> Bigger than we thought it was going to be”: the ICJ’s Advisory Opinion and the 1.5°C Temperature Obligation

Bigger than we thought it was going to be”: the ICJ’s Advisory Opinion and the 1.5°C Temperature Obligation

09 Sep 2025

Cornerstone Climate, Planning and Environment

LexisNexis has published a detailed analysis by Estelle Dehon KC of the International Court of Justice (ICJ) Advisory Opinion (AO), available here (behind paywall). The article covers:

The remarkable genesis of the Advisory Opinion with the initiative of the Pacific Island Students Fighting Climate Change
Summary of main findings
Why the AO is significant for climate justice
What will happen next and likely timescales
Domestic legal and political repercussions
Corporate and financial implications

There is much more to be said about the Advisory Opinion. In this article, Estelle covers one of the ICJ’s key determinations – the 1.5°C temperature goal – and what it means for states’ legal obligations concerning climate change.

But First, Why COPs do Matter

In two different ways, The ICJ’s AO answers a question about what value, if any, the Conferences of the Parties (COPs) under the UN Framework Convention on Climate Change, still hold, given that this year will be the 30^th COP, but global greenhouse gas (GHG) emissions continue to rise, as does attendance at the COPs by fossil fuel interests.

First, from a political perspective, the Pacific Island Students used the COP process to excellent effect. Having secured the political backing of Vanuatu for making the request for the AO via the UN General Assembly (UNGA), the Pacific Island Students reached out to young people across the world, setting up “fronts” in many countries, often countries that felt voiceless on climate change. They got involved in the COP process early on, as it allowed them to meet within those two weeks with all of the state parties from whom they needed to secure support for the UNGA vote to trigger the ICJ AO process. They were obviously very effective advocates. They needed 94 states, but the vote was “bigger than we thought it was going to be”: more than 130 supported the resolution. A flavour of their advocacy can be gleaned from their ICJ AO Delivery Toolkit, available here.

Second, from a legal perspective, the key decisions arising from the COPs – such as the Glasgow Climate Pact at the COP26 – amount to “subsequent agreements” by the parties under the Vienna Convention on the Law of Treaties regarding the interpretation of the Paris Agreement. The Glasgow Climate Pact recorded the parties’ agreement that ““the impacts of climate change will be much lower at the temperature increase of 1.5°C compared with 2°C” and their “resolve to pursue efforts to limit the temperature increase to 1.5°C” (see decision 1/CMA.3, Glasgow Climate Pact, 13 November 2021). Then, at the First Global Stocktake, this resolve was repeated and all parties encouraged to submit “ambitious, economy-wide emission reduction targets, covering all greenhouse gases, sectors and categories and aligned with limiting global warming to 1.5°C, as informed by the latest science, in the light of different national circumstances”(decision 1/CMA.5, 13 December 2023).

As a result, the Court came to one of the standout conclusions of the AO:

“Accordingly, the Court considers the 1.5°C threshold to be the parties’ agreed primary temperature goal for limiting the global average temperature increase under the Paris Agreement. The Court adds that this interpretation is consistent with Article 4, paragraph 1, of the Paris Agreement, which requires that mitigation measures be based on the ‘best available science’.” [§224]

How Meaningful is the 1.5°C Threshold?

Some have questioned whether there is a disconnect between the Court and the climate ‘reality’ and whether it was strategically sensible to make the 1.5°C threshold the primary temperature goal under the Paris Agreement. In January and February this year, there were a swathe of reports as various scientific bodies reported that, for the first time, the 1.5°C threshold had been breached for a full year. Since then, further weather records have been broken and there is every indicating the warming trend will continue. So does it make sense for the Court to find a legal obligation on all states, across a very large number of international treaties, to achieve a temperature goal which already seems out of reach?

The answer is emphatically yes, for a number of reasons.

First, the Paris Agreement temperature threshold is about a long term temperature increase. That requires measurements to show the average increase over a period of 20 – 30 years. Europe’s Copernicus observation agency said in July 2024 that if warming continued at the pace then recorded, they expected the 1.5°C long-term threshold to be breached in the early 2030s approximately.

