International Shipping Decarbonization Through a Two-Level Game Lens: The IMO Net-Zero Framework and Conditions for a More Durable Agreement Space

International shipping decarbonization has expanded beyond technical and operational regulation to include contested market-based bargaining. The proposed International Maritime Organization (IMO) Net-Zero Framework (NZF) combines a marine fuel standard with an economic compliance mechanism. Drawing on IMO submissions, voting records, official statements, and domestic and sectoral materials, this two-level game analysis examines why support sufficient to approve the draft for circulation in April 2025 did not translate into timely adoption later that year. The evidence suggests that the initial agreement space was not durable enough to support adoption on the planned timetable. At Level I, bargaining involved three linked dimensions: rule-making authority, responsibility allocation, and transition pathway design. At Level II, domestic, regional, and sectoral constraints included regulatory credibility and investment signals for the European Union; sovereignty and cost concerns for the United States; trade exposure and disproportionate impacts for trade-exposed emerging economies; ambition and revenue-supported transition for small island developing States; and fuel availability, fleet competitiveness, and legal certainty for energy exporters and open registries. A coalition sufficient at one procedural stage may therefore not remain sufficient at the next. A more durable agreement space would likely require an IMO-centered regulatory core, targeted enabling support, an ambition floor with compliance flexibility, and sequenced implementation and review.