International cooperation on climate change mitigation is hampered by the public goods nature of ecosystem services, incentivizing free-riding. The limited capacity of the atmosphere to absorb greenhouse gases necessitates significant emission reductions, but the costs of mitigation often outweigh immediate benefits, leading to delays in cooperation. Existing large-scale negotiations, like those in Kyoto, Copenhagen, and Doha, have failed to establish effective, widely adopted legally binding agreements. While bilateral and pledge-and-review agreements (like the Paris Agreement) have had some success, they rely on voluntary commitments, which may be insufficient to prevent severe climate change. Building-block approaches, focusing on localized agreements, offer a potential solution but risk slow global adoption. This research explores whether a strategic shift from local to global agreements can accelerate mitigation efforts.

The paper reviews existing literature on climate change mitigation strategies, highlighting the challenges of achieving global cooperation due to free-riding and the costs of mitigation. It discusses the limitations of previous large-scale negotiations and the potential of building-block approaches that leverage localized agreements. Game-theoretic models and common pool resource analyses are cited to support the argument that localized agreements might succeed where global cooperation fails. However, these approaches can be slow to achieve global-scale mitigation due to the need for multiple group adoptions and limited influence over late-adopting players.

The authors use a stochastic model to simulate the dynamics of climate change mitigation across multiple groups of players (representing countries or coalitions). Players can choose between a stringent mitigation strategy (aligned with a ≤2°C warming scenario) or minimal mitigation (aligned with a >2°C warming scenario). The model incorporates several key factors: The cost of stringent mitigation, which is assumed to substantially outweigh immediate benefits; the probability of random policy choices (representing unpredictable events); and the effect of legally binding agreements (local or global) that sanction non-mitigators. The model considers different scenarios, including local agreements only, global agreements only, a shift from local to global agreements, and simultaneous attempts at both. The simulations vary parameters such as group size, inter-group rivalry, and economies of scale to assess their impact on mitigation outcomes. The model is designed to represent a worst-case negotiation scenario where the costs of mitigation significantly outweigh short-term benefits.

The simulation results demonstrate that premature global-scale efforts to establish legally binding agreements often fail to accelerate cooperation. In contrast, local agreements can successfully establish mitigation as a norm within groups, eventually leading to widespread global adoption. A timely shift from local to global agreements, once a sufficient proportion of players have adopted mitigation locally, significantly accelerates the process. This is because global agreements effectively sanction non-mitigating groups, pushing them to cooperate. The benefits of this approach are amplified by economies of scale (where increased adoption reduces per-unit costs) and diminished by inter-group rivalry (where competition between groups hinders cooperation). The study also explores the effect of unequal emissions and diplomatic influence, finding that while these factors introduce variability, a shift from local to global agreements remains the most effective strategy. The timing of the shift from local to global agreements is crucial. Premature shifts can delay mitigation, while delayed shifts may squander the benefits of global agreements. The most effective threshold for switching typically corresponds to the quorum required to enact global agreements (50%). The paper also analyzes scenarios of concurrent local and global agreement attempts; these have limited effects when high constitutive sanctioning costs apply; otherwise, a more graduated shift where local and global agreement attempts overlap might be more beneficial.

The findings strongly support the argument that a phased approach to climate change mitigation is most effective. By focusing initially on localized agreements to build momentum and establish a foundation of committed actors, it becomes possible to subsequently leverage the power of global agreements to bring in the remaining, hesitant players. The study highlights the scale-specific roles of mitigation incentives, emphasizing the importance of local agreements in establishing initial commitments and global agreements in achieving widespread adoption. The results demonstrate that addressing the challenges posed by economies of scale and inter-group rivalry requires a multi-scale approach, utilizing both local and global incentives. The model's worst-case scenario assumptions strengthen the significance of the results, suggesting the effectiveness of the proposed strategy even under challenging conditions. The success of this approach is contingent on the timing of the shift from local to global strategies. It also considers scenarios with unequal emissions and diplomatic influence, demonstrating the robustness of the proposed strategy under varying conditions.

This research demonstrates the significant advantage of a well-timed shift from local to global legally binding agreements for accelerating climate change mitigation. The results suggest that policymakers should prioritize establishing robust local agreements as a foundation for subsequent global cooperation. Future research could explore the optimal timing of the shift in different contexts and further investigate the interactions between local and global mitigation efforts, such as the role of decentralized sanctioning and positive incentives.

The model simplifies several aspects of climate change negotiations, including the continuous nature of mitigation commitments (the model uses a binary approach), variations in mitigation costs and benefits among players, and the specific forms of local and global agreements. Furthermore, the model doesn't explicitly model the dynamics of climate change itself or the influence of climate projections on policy decisions. These simplifications could affect the quantitative predictions but do not negate the overall conclusion that a strategic shift in agreement scale can significantly improve outcomes.