“An existential problem of planetary proportions”: what does the International Court of Justice’s advisory opinion on climate change mean for private practice lawyers?

By Tom Cummins, Senior Counsel, Ashurst LLP.

“Historic”.

“Groundbreaking”.

“Seismic”.

“Landmark”.

This is a selection of the epithets used to describe the “advisory opinion on the obligations of states in respect of climate change” issued by the International Court of Justice, the principal judicial organ of the United Nations, in July 2025.  The ICJ was responding to an April 2023 resolution of the General Assembly of the United Nations requesting it to weigh in on how climate change impacts international law.   

The opinion produced rare excitement in public international lawyers and climate activists, but to what extent has it registered with private practice lawyers in the UK?  What does the opinion really mean to them?

On 22 October 2025 I was part of a panel at Ashurst discussing this question.  This blog summarises a few points from the event.

First, what does the advisory opinion actually say?  In 140 pages of carefully reasoned, citation-heavy analysis, the 15 judges of the ICJ concluded that states have wide ranging obligations under international law to protect the climate system and environment from anthropogenic greenhouse gas (GHG) emissions  for present and future generations.  They further concluded that states may face a panoply of legal consequences for causing harm to the climate system.

Is this a big deal?  The opinion is advisory, not binding on any state, or other actor.  It is concerned with the obligations of states, not the private actors (e.g. companies or banks) who drive much of the world’s economic activity.  It was produced at a time when the standing of international law, and its notions of mutual rights and obligations between states, may be at its lowest ebb since the Second World War.

And yet,  the opinion is unanimous – reflecting shared conclusions of 15 judges from a range of cultures and legal systems.  It is thorough.  It endorses climate science, and the reports of the Intergovernmental Panel on Climate Change as the “best available science on the causes, nature and consequences of climate change”.  It reflects an acceptance that courts can deal with difficult issues of climate law.  They do not raise issues that are legally or technically insurmountable.  And the opinion draws together the strands from a wide examination of sources of law to root states’ climate obligations in international human rights, environmental, and customary, international law.

It is, then, from a legal perspective, a big deal.

But what does it mean for private practice lawyers specifically?  Private practice lawyers advise across a broad range of disciplines and sectors.  My background is in corporate and business law, and so my perspective is informed by that lens.

What do private practice lawyers do?  They advise and counsel their clients through the legal landscape, identifying and managing risks, and helping clients maximise opportunities.  The risks and opportunities that arise from the advisory opinion will depend on what the opinion causes others to do.  The opinion is not self-executing.  The question is what, if any changes, will follow from it in the form of actions by governments, courts or other actors.  How will it manifest itself in domestic laws and regulation?

The implications of the opinion may be various.  The judges’ conclusion that the aspiration in the 2015 Paris Agreement to limit temperature increases to 1.5 degrees above pre-industrial levels had hardened into the parties’ primary temperature goal may influence states’ nationally determined contributions (NDCs), and conduct at conferences of the parties under the United Nations Framework Convention on Climate Change (COPs).  The opinion’s discussion of liability for states who fail to regulate the quantity of emissions caused by private actors in their jurisdiction may drive increased regulation, covering aspects such as disclosure, transition plans, financing, subsidies, tax, and licensing of projects.

Claimants in public law litigation, such as judicial review in the UK, or constitutional challenges in other jurisdictions, are likely to draw on the opinion to support claims that governments have acted unlawfully.  The same is true of private law litigation, with challenges to the emitting activities of companies and claims for compensation for emissions likely to cite the opinion.  The world of investment arbitration – where foreign investors seek compensation from host states – is likely also to be affected by the opinion, perhaps in the form of states relying on the opinion as a defence to compensation claims for measures mandating reduced emissions.

It is not hard to see how this landscape will inform the day-to-day work of private practice lawyers.  The projects lawyer advising on a new development may need to take into account new regulation aimed at curbing GHG emissions, or related case law.  The M&A or finance lawyer advising on a transaction may need to consider the risks of climate change or related legal developments on the protections sought in contractual documents.  Disputes lawyers (like me) will need to think about how the underlying public law framework reflected in the opinion may flow through into decisions by courts or arbitral tribunals.

Lawyers in England and Wales will also do well to look up the Law Society’s April 2023 guide to the  “Impact of climate change on solicitors”, in the light of the opinion.  That document provides a short overview of how climate issues may impact solicitors’ practices and duties.  The ICJ’s examination of the intersection between climate change and law is a valuable complement to that document.

The ICJ’s opinion concludes with a resonant section which points to both the limitations of the law, and its opportunity.  It is worth quoting in some detail:

The questions posed by the General Assembly represent more than a legal problem: they concern an existential problem of planetary proportions that imperils all forms of life and the very health of our planet. International law, whose authority has been invoked by the General Assembly, has an important but ultimately limited role in resolving this problem. A complete solution to this daunting, and self-inflicted, problem requires the contribution of all fields of human knowledge, whether law, science, economics or any other. Above all, a lasting and satisfactory solution requires human will and wisdom — at the individual, social and political levels — to change our habits, comforts and current way of life in order to secure a future for ourselves and those who are yet to come. Through this Opinion, the Court participates in the activities of the United Nations and the international community represented in that body, with the hope that its conclusions will allow the law to inform and guide social and political action to address the ongoing climate crisis.

The “solution” to the climate crisis – the global energy transition, and climate adaptation – requires bankable projects, effective risk allocation, compliant structures, enforceable contracts, and efficient dispute resolution.

None of this can happen without the involvement of the law and the in-house, and private practice, lawyers who practise it.