We empirically test the role of membership rules and voting schemes for climate change coalitions with the STAbility of COalitions model (STACO). The model comprises twelve world regions and captures long-run effects of greenhouse gas accumulation. We apply three stability concepts that capture the notion of open membership and exclusive membership with majority and unanimity voting. We show that exclusive membership leads to superior outcomes than open membership and that unanimity voting is preferable to majority voting in welfare and environmental terms. Our results suggest restricting membership in future international environmental agreements and they provide a rationale for unanimity voting as applied in many international organizations.