This study explores how conditional cooperation strategies affect collective action in a public good game, particularly those resembling international climate negotiations. We rely on a laboratory economic experiment with about 1000 participants to investigate the relationship between group size and cooperation under a baseline and a common commitment condition. Results show that common commitment mechanisms significantly enhance and stabilize cooperation, even in larger groups, provided the group marginal return is sufficiently high. These findings have critical implications for designing international climate agreements and other public good provision systems.