Climate Decision

The International Court of Justice has held that there is a duty to exercise due diligence to

address climate change as a matter of customary environmental law:

131. The Court turns now to consider the applicability and relevance of customarym

international law. In the context of climate change, (a) the duty to prevent significant

harm to the environment requires particular attention, as does (b) the duty to co-operate

for the protection of the environment.

(a) Duty to prevent significant harm to the environment

132. Participants generally agree that States have a duty under customary international

law to prevent significant harm to the environment. Indeed, the Court has recognized that

“[a] State is . . . obliged to use all the means at its disposal in order to avoid activities

which take place in its territory, or in any area under its jurisdiction, causing significant

damage to the environment of another State”(Pulp Mills on the River Uruguay

(Argentina v. Uruguay), Judgment, I.C.J. Reports 2010 (I), p. 56, para. 101).

133. As concerns the applicability of the duty to prevent significant harm to the

environment in the context of climate change, participants expressed two positions. Most

participants affirmed that this duty is applicable to climate change, relying, inter alia, on

the general nature of the no harm principle from which the Court derived the duty to

prevent significant harm to the environment (see Corfu Channel (United Kingdom v.

Albania), Merits, Judgment, I.C.J. Reports 1949, p. 22). Other participants argued that

this duty does not apply in the climate change context. They maintained, inter alia, that it

is confined to instances of direct cross-border harm, as addressed by

the Court in the past, and that climate change is a process which, by its cumulative and

global nature, is distinct from more specific processes resulting in transboundary harm.

134. In its Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons,

the Court recognized that

“[t]he existence of the general obligation of States to ensure that activities within

their jurisdiction and control respect the environment of other States or of areas

beyond national control is now part of the corpus of international law relating to

the environment” (I.C.J. Reports 1996 (I), pp. 241-242, para. 29).

This jurisprudence affirms that the duty to prevent significant harm to the environment is

not confined to instances of direct cross-border harm and that it applies to global

environmental concerns. Therefore, the customary duty to prevent significant harm to the

environment also applies with respect to the climate system and other parts of the

environment.

135. The duty to prevent significant harm to the environment is an obligation to act with

due diligence (see Pulp Mills on the River Uruguay (Argentina v. Uruguay), Judgment,

I.C.J. Reports 2010 (I), pp. 55-56, para. 101, and p. 79, para. 197). As the Court has held,

while an obligation to prevent “is one of conduct and not one of result, in the sense that a

and technological information, as well as

relevant rules and international standards, and which vary depending on each State’s

respective capabilities. Other elements of the required conduct include undertaking risk

assessments and notifying and consulting other States, as appropriate. The Court will

address these elements more specifically below (see paragraphs 280-300).

137. The determination of what is required by due diligence ultimately “calls for an

assessment in concreto” of what is reasonable under the specific circumstances in which

a State finds itself (Application of the Convention on the Prevention and Punishment of

the Crime of Genocide (Bosnia and Herzegovina v. Serbia and Montenegro), Judgment,

I.C.J. Reports 2007 (I), p. 221, para. 430). This does not exclude the identification of a

required standard of conduct at a general level, depending on the overall character of the

risk to the part of the environment in question. This is particularly apposite with respect

to climate change because the specific character of the risk of significant harm to the

climate system is indisputably established. The best available science, as presented by the

IPCC, confirms that cumulative GHG emissions are the primary source of risks arising

from anthropogenic climate change (see paragraphs 72-87 above). All States contribute to

that risk, albeit to significantly differing degrees, and all States are affected by the

cumulative effects of GHG emissions, depending on their respective situations. Climate

change therefore poses a quintessentially universal risk to all States. This risk is of a

general and urgent character, requiring the identification of a corresponding general

standard of conduct, to be applied subject to the principle of common but differentiated

responsibilities and respective capabilities.

