The 1.5°C Temperature Target as a Means to an End: Key Implications of the ICJ’s Interpretation of the Paris Agreement’s Overall Objective

Introduction

In its 2025 Advisory Opinion on Obligations of States in Respect of Climate Change (AO), the International Court of Justice (ICJ) gave its imprimatur to the aim of limiting global temperature rise to 1.5°C above pre-industrial levels as the current ‘consensus target’ of the Paris Agreement (para 224), consciously abandoning the target of ‘well below 2°C’. This is in line with similar conclusions drawn by the European Court of Human Rights, the International Tribunal on the Law of the Sea and the Inter-American Court of Human Rights.

The Court’s endorsement of the 1.5°C target is significant. The Paris temperature target plays a crucial role in the interpretation of all provisions of the Paris Agreement and, in line with the interpretative principle of systemic integration employed by the ICJ, it also informs the conduct required of states in fulfilling their other obligations under international law. This aspect of the Court’s Opinion has rightly attracted the attention of various commentators (here, here and here).

We wish to draw attention to one aspect of the Court’s Opinion that has not yet been highlighted: the Court’s emphasis, in paragraph 223, that the 1.5°C temperature target represents a concretization of the overall objective of the United Nations Framework Convention on Climate Change (UNFCCC), which, pursuant to Article 2, is to stabilize concentrations of greenhouse gases (GHGs) in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. It is this latter objective that constitutes the object and purpose of the climate change treaty regime as a whole, which encompasses the Paris Agreement as well as the UNFCCC and the 1997 Kyoto Protocol (paras 187-195, AO).

In what follows, we first succinctly address the Court’s findings on this point and consider how the Court’s approach aligns with scientific understanding of the 1.5 °C target, before discussing a number of key legal implications that follow from viewing the 1.5°C target not as the ultimate objective of legally mandated climate action under international law, but rather as a means to an end.

The Court’s findings on the object and purpose of the climate change treaty framework

In paragraph 197 of the AO, the Court emphasizes that the ‘ultimate objective’ of the UNFCCC, as set out in Article 2, is to achieve ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system’. This places mitigation at the centre of the climate change regime.

The Paris Agreement, in turn, explicitly aims to enhance the implementation of the UNFCCC, ‘including its objective’, and ‘to strengthen the global response to the threat of climate change’ (Paris Agreement, Article 2). This recursive relationship is confirmed in the preamble: the Agreement was concluded ‘[i]n pursuit of the objective of the Convention, [and is] guided by its principles.’ From this vantage point, the ICJ finds that:

[T]he temperature goal in the Paris Agreement is intended to promote the overall object and purpose of the climate change treaty framework, namely “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system” (UNFCCC, Article 2). The Court considers that the Framework Convention’s overall objective constitutes the object and purpose of the Paris Agreement, with the temperature goal providing a means for achieving this object and purpose. (para 225)

The Court makes a similar finding in relation to the aim, formulated in Article 4(1) Paris Agreement, of reaching a global peak of GHG emissions ‘as soon as possible’ which it also construes as being in furtherance of the UNFCCC’s ‘ultimate objective’ of stabilization of GHG concentrations in the atmosphere so as to prevent dangerous anthropogenic interference with the climate system (para 194).

The priority to be given to emissions reduction in states’ fulfilment of their legal obligations is clearly reflected in the text of the climate treaties. Under the UNFCCC, states have an obligation to adopt national policies and take corresponding measures that can achieve “the mitigation of climate change, by limiting its anthropogenic emissions of greenhouse gases and protecting and enhancing its greenhouse gas sinks and reservoirs” (UNFCCC, Article 4(2)(a)). In this respect, the ICJ’s AO clarifies that the commitments of the parties under the Paris Agreement are not merely procedural, but purposive. Specifically, states’ commitments in their nationally determined contributions (NDCs) as well as their national measures of implementation must ‘be capable of making an adequate contribution’ to achieving the temperature goal and limiting GHG emissions (AO, para 242).

The 1.5°C target and science

Framing the 1.5°C temperature target as a means to an end—rather than an end in and of itself—aligns with the scientific understanding of the 1.5°C temperature target. In climate science, the relationship between global mean temperature and human-caused climate impacts is undisputed. Global mean temperatures have been well observed for over a century, and the rise in mean temperature since the Industrial Revolution is approximately linearly related to the totality of anthropogenic CO2 emissions, mostly caused by the burning of fossil fuels. This constitutes unequivocal evidence of a direct link between global temperature and human activity.

