Tom Hickman | Climate Justice
Ten years after it was adopted, the Paris Agreement remains the world’s best hope for avoiding the catastrophic consequences of human-induced climate change. The stated objective of the agreement, to which 195 states and the European Union committed, was ‘holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C’.
But the global consensus is fraying. Political parties in many countries have said they will abandon commitments to reach net zero by 2050 or otherwise reduce the ‘nationally determined contributions’ (NDCs) required by the Paris Agreement. On his first day in office, President Trump issued an executive order withdrawing the United States from the agreement for a second time.
At least three weaknesses in the Paris Agreement have facilitated back-pedalling on climate goals. First, the agreement is expressed primarily in terms of aims rather than duties. Where the drafting unambiguously lays down mandatory requirements, they are framed as procedural or formal in character. At the heart of the agreement is an obligation on parties to ‘prepare, communicate and maintain’ NDCs. But there are no express requirements for the content of NDCs. Similarly, parties are required to ‘pursue’ domestic measures ‘with the aim of achieving such contributions’: this is the language of ambition not obligation.
The second weakness is the absence of any specified legal consequences for states falling behind the Paris Agreement’s demanding objectives. The prescribed consequences for poor progress are political: a five-yearly global stocktake and the attentions of an expert compliance committee with no penal powers.
Third, countries can withdraw from the agreement, giving a year’s notice. So far, only the US has exercised that right, but it is the second biggest emitter of greenhouses gases after China. The possibility that other countries might take similar action is an ever-present risk to the system.
In 2023, the United Nations General Assembly adopted a resolution to ask the International Court of Justice (ICJ) to clarify, first, the obligations of states under international law to ensure protection of the climate system from anthropogenic emissions of greenhouse gases and, second, the legal consequences for states whose actions have caused significant harm to the environment. The resolution was tabled by Vanuatu, a nation made up of an archipelago of dozens of volcanic islands, which is consistently ranked as the country with the highest disaster risk. The campaign had been started by a class of law students at the University of the South Pacific and gained the endorsement of the Pacific Islands Forum in 2022.
Eleven international organisations and 96 states made oral submissions at a hearing in the Hague in December 2024, and hundreds of written submissions were filed. This was the highest level of participation in proceedings before the ICJ in the court’s history. The court’s unanimous opinion, delivered last July, powerfully reinforces the Paris Agreement regime.
The ambiguities concerning state obligations were largely removed. The court interpreted the agreement as containing a coherent set of complementary legal obligations directed to, and capable of attaining, the objective of ‘providing an effective response’ to the ‘urgent threat’ of climate change.
It is an established principle of treaty interpretation that subsequent agreement of the parties as to how the treaty terms should be understood must be taken into account in interpreting its intended meaning and effect. Although 1.5°C was expressed in the original agreement as an additional target, subsequent meetings have endorsed the view that it was the primary goal. The International Panel on Climate Change has concluded with a high degree of confidence that warming of 1.5°C is not safe for most nations and communities. The court was thus able to lock in the 1.5°C threshold as the objective to which other obligations under the treaty are directed.
The court then turned to the mechanism for achieving this objective: namely, setting and complying with NDCs. While recognising that the text was ‘silent’ on the content of NDCs, the court concluded that this did not mean states were bound only by good faith in setting them. The court pointed to a paragraph in the agreement which says that NDCs will be progressively demanding and reflect a state’s ‘highest possible ambition’. This is ‘not to be read as merely hortatory, as suggested by some participants … the term “will” is used here in a prescriptive sense.’ The principle of due diligence means that ‘each party has to do its utmost to ensure that the NDCs it puts forward represent its highest possible ambition in order to realise the objectives of the agreement’ – i.e. ‘to hold the increase in the global average temperature to below 1.5°C’.
The court also found implied obligations on states to comply with their NDCs. Under a stringent standard of due diligence – required because the risks ‘escalate with every increment of global warming’ – parties have an obligation to pursue domestic measures that are ‘reasonably capable of achieving the NDCs set by them’. For the UK and the European Union, this limits the scope for backsliding on net zero.
The court did not directly confront the problem of state withdrawal from the Paris Agreement, or seek to interpret the agreement in a manner that restricted that right, but it did interpret wider principles of international law to achieve the same result.
There is an established obligation on states under customary international law to use all means at their disposal to prevent activities that cause significant damage to the environment of a neighbouring state, such as by damming or polluting a river. The UK and other countries tried to argue that this principle ‘has no application’ to the harms caused by climate change, since the damage from anthropogenic greenhouse gas accumulations is caused not by individualised state action but by the aggregate consequences of varied and diffuse activities.
The ICJ rejected such arguments, holding that customary international law requires all states to act with due diligence, using all means at their disposal, to prevent activities that cause significant harm to the climate system. This includes an obligation to co-operate with each other to achieve this result. Countries that are in ‘full compliance’ with climate change treaties, including the Paris Agreement, would be able to demonstrate substantial compliance with that duty under customary international law. By contrast, any state that is not party to the Paris Agreement and does not apply equivalent measures will have the ‘full burden of demonstrating that its policies and practices are in conformity with its customary obligations’. In this way, the court closed off the possibility of states withdrawing from the Paris Agreement framework without exposing themselves to being found to be committing internationally wrongful acts.
The court also set out a path by which heavily emitting states may be found responsible to states that are harmed by climate change:
Each injured state may separately invoke the responsibility of every state which has committed an internationally wrongful act resulting in damage to the climate system and other parts of the environment.
While there may be challenges in establishing a causal link between internationally wrongful acts and particular incidents of harm, this is not impossible. The approach bears similarities with the way domestic courts have found employers liable for diseases suffered by employees who have been exposed to dangerous emissions or toxins at work even when that may not have been the only source. The courts have found employers to be liable where their wrongful acts have materially contributed to exposure. The ICJ has charted a similar solution to a similar problem. The message is clear: the complexities of climate change will not save states that fail to comply with their international obligations.
The weaknesses in the Paris Agreement were therefore substantially removed: the court clarified the obligations of states and identified stringent requirements to set and attain NDCs; it established a powerful incentive for states to remain parties to the Paris Agreement; and it explained the hazard of liability for international wrongful acts if states decelerate attempts to achieve the 1.5°C goal.
The ICJ opinion has advisory status, but it represents the highest authority on the interpretation of customary international law and on the various treaties examined by the court. It will be treated as authoritative by domestic courts and international tribunals alike. Though international law does not have direct application in the UK, there are many contexts where it is relevant to interpreting the requirements of domestic law. In the past, the courts have afforded considerable latitude to the government in interpreting the requirements of the Paris Agreement. Following the ICJ’s opinion, that latitude ought to reduce.
It is frequently pointed out that the climate crisis cannot be solved by litigation or by judges. While obviously true, the point can be overstated. One of the purposes of international law is to protect states from harm caused by other states, and governments have chosen to co-ordinate their efforts to combat climate change through treaties. Laws and the impartial adjudication of laws by courts are therefore central to the global response to climate change.
While the entire regime of international law is experiencing profound challenges, states ultimately require international law to co-ordinate their actions. Most of the international community remains committed not only to the rules-based order but to the Paris Agreement. For as long as that remains the case, the ICJ’s contribution will be substantial. It is also significant that the ICJ proceedings would never have occurred had it not been for pressure from some of the world’s smallest, most isolated and most at risk communities. The story of the ICJ’s opinion shows how international law and legal process can sometimes empower communities to have an impact beyond the possibilities of politics.