138. Under these circumstances, the Court recognizes that the standard of due diligence

for preventing significant harm to the climate system is stringent (see Climate Change,

Advisory Opinion, ITLOS Reports 2024, pp. 91-92, para. 241, pp. 92-93, para. 243, p. 94,

para. 248, pp. 137-138, paras. 398-400, and pp. 152-158, para. 441). Moreover, as the

Court has explained, due diligence “entails not only the adoption of appropriate rules and

measures, but also a certain level of vigilance in their enforcement and the exercise of

administrative control” (Pulp Mills on the River Uruguay (Argentina v. Uruguay),

Judgment, I.C.J. Reports 2010 (I), p. 79, para. 197). As concerns climate change, a

heightened degree of vigilance and prevention is required.
139. The Court concludes that the duty of States to prevent significant environmental

harm applies in the context of climate change and that this duty forms part of the most

directly relevant applicable law.

Obligation of States with Regard to Climate Change, International Court of Justice (July 23,

2025) at 48-51.

The Court also held the obligation to address climate change is a “core” human right

protected under the human rights treaties. Id. at 51-52. It held further that “the principles of

sustainable development, common but differentiated responsibilities and respective capabilities,

equity, intergenerational equity, the precautionary approach or principle, and the “polluter pays”

principle” were a part of this customary international law binding on all nations. Id. at 52-56.

Finally, and very significantly, the Court held that these general principles of international

law are not limited by or displaced by the UNFCCC, Kyoto Protocol or the Paris Agreement. Id.

at 56-58

State cannot be under an obligation o succeed, whatever the circumstances, in

preventing” harm, “the obligation of [States] is rather to employ all means reasonably

available to them, so as to prevent [harm] so far as possible”(Application of the

Convention on the Prevention and Punishment of the Crime of Genocide (Bosnia

and Herzegovina v. Serbia and Montenegro), Judgment, I.C.J. Reports 2007 (I), p. 221,

para. 430).

136. The conduct required by due diligence has several elements. These elements include

States taking, to the best of their ability, appropriate and, if necessary, precautionary

measures, which take account of scientific and technological information, as well as

relevant rules and international standards, and which vary depending on each State’s

respective capabilities. Other elements of the required conduct include undertaking risk

assessments and notifying and consulting other States, as appropriate. The Court will

address these elements more specifically below (see paragraphs 280-300).

137. The determination of what is required by due diligence ultimately “calls for an

assessment in concreto” of what is reasonable under the specific circumstances in which

a State finds itself (Application of the Convention on the Prevention and Punishment of

the Crime of Genocide (Bosnia and Herzegovina v. Serbia and Montenegro), Judgment,

I.C.J. Reports 2007 (I), p. 221, para. 430). This does not exclude the identification of a

required standard of conduct at a general level, depending on the overall character of the

risk to the part of the environment in question. This is particularly apposite with respect

to climate change because the specific character of the risk of significant harm to the

climate system is indisputably established. The best available science, as presented by the

IPCC, confirms that cumulative GHG emissions are the primary source of risks arising

from anthropogenic climate change (see paragraphs 72-87 above). All States contribute to

that risk, albeit to significantly differing degrees, and all States are affected by the

cumulative effects of GHG emissions, depending on their respective situations. Climate

change therefore poses a quintessentially universal risk to all States. This risk is of a

general and urgent character, requiring the identification of a corresponding general

standard of conduct, to be applied subject to the principle of common but differentiated

responsibilities and respective capabilities.

138. Under these circumstances, the Court recognizes that the standard of due diligence

for preventing significant harm to the climate system is stringent (see Climate Change,

Advisory Opinion, ITLOS Reports 2024, pp. 91-92, para. 241, pp. 92-93, para. 243, p. 94,

para. 248, pp. 137-138, paras. 398-400, and pp. 152-158, para. 441). Moreover, as the

Court has explained, due diligence “entails not only the adoption of appropriate rules and

measures, but also a certain level of vigilance in their enforcement and the exercise of

administrative control” (Pulp Mills on the River Uruguay (Argentina v. Uruguay),

Judgment, I.C.J. Reports 2010 (I), p. 79, para. 197). As concerns climate change, a

heightened degree of vigilance and prevention is required.