In its case law the International Court of Justice has repeatedly suggested the idea that rules of customary international law (CIL) do not operate in a vacuum but, instead, are to be understood against the background of other rules of the international legal system. This observation, although somewhat unsurprising, shows that the sources of international law exist in close interconnection – something that is also visible if one looks at the rules of interpretation contained in Article 31 of the Vienna Convention on the Law of Treaties. Accordingly, ‘any relevant rules of international law applicable in the relations between the parties’ must be taken into account, together with the context, when interpreting treaty provisions. The question addressed in this chapter is whether or not the same can be said of the interpretation of customary rules. In other words, if we look at the practice of international courts and tribunals, is it possible to reach the conclusion that CIL rules, too, must be interpreted with the cognizance of any relevant rules of international law applicable between the parties?

Chapter 3 addresses the suggestion that for a special regime to exist, there must be a mutual engagement of community members. As the chapter argues, in the context of international law, a mutual engagement among a group of international law specialists can be inferred from their participation in a distinct legal discourse, and from their further specialization and distinct way of ascribing functions to legal agents. More specifically, it can be inferred from: the publication of specialized international law journals; the way of organization of conferences and workshops; the creation of inter-governmental organization; the work of NGOs; the specialized research profile of international scholars and description of chairs; their separation of tasks and division of labour; and the function that they ascribe to the judiciary and to the international legal scholar.

This chapter addresses the suggestion that for a special regime to exist, community members must be engaged in a joint enterprise. In the context of international law, to claim that a group of international law specialists is engaged in a joint enterprise is to assert that they do what they do based on the idea that some certain state of affairs is desirable. As Chapter 4 argues, in the context of international law, the existence of such a presupposition can be inferred from the actual pursuit of those specialists of a distinct state of affairs, and the way in which they perform assignments.

This chapter examines the role of the state in collaborative governance as well as the mix between hard (regulation) and soft modes of governance (orchestration) to achieve decarbonization in Sweden after the adoption of the 2017 Climate Policy Framework. The chapter focuses on state-led transformation and critically examines Sweden’s progress toward its overarching goal to become the first fossil-free welfare state by 2045. It investigates Sweden’s national strategies and governance modes to achieve decarbonization and overcome carbon lock-ins through institutional, economic, technological, and behavioral transformation. It concludes that Sweden’s path to decarbonization – like many other countries – resembles more of an incremental transition limited to certain sectors rather than the wholesale transformation toward achieving a fossil-free society.

The ILC Study Group on Fragmentation of International Law asserted that the proliferation of special regimes poses a threat to the unity of the international legal system. Chapter 6 challenges this assumption. It builds on the distinction made in epistemology between knowledge-that and knowledge-how. The idea of a special regime as a community of practice makes it a system of knowledge-how. As such, it is compatible with all of the legal positivist’s, legal realist’s and legal idealist’s conceptions of an international legal system, which either see it as a system of knowledge-that or a combination of a system of knowledge-that and a system of knowledge-how. In the former case, in no way does the proliferation of special regimes affect the unity of the international legal system. In the latter case, the unity of the international legal system is indeed affected, but only in a positive sense, as it increases the efficacy of international law relative to the assumed legal ideal.

The process of identifying and interpreting norms of customary international law, while appearing to be primarily based on an inductive analysis of state practice and opinio juris, is sometimes a deductive exercise based on logic and reason. Logic permeates every decision in international law. Logic manifests itself inherently throughout the process and can be identified in all steps of reasoning in identifying, interpreting and applying customary international law. Logic, however, can constitute the application of either an inductive or deductive inference. This chapter focuses on situations in which the International Court of Justice (ICJ) and the Permanent Court of International Justice (PCIJ) applied a deductive approach, identifying or interpreting norms of customary international law without seeming to consult state practice and opinio juris. Specifically, it considers whether norms that can be reasonably inferred or deduced from existing rules, or that are simply logical for the operation of the international legal system, can be identified as norms of customary international law under a complementary, supplementary or distinctive interpretive approach.

This chapter addresses the chronological paradox of customary international law (CIL). The paradox is that for a new customary rule to be created states must believe that the law already obligates the behaviour specified in that rule (opinio juris). However, the behaviour in question can only be legally required once that rule has been created. As a result, creating a new customary rule would be impossible, or at the very least an incoherent process. This chapter challenges this conclusion. In addressing the chronological paradox, it provides a coherent interpretation of the creation of new customary rules. It argues that the sense of legal obligation (opinio juris) emerges from the general principle of good faith. Good faith leads to legal obligations, which compel a subgroup of states to engage in specific behaviour. Then, as a result of this subgroup’s repeated behaviour, a new customary rule emerges, obligating the entire community of states to act accordingly. To explain the shift from good faith to legal obligations and from legal obligations to customary rules, the chapter draws on interpretivism, social ontology and contemporary research on constitutive rules.

