that states have a duty to prevent environmental harm, whether caused by private sector activities or countries themselves. 

Building on this momentum, engagement with the ICJ proceedings was unprecedented. The hearing attracted 91 written submissions and oral interventions from 96 states, making it the largest case in the ICJ’s history.

Findings 

The ICJ’s Opinion sets out a series of legal findings that clarify how international law applies to the climate change. These include: 

1. Countries have a duty  

The ICJ’s 15 Judges were unanimous in their decision. Countries and governments are required to protect people and the environment from climate change. The ICJ recognised that climate change is “an existential problem of planetary proportions that imperils all forms of life and the very health of our planet.”  

To tackle the “urgent and existential threat” posed by climate change, the ICJ affirmed that countries must apply a high standard of care, planning and action, and must use “all means at their disposal” to prevent climate change. When a country fails to meet these standards, it may be considered to have committed an “internationally wrongful act”.  

2. Countries can be liable 

In practice, the ICJ confirmed that this means that countries could be considered liable under international law and that other countries could take legal action against them. The potential remedies for any such legal action include requiring countries to cease their harmful activities, rectify the problems they have caused, or provide compensation.  

For small island states who face imminent threats from rising sea levels and extreme weather events, this empowers them to demand change.  

3. Countries must act on fossil fuels 

Climate science was placed at the very heart of the ICJ’s opinion and the science is clear. Fossil fuels are the leading cause of greenhouse gas emissions and climate change. Any attempts to combat climate change will require countries to be “vigilant” over private companies producing fossil fuels within their territory. The ICJ said that countries must adequately regulate private actors’ activities and could be held liable for damage caused by their emissions if they failed to do so.  

In a significant development for the oil and gas industry, the ICJ identified specific climate-harming activities — such as fossil fuel production, consumption, exploration licensing, and subsidies — that could now be considered internationally wrongful acts under its landmark opinion. 

In many cases, countries that produce large amounts of fossil fuels tried to argue that the law would not be applicable to them. However, the court decided early on in the case that its remit would not be restricted to treaties that required counties to sign-up to them. Instead, the ICJ made clear that environmental duties do not only stem from treaties. It stated that they also come from customary international law (legal principles developed through consistent state practice) which bind all countries, regardless of whether they have ratified specific treaties. 

Corporate Accountability

While the ICJ focused on state duties, the force of its opinion indirectly strengthens the framework for corporate accountability. International instruments such as the UN Guiding Principles on Business and Human Rights, and regional laws such as the European Union’s Corporate Sustainability Due Diligence Directive, already require businesses to avoid harming human rights and the environment. 

It will be interesting to see whether these duties will be interpreted more strictly in light of the ICJ’s clear assertion of a high level of due diligence needed to tackle climate issues and what this might look like in practice. It could also encourage the UK to re-open the question of stronger domestic due diligence. 

Cases against the UK Government 

In March 2025, Leigh Day represented Oceana in a High Court challenge against the UK Government concerning its issuing of 28 oil and gas exploration licences. Oceana argued that the government failed to adequately consider the exacerbation of the climate crisis resulting from greenhouse gas emissions associated with the combustion of extracted fossil fuels.  

In another case, Leigh Day acts for Friends of the Earth in a legal challenge regarding the UK Government’s national climate adaptation plans. It is alleged that the government breached its statutory obligations under the Climate Change Act 2008 by failing to extend necessary civil protections to vulnerable groups, including elderly individuals and those residing in geographically at-risk areas.  

The ICJ’s advisory opinion is expected to play a significant role in future cases of this nature, especially where the central issue is whether the UK government has properly considered its legal obligations prior to implementing policies or authorising activities that may pose a threat to the climate. 

Conclusion 

The ICJ opinion makes clear that tackling climate change will demand real change from governments, businesses and individuals. It sets the strongest legal framework yet for holding states to account and could shape policy, empower communities and support legal challenges against those causing harm. 

It also proves that a small group of students from climate-threatened islands can reshape international law at the highest level.