On 23 July 2025, in a decision that’s already being called historic, the International Court of Justice (ICJ) issued its long-awaited advisory opinion on states’ obligations in relation to climate change. The key takeaway being states can’t sit on their hands while the planet burns. Failing to act on climate change can now amount to an internationally wrongful act, with real legal consequences.

Let’s be clear, what recently took place at the ICJ is a watershed moment, not just for international environmental law, but for all of us working or aspiring to work in the legal field. Whether you're headed into practice areas such as climate litigation, public international law, corporate, environmental, social and governance (ESG) or just curious about how courts are shaping the future, this new development changes the game.

Let’s unpack what this all means, not just in terms of black letter law, but in terms of where the legal profession is headed.

From the Pacific Islands to the Hague – how we got here

This opinion didn’t come out of nowhere. It’s the result of years of grassroots effort led by students from the University of the South Pacific, backed by Pacific Island nations and climate-vulnerable communities. Their ask is for the United Nations General Assembly to request an ICJ advisory opinion clarifying:

• the legal duties of states to act on climate change; and
• what happens when they don’t.

Fast forward two years (with more than 90 states and organisations weighing in), we now have the court’s answer. Although it's not technically binding yet, it's bold, it's clear and it's legally powerful.

The core message – climate inaction is a legal breach

The bottom line is that the ICJ has confirmed that international law already obliges states to take strong, science-based action to mitigate and adapt to climate change.

This isn’t about vague aspirations or political promises, but about legal obligations grounded in:

• climate treaties (UNFCCC, Kyoto Protocol, Paris Agreement);
• customary international law; and
• international human rights law.

What’s more, the ICJ rejected the idea that climate law begins and ends with treaties. States also have duties under broader legal frameworks, especially the customary law principles of preventing significant environmental harm, acting with due diligence and cooperating in good faith.

The court emphasised that these duties aren't just about emissions from a government-run power plant. They extend to regulating private actors, including corporations. That means states that allow polluting industries to run wild, or that subsidise fossil fuel extraction, can be held responsible under international law.

The 1.5°C target is now legally meaningful

One of the most significant parts of the opinion is the ICJ's treatment of the 1.5°C global warming limit from the Paris Agreement. The court didn’t just call it a goal, but also a legally binding benchmark.

In translation, this means that countries’ climate plans, also known as their Nationally Determined Contributions, must reflect their highest possible ambition and align with that 1.5°C limit. Falling short isn’t merely bad policy anymore, it could be illegal.

What counts as a wrongful act?

The ICJ made it clear that greenhouse gas emissions themselves aren't automatically illegal. However, failing to regulate emissions, or actively supporting them (for example fossil fuel subsidies, new oil and gas licenses) can breach international law.

This means countries are officially on legal thin ice if they continue to:

• approve new fossil fuel extraction projects;
• provide public financing or subsidies for fossil fuels; and
• ignore their duty to assess and prevent transboundary climate harm.

This is especially significant for large emitters and fossil fuel-producing states. The court has in effect put them on notice – any expansion of fossil fuel infrastructure equates legal risk.

What this means for you – the next generation of legal minds

If you’re a young legal professional or law student thinking about your future, here’s why this moment matters:

• Climate litigation is exploding. From national courts and international tribunals to human rights bodies, the doors are open.
• The law is no longer silent on climate. The ICJ just gave a loud, clear voice to international law’s climate obligations.
• The legal profession continues to evolve. Issues around ESG, climate risk, public interest litigation aren’t simply side topics, they’re central to modern lawyering.

Whether you’re planning to advise governments, sue polluters, draft climate laws or help companies stay compliant, the ICJ opinion is required reading.

Final thoughts – the law Is catching up with the climate crisis

What’s so powerful about this moment is that it reaffirms the law’s role in shaping our future. For too long, climate change has felt like a political problem, a scientific issue, or a corporate headache. But it’s also, and now indisputably, a legal matter.

For you, future barrister, solicitor, advocate or judge, are now part of the generation that must carry this forward. The world’s highest court has spoken. Now it’s time to turn legal principles into real-world accountability.