The importance of the threshold being a long term increase is not a sophistry about measurement. It means that rapid action may still be able to avert the breach of the long-term threshold, even if there are a number of years that are warmer than 1.5°C. In effect, it makes those years into important indicators of the need to scale up action (canaries in the coalmine) rather than excuses for scaling back action because of exceedance of the threshold.

The ICJ’s approach in the AO means that this has direct implications for the lawfulness of states’ actions. This flows from the Court’s reasoning on obligations of result and obligations of conduct, which is very interesting: the Court emphasised that both types of obligation may result in responsibility of a State for a breach of the relevant obligation, to a certain extent collapsing a strict distinction between the two:

“[T]he Court finds it useful here to note that, in the case of an obligation of conduct, a State acts wrongfully if it fails to use all means at its disposal to bring about the objective envisaged under the obligation, but will not act wrongfully if it takes all measures at its disposal with a view to fulfilling the obligation even if the desired objective is ultimately not achieved. In the case of an obligation of result, a State acts wrongfully if it fails to bring about the result required under the obligation. At the same time, it cannot be said that an obligation of result, such as an obligation to ‘adopt national policies and take corresponding measures on the mitigation of climate change’, will be met merely by the adoption of any policies and the taking of corresponding measures. To comply with this obligation of result, the policies so adopted and the measures so taken must be such that they are able to achieve the required goal. In other words, the adoption of a policy, and the taking of related measures, as a mere formality is not sufficient to discharge the obligation of result.” [§208]

This is strengthened by the Court’s conclusions on state responsibility in relation to the activities of private actors: “[A] State may be responsible where, for example, it has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction.” [§428]. This could require states to take regulatory or other action in relation to the most GHG-emitting companies and corporate entities.

Accordingly, a year (or years) of exceedance of the 1.5°C temperature threshold raises the bar for what will be required in the policies, regulatory decision and actions of states, both in relation to the obligation of outcome in meeting the temperature goal and in relation to any linked obligations of conduct (such as the preparation of Nationally Determined Contributions). This very strongly connects the science and the legal obligations that animate the requirements of the Paris Agreement.

Second, even if there were to be “overshoot” of the 1.5°C temperature threshold, rapid reductions in emissions, combined with GHG removals, could reduce warming below the threshold again. Scientific studies by the Institute for Applied Systems Analysis and the Potsdam Institute for Climate Impact Research, including Imperial College London researchers, shows that the more swiftly such a reversal can be achieved, the more likely dangerous tipping risks can be limited.

Third, a legal target that has been missed still remains important and still retains legal force, particularly where every fraction of a degree of overshoot matters, because the harms scale with every fraction. Every tonne of carbon counts even more so when it is contributing to fractions of a degree of warming overshooting 1.5. In this regard,

If the long term 1.5°C temperature threshold were breached, there would remain obligations under Article 2 to hold any temperature rise as close to 1.5°C as possible (which is what keeping the temperature rise “well below 2°C” means, in light of the ICJ’s AO), which would affect the extent and timing of the steps states would be required to take.

Finally, it affects remedies and brings into play another of the headline-grabbing elements of the AO: the possibility that states may have to pay compensation. While the Court did not go as far as some had hoped in relation to setting out the legal consequences of breaching climate change obligations – the Court kept the discussion quite high-level, rather than giving detail of what the various remedies may mean in specific terms – the Court did address remedies such as the principles of cessation and mitigation (ie any ongoing violation would have to stop and immediate steps taken to mitigate the harm). It also addressed the obligation of restitution (for example, rebuilding destroyed infrastructure), but recognised that this type of restitution may not be possible in certain circumstances, so compensation may be required instead. For the most polluting developed nations, failure to take the requisite steps to reduce emissions and render them most culpable for any breach of the 1.5°C temperature threshold, which would give rise to a new legal risk of potentially having to pay compensation to states harmed by the dangerous impacts of such a changed climate.

Conclusion

The ICJ’s Advisory Opinion on climate change is a landmark in the evolution of international law, which has the capacity to reshape legal argument, political negotiation and corporate strategy. While the precise impacts will be multitudinous, one point is clear: the era of climate accountability has arrived.