Diffusion holds the key to both the mechanism of carbon emission and a solution to the problem of emission excesses. In essence, diffusion represents spatial dependence through connectivity between states and affects their policies or even regulations entailed in the framework of global governance. Even though it is of critical importance to climate governance in influencing trust and incentives for cooperation, diffusion has received limited attention from international relations analysts of climate change. Using spatial modeling and systemic international relations theories, we uncover that, on average, diffusion adversely affects other states’ emission efficiency and that emission by states with competitive trading activity is a major source of the adverse diffusion. This result holds even if international and domestic countervailing factors are taken into account. An in-sample simulation analysis confirms that, for better climate governance, the adverse diffusion can be neutralized by a coalition of numerous trading states, rather than by a limited number of large states (e.g., G20).