Recent international court rulings have clarified States’ due diligence obligations in climate policy, emphasizing proactive efforts to mitigate environmental harm rather than guaranteeing specific outcomes.

Climate change, driven by escalating greenhouse gas (GHG) emissions, represents one of humanity’s most pressing challenges. While States are not directly liable for failing to meet specific targets—even under obligations owed universally (erga omnes)—they can still be held accountable under the international law’s due diligence principle. This principle requires States to make reasonable efforts toward achieving climate goals. Recent judicial decisions have provided a clearer understanding of these obligations, particularly in the context of climate policy. This article explores how these rulings define and refine the due diligence requirements for States.

The European Court of Human Rights (ECtHR) in Verein KlimaSeniorinnen v. Switzerland and the International Tribunal for the Law of the Sea (ITLOS), in its Advisory Opinion on Climate Change, have both made significant contributions to the discourse on State responsibilities. These rulings establish a consistent standard of conduct that spans human rights and maritime law, emphasizing the expectation that States act diligently to mitigate the adverse effects of climate change.

In international law, due diligence is interpreted in two primary ways. First, it is seen as a standard of conduct guiding how States are expected to fulfill their obligations. Second, as articulated by legal scholar Dupuy, it is considered an obligation to exert significant effort to mitigate risks associated with specific situations. Within international environmental law, due diligence is particularly crucial to the prevention principle, which obligates States to take proactive measures to prevent ecological harm.

This principle is embedded in various treaties, such as Article 2 of the Long-Range Transboundary Air Pollution Convention (1979) and Article 3 of the International Law Commission’s Draft Articles on Preventing Transboundary Harm from Hazardous Activities. These legal frameworks mandate States adopt and implement measures that protect the environment, integrating conduct standards and substantive obligations.

The scope of due diligence has been thoroughly examined in international litigation. For example, in the Costa Rica v. Nicaragua case, Judge Ad Hoc Dugard argued that due diligence is a standard of conduct derived from the prevention principle rather than a distinct obligation. In contrast, Judges Owada and Donoghue interpreted due diligence as a substantive obligation that could be fulfilled through measures such as environmental impact assessments.

The Seabed Disputes Chamber’s 2011 Advisory Opinion further reinforced the notion that due diligence is a substantive obligation that evolves with advancements in science and technology and the increasing risks associated with certain activities. This perspective contrasts with more recent rulings by the ECtHR and ITLOS, which have offered more explicit definitions of due diligence obligations, particularly in climate change.

The ITLOS, in its Advisory Opinion on Climate Change, clarified States’ obligations under the United Nations Convention on the Law of the Sea (UNCLOS) regarding marine pollution exacerbated by climate change. Despite the different legal frameworks involved, the ITLOS and ECtHR judgments converge on understanding due diligence as an obligation of conduct rather than a result. ITLOS’s Advisory Opinion on Article 194(1) of UNCLOS emphasized that States must make their best efforts to prevent, reduce, and control marine pollution without being required to guarantee specific outcomes.

This aligns with the Seabed Disputes Chamber’s 2011 Advisory Opinion, which framed due diligence as a conduct-based obligation. Similarly, the ECtHR, in interpreting Article 8 of the European Convention on Human Rights (ECHR), underscored that due diligence entails taking timely and appropriate actions to reduce GHG emissions without mandating the achievement of specific reduction targets.

The ITLOS and ECtHR have set a high bar for state conduct, requiring adopting and implementing effective climate policies that align with contemporary international goals and commitments, including those outlined by the Intergovernmental Panel on Climate Change (IPCC) and the Paris Agreement. The ITLOS stressed that given the severe threats posed by climate change, mere efforts are insufficient—States must demonstrate a rigorous commitment to mitigating these threats.

The ECtHR similarly demanded that States undertake prompt and consistent actions aimed at significant GHG reductions to achieve net neutrality within the next three decades. To support this, the court outlined a compliance framework that includes setting and regularly updating specific emission reduction targets. Both courts recognized that while compliance should be tailored to each State’s unique circumstances, due diligence ultimately focuses on the actions taken by States, not on achieving specific outcomes. This ensures that efforts commensurate with a State’s capabilities can mitigate its responsibility for contributing to climate change.

The rulings from the ITLOS and ECtHR have established clear due diligence obligations for States regarding climate change, defining these as distinct duties rather than mere compliance standards. These judgments clarify State responsibilities without requiring specific treaty provisions to enforce GHG reduction efforts, allowing flexibility in setting and pursuing emission targets. However, while these rulings outline the scope of State duties, they stop short of addressing the consequences of non-compliance—a gap that may soon be addressed by the Inter-American Court of Human Rights and the International Court of Justice as they delve further into this evolving area of international law.