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 printed an in depth evaluation by Estelle Dehon KC of the International Court of Justice (ICJ) Advisory Opinion (AO), obtainable right here (behind paywall). The article covers:

• The exceptional genesis of the Advisory Opinion with the initiative of the Pacific Island Students Fighting Climate Change
• Summary of primary findings
• Why the AO is critical for local weather justice
• What will occur subsequent and certain timescales
• Domestic authorized and political repercussions
• Corporate and monetary implications

There is way more to be stated in regards to the Advisory Opinion. In this text, Estelle covers one of many ICJ’s key determinations – the 1.5°C temperature purpose – and what it means for states’ authorized obligations regarding local weather change.

But First, Why COPs do Matter

In two other ways, The ICJ’s AO solutions a query about what worth, if any, the Conferences of the Parties (COPs) underneath the UN Framework Convention on Climate Change, nonetheless maintain, provided that this yr would be the 30^th COP, however international greenhouse fuel (GHG) emissions proceed to rise, as does attendance on the COPs by fossil gas pursuits.

First, from a political perspective, the Pacific Island Students used the COP course of to wonderful impact. Having secured the political backing of Vanuatu for making the request for the AO by way of the UN General Assembly (UNGA), the Pacific Island Students reached out to younger folks the world over, organising “fronts” in lots of nations, usually nations that felt unvoiced on local weather change. They acquired concerned within the COP course of early on, because it allowed them to fulfill inside these two weeks with all the state events from whom they wanted to safe assist for the UNGA vote to set off the ICJ AO course of. They have been clearly very efficient advocates. They wanted 94 states, however the vote was “bigger than we thought it was going to be”: greater than 130 supported the decision. A flavour of their advocacy will be gleaned from their ICJ AO Delivery Toolkit, obtainable here.

Second, from a authorized perspective, the important thing selections arising from the COPs – such because the Glasgow Climate Pact on the COP26 – quantity to “subsequent agreements” by the events underneath the Vienna Convention on the Law of Treaties relating to the interpretation of the Paris Agreement. The Glasgow Climate Pact recorded the events’ settlement 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 resolution 1/CMA.3, Glasgow Climate Pact, 13 November 2021). Then, on the First Global Stocktake, this resolve was repeated and all events inspired 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”(resolution 1/CMA.5, 13 December 2023).

As a consequence, the Court got here to one of many 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 or not there’s a disconnect between the Court and the local weather ‘reality’ and whether or not it was strategically wise to make the 1.5°C threshold the first temperature purpose underneath the Paris Agreement. In January and February this yr, there have been a swathe of experiences as numerous scientific our bodies reported that, for the primary time, the 1.5°C threshold had been breached for a full yr. Since then, additional climate data have been damaged and there may be each indicating the warming pattern will proceed. So does it make sense for the Court to discover a authorized obligation on all states, throughout a really giant variety of worldwide treaties, to realize a temperature purpose which already appears out of attain?

The reply is emphatically sure, for numerous causes.

First, the Paris Agreement temperature threshold is a couple of long run temperature improve. That requires measurements to indicate the typical improve over a interval of 20 – 30 years. Europe’s Copernicus commentary company stated in July 2024 that if warming continued on the tempo then recorded, they anticipated the 1.5°C long-term threshold to be breached within the early 2030s roughly.

The significance of the edge being a long run improve shouldn’t be a sophistry about measurement. It signifies that fast motion should still be capable of avert the breach of the long-term threshold, even when there are a selection of years which are hotter than 1.5°C. In impact, it makes these years into necessary indicators of the necessity to scale up motion (canaries within the coalmine) reasonably than excuses for scaling again motion due to exceedance of the edge.

The ICJ’s strategy within the AO signifies that this has direct implications for the lawfulness of states’ actions. This flows from the Court’s reasoning on obligations of consequence and obligations of conduct, which may be very fascinating: the Court emphasised that each forms of obligation might lead to duty of a State for a breach of the related obligation, to a sure extent collapsing a strict distinction between the 2:

“[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 duty in relation to the actions of personal 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 might require states to take regulatory or different motion in relation to probably the most GHG-emitting firms and company entities.

Accordingly, a yr (or years) of exceedance of the 1.5°C temperature threshold raises the bar for what might be required within the insurance policies, regulatory resolution and actions of states, each in relation to the duty of final result in assembly the temperature purpose and in relation to any linked obligations of conduct (such because the preparation of Nationally Determined Contributions). This very strongly connects the science and the authorized obligations that animate the necessities of the Paris Agreement.

Second, even when there have been to be “overshoot” of the 1.5°C temperature threshold, fast reductions in emissions, mixed with GHG removals, might scale back warming under the edge once more. Scientific research by the Institute for Applied Systems Analysis and the Potsdam Institute for Climate Impact Research, together with Imperial College London researchers, reveals that the extra swiftly such a reversal will be achieved, the extra probably harmful tipping dangers will be restricted.

Third, a authorized goal that has been missed nonetheless stays necessary and nonetheless retains authorized pressure, significantly the place each fraction of a level of overshoot issues, as a result of the harms scale with each fraction. Every tonne of carbon counts much more so when it’s contributing to fractions of a level of warming overshooting 1.5. In this regard,

If the long run 1.5°C temperature threshold have been breached, there would stay obligations underneath Article 2 to carry any temperature rise as near 1.5°C as attainable (which is what holding the temperature rise “well below 2°C” means, in mild of the ICJ’s AO), which might have an effect on the extent and timing of the steps states can be required to take.

Finally, it impacts cures and brings into play one other of the headline-grabbing components of the AO: the chance that states might must pay compensation. While the Court didn’t go so far as some had hoped in relation to setting out the authorized penalties of breaching local weather change obligations – the Court stored the dialogue fairly high-level, reasonably than giving element of what the assorted cures might imply in particular phrases – the Court did tackle cures such because the ideas of cessation and mitigation (ie any ongoing violation must cease and rapid steps taken to mitigate the hurt). It additionally addressed the duty of restitution (for instance, rebuilding destroyed infrastructure), however recognised that any such restitution will not be attainable in sure circumstances, so compensation could also be required as a substitute. For probably the most polluting developed nations, failure to take the requisite steps to scale back emissions and render them most culpable for any breach of the 1.5°C temperature threshold, which might give rise to a brand new authorized threat of doubtless having to pay compensation to states harmed by the damaging impacts of such a modified local weather.

Conclusion

The ICJ’s Advisory Opinion on local weather change is a landmark within the evolution of worldwide legislation, which has the capability to reshape authorized argument, political negotiation and company technique. While the exact impacts might be multitudinous, one level is evident: the period of local weather accountability has arrived.