Hence, global governance can be understood as the governance of various types of global issues at the global level, including horizontal coordination among a variety of global actors not limited to nation-states. Many studies have acknowledged the limits and weaknesses of the existing global governance framework in the management of sustainability concerns, and they contend that it should be reformed. The most ambitious proposal for global governance would be to set up a new authoritative single entity, exemplified by the constitutional approach described by Mitrany (1933). Others have suggested more modest alternatives. Haas (2004) presented a “network model of decentralized global governance”, a form of global governance based on diffuse networks of diverse actors.