Considering that global mean temperature is clearly related to the ultimate goal to be achieved through the climate change treaty framework—namely, the prevention of dangerous anthropogenic interference with the climate system—a 1.5°C temperature target for the rise of global temperatures constitutes, without a doubt, a meaningful target in the pursuit of this objective. Still, a temperature target does not capture all climate impacts resulting from GHG emissions, such as ocean acidification, or rates of change. For this reason scientists have pointed to an important limitation of global temperature targets: they may mask the link between GHG emissions and climate impacts and, thereby, fail to effectively communicate the urgency of reducing CO2 emissions.

What is more, various scientific publications outline that there exist other possible targets besides the 1.5°C temperature target that could, in theory, be pursued to achieve the ultimate objective of the climate change treaty regime. Examples include the avoidance of tipping points such as loss of the Greenland ice sheet, or the imposition of specific limits on GHG emissions or on the rate and velocity of temperature change. This underlines that the 1.5°C temperature target can be used as a proxy for harm to the climate system, but should not be viewed as a substitute for the aim of limiting anthropogenic GHG emissions so as to prevent dangerous climate change.

Key implications for the obligations arising under the climate change treaty framework

The object and purpose of a treaty plays a key role in the interpretation of the obligations it imposes. According to Article 31 of the Vienna Convention on the Law of Treaties, and as highlighted in the AO, provisions in the UNFCCC and the Paris Agreement ‘are to be interpreted and applied’ in light of their ultimate objective: the stabilization of greenhouse gas concentrations in order to prevent dangerous interference with the climate system (AO, para 197). In what follows, we highlight several key implications that may follow from this framing of the object and purpose of the Paris Agreement, with the 1.5°C temperature target functioning as a means to achieve the climate change treaty regime’s ultimate objective.

1. Limiting GHG emissions in order to prevent dangerous interference with the climate system is the common goal to be collectively pursued by states

Stabilization of GHG concentrations in the atmosphere with the aim of preventing harm to the climate system is the ultimate common goal shared by all parties to the UNFCCC and the Paris Agreement. This goal can only be achieved collectively, and parties have an obligation to co-operate in fulfilling their treaty obligations. As the Court indicates, ‘co-operation is indispensable in the field of climate change’ and the customary duty to co-operate in the protection of the environment is reflected in several provisions of the climate change treaties (para 215).

This common goal informs the conduct that is required from each state party. In this respect, the Court notes that states have a due diligence obligation to ensure that their national mitigation measures, ‘when taken together’, are capable of achieving the 1.5°C temperature target as well as ‘the overall objective’ of stabilization of greenhouse gas levels (para 245). Such reasoning illustrates the close connection between the notions of obligations of conduct and result (see AO, para 207), and that the distinction between them is ‘not necessarily a strict one’, as the collective result that is to be pursued by states informs the content of each state’s obligation of conduct.

2. Legal limitations to the use of geoengineering to meet mitigation obligations

Since the object and purpose of the Paris Agreement must inform the type of action that states are required to take while performing their treaty obligations, achieving the temperature target is not, in itself, a sufficient indicator that states are complying with their mitigation obligations. If the 1.5°C target is to be understood as a ‘means’ to achieving the goal of limiting GHG emissions to prevent dangerous anthropogenic interference with the climate system (AO, para 225), this target can only continue to guide states’ action for as long as it still contributes to that goal.

This finding is in line with scientific studies, which underline that warming thresholds can be used as proxies for harm to the climate and ecosystems, but emphasize that the two should not be equated. In fact, some climate impacts are directly caused by increasing concentrations of GHG emissions in the atmosphere, even without considering any particular level of warming, such as ocean acidification.

Importantly, this means that the adoption by states of national measures and strategies that pursue holding global temperatures below 1.5°C might not satisfactorily fulfil their obligations under the Paris Agreement if, in addition, they do not also seek to reduce their GHG emissions. An example of such proposed measures would be the deployment of solar radiation modification or solar geoengineering, a technology that is increasingly being researched and tested and has attracted much public and political interest, particularly as climate impacts are becoming more severe and frequent and mitigation action is lagging behind. While this technology could be used to temporarily reduce global temperatures and prevent certain effects of global warming, it fails to address the main driver of climate change, namely, anthropogenic GHG emissions. In addition, the risks of solar geoengineering for human and natural systems are well documented, including in a recent independent expert report to the European Commission published in the context of the Scientific Advice Mechanism. Such risks include changes in precipitation and temperature patterns, changes in land and ocean ecosystems, the hampering of food production, and threat of ‘termination shock’ (if technologies suddenly fail or stop being deployed)—which could amplify, rather than restrict, harm to the climate system that states are obliged to prevent. Such risks indicate that reliance on geoengineering as an alternative to reducing GHG emissions is not in line with parties’ mitigation obligations under the climate treaties, when these are interpreted in line with their overall object and purpose.

3. The imperative of phasing out fossil fuel

Taking the goal of limiting GHG emissions seriously also means that states may be in breach of their Paris Agreement obligations if they fail to take sufficient steps to phase out fossil fuels, which includes regulating the fossil fuel industry and limiting its emissions.

As the Court warned,

[f]ailure of a State to take appropriate action to protect the climate system from GHG emissions — including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies — may constitute an internationally wrongful act which is attributable to that State. (para 427)

It is well-known that fossil fuels are the main cause of climate change. According to the IPCC AR6 Synthesis Report (at page 20), CO2 emissions from existing fossil fuel infrastructures already exceed the remaining carbon budget that would allow for even a 50% chance of meeting the 1.5°C temperature target—which, it should be said, is the minimum to allow us to strive towards the objective of preventing dangerous anthropogenic interference with the climate system. Considering this scientific reality, the urgent phasing out of fossil fuels should constitute the main pillar of effective harm prevention measures, and it should be clear that permitting the exploitation of additional fossil fuel sources or increasing fossil fuel consumption is manifestly incompatible with the object and purpose of the climate change treaty framework.

4. Continuing obligations when the 1.5°C temperature goal is exceeded

Global mean temperature has already increased by around 1.4°C since pre-industrial times. With present emission pathways, and even if we assume that all states fully implement their emissions reduction targets, it is more likely than not that the 1.5°C warming limit will be reached and exceeded imminently. Recent studies indicate that a time where this threshold will be surpassed is fast approaching (what is usually known as ‘overshoot’).

The emphasis by the Court on the ultimate objective of limiting GHG emissions and preventing harm to the climate system helps to clarify that, if (or when) this ‘overshoot’ scenario materializes, states’ obligations under the Paris Agreement will not cease to exist, nor will its objectives become futile. To the contrary, states will continue to be bound by a due diligence obligation to prevent, through a rapid and significant reduction of their GHG emissions, additional harm to the climate system. This is a ‘stringent’ obligation, as the Court indicates (AO, para 254), on account of the severity of the associated risks and projected impacts of climate change increasing with every increment of global warming.  Accordingly, states bear the obligation to limit the magnitude and length of any overshoot, aiming to get back to a 1.5°C world as soon as possible—or, if that is no longer possible, to continue preventing all additional harmful effects of anthropogenic climate change.

Conclusion and outlook: implications beyond the climate change treaty framework

The ICJ’s 2025 Advisory Opinion marks a pivotal development in international climate change law. The Court has firmly identified 1.5°C as the consensus temperature target under the Paris Agreement but, instead of elevating it as an isolated benchmark of the treaty regime, it clarified that the 1.5°C target is not an end in itself but a tool to guide states toward their primary objective—the reduction of GHG emissions.

This framing of the 1.5°C target by the Court as a means to an end has important implications for the obligations of states parties to the Paris Agreement. It makes stabilizing GHG concentrations in the atmosphere at a level to prevent dangerous anthropogenic interference with the climate system the common goal to be collectively pursued by parties. It has placed legal constraints on the use of certain technologies such as geoengineering to lower global temperature if no measures are taken to also limit GHG emissions. It has underscored the legal urgency of phasing out fossil fuels. And it has clarified that states’ obligations persist—if anything, more stringently than ever—in a scenario of overshoot.

Such implications extend well beyond the climate change treaty framework. In its Opinion, the Court finds that the concentration of atmospheric GHG at dangerous levels is the primary cause of ‘harm to the climate system’ which is, in itself, a form of environmental harm (AO, para 278). Consequently, the objective of stabilizing GHG emissions in order to prevent dangerous anthropogenic interference with the climate system also informs the obligations of states that are not party to the climate change treaties, but are bound to the customary law duty not to cause significant transboundary environmental harm. This duty is applicable to every individual state, regardless of the fact that the risk of harm associated with climate change is ‘a consequence of a combination of activities by different States’ (AO, para 277). In fact, the circumstance that anthropogenic climate change is caused by the cumulative effects of actions undertaken by all states (and private actors) adds an additional layer to states’ obligations, requiring them to co-operate with one another in the fulfilment of their obligations. This finding further highlights the absolute imperative that all states develop a coordinated and co-operative response that can avert the mounting risks for humans and the planet by effectively limiting concentrations of GHG emissions in the atmosphere.