This chapter provides an overview of the aims and research questions guiding this book. It introduces key terms and concepts and outlines the main contributions of the book. The chapter explains why the complex relations between state and non-state actors are crucial to understanding the implementation of the Paris Agreement. It provides a background to understanding the role of collaborative climate governance in the post-Paris governance landscape by highlighting the international context and describing Sweden’s climate policy framework. Finally, it provides a brief overview of each chapter in the book.

When faced with the inevitable task of interpreting customary international law (CIL), what should a court do and what should it consider? Should it engage in an ‘inductive’ process of sifting through available evidence of state practice and opinio juris or a deductive process designed to reason logically from principles embedded in the rule? Should the court invoke something like the rules of treaty interpretation with their focus on good faith, ordinary meaning, context, and object and purpose? International law doctrine falls short here. Figuring out how to interpret and apply custom requires a theory of custom, a focus on the normative stories we tell. This reveals that there is not just one story explaining why custom should be a source of law, but multiple stories. What we call ‘custom’ may represent or draw from at least three different sources of law: Negotiated Law, Legislated Law and Adjudicated Law. This chapter aims to show that the non-treaty rules derived from each draw on different sources of legitimacy, operate according to different logics, dictate different methods of interpretation, and favour different methods for resolving disputes.

The risk of possibly conflicting norms of customary international law (CIL) has received increased attention in recent international legal scholarship and practice. In the absence of commonly accepted or authoritative rules of conflict that may mitigate the ramifications which stem from a possible clash of opposing obligations under CIL, legal scholars and practitioners alike almost instinctively turn towards competent (judicial) authorities, thereby seeking advice on how to strike a balance between conflicting norms in conformity with applicable legal frameworks and regimes. Thus far, international courts and tribunals, including the International Court of Justice, have regularly refrained from outlining, conceiving and imposing coherent analytical and prescriptive means of establishing an equilibrium between (partially) opposing norms of CIL in situ. This chapter argues that international adjudicative bodies could contemplate resorting to the German constitutional law principle of practical concordance (praktische Konkordanz) and thereby draw on a legal methodology that has become well established and is regularly applied by the German Constitutional Court when ruling on fundamental rights.

This chapter explores net-zero emission targets in Swedish municipalities. Based on a detailed examination of over 300 local climate and environment plans, the chapter both maps and evaluates the quantity and quality of net-zero emissions targets in Swedish municipalities, as well as how they relate to the national climate mitigation goals. It identifies 39 municipalities with net-zero emission targets. The targets range from highly specific with intermediary goals and plans for how to deal with residual emissions, to lofty, one-sentence visions without any further specifications. The findings are subsequently discussed whether aspirational goals – such as net-zero emission targets – could have an impact on the climate policy of a municipality or whether more specific goals are more effective. The chapter concludes that net-zero emission targets are still in their infancy in Swedish municipalities with large heterogeneity and gaps in how the goals are defined, what they include in terms of GHGs and sectors, and whether they also include measures for dealing with residual emissions.

Customary international law (CIL) is particularly vulnerable to the accusation that it is no more than ‘mere assertion’, a creation of the courts, if not downright fantasy. Yet it is in CIL that one finds the strongest claim to objectivity in international law. It is expressed in the doctrine that one of the elements of CIL is state practice, which represents the ‘objective’ element of CIL. It is thought to supplement the ‘subjective’ or ‘psychological’ element of CIL: opinio juris. This chapter argues that the notion of state practice as a set of ‘material facts’ that should be ‘identified’ and from which customary norms can be ‘induced’ is grounded in obsolete epistemology. The identification of state practice is more adequately described as a selection of what deserves to be counted as state practice. It is argued that the starting point for this selective process is opinio juris. Opinio juris does not come after the fact, as a subjective feeling of obligation that is superadded to a set of otherwise objective facts. Opinio juris is the indispensable conceptual framework without which habits and usages cannot even be ‘seen’ as state practice.

Interpretation is ubiquitous in everyday life. We constantly interpret a variety of objects. Interpretation is central to the practice of international law, too. Arguing about international law’s content is the everyday business of international lawyers, and this often includes arguing about the existence and content of norms of customary international law (CIL). Although a number of scholars recognise that CIL can be interpreted, disagreements remain as to the precise methods and extent of CIL interpretation. Such disagreements are born of a common concern to secure competently made, coherent and accurate interpretations of CIL, given the latter’s non-textual nature. This chapter aims to explore in a preliminary manner two related questions regarding CIL interpretation: (1) Is it necessary, or even possible, to strive towards coherence in the interpretation of CIL? (2) Are there any possible indicators of (in-)coherence in that respect? Providing answers to these questions depends on how one understands coherence in the first place, including its relation to legal reasoning. A substantial part of the chapter will therefore deal with that as well.