This chapter provides a comparative assessment of climate policies, policy instruments, and greenhouse gas emissions reduction targets across subnational jurisdictions in the United States, Canada, Germany, Spain, and the UK that joined the Under2 Coalition at different timepoints. The analysis reveals that while founding members generally demonstrate sustained climate policy effort, the relationship between membership timing and climate ambition is mediated by domestic political contexts, institutional capacity, and structural factors. Under2 Coalition membership functions primarily as an amplifying mechanism rather than a transformative catalyst, reinforcing existing climate policy trajectories through reputational incentives, policy learning networks, and international visibility. Evidence of policy diffusion emerges through adoption of California’s vehicle emission standards, institutional emulation of climate laws, and transfer of innovative mechanisms like Scotland’s Climate Justice Fund. However, some early joiners match or exceed founding member performance, while certain later joiners demonstrate a remarkable acceleration of policy-based climate action, challenging simple assumptions about first-mover advantages. Political transitions, constitutional arrangements, and economic factors prove more decisive than coalition entry sequencing in determining actual policy outcomes.