The issue of US ratification of international environmental treaties is a recurring obstacle for environmental multilateralism, including the climate regime. Despite the perceived importance of the role of the US to the success of any future international climate agreement, there has been little direct coverage in terms of how an effective agreement can specifically address US legal participation. This paper explores potential ways of allowing for US legal participation in an effective climate treaty. Possible routes forward include the use of domestic legislation such as section 115 (S115) of the Clean Air Act (CAA), and the use of sole-executive agreements, instead of Senate ratification. Legal participation from the US through sole-executive agreements is possible if the international architecture is designed to allow for their use. Architectural elements such as varying legality and participation across an agreement (variable geometry) could allow for the use of sole-executive agreements. Two broader models for a 2015 agreement with legal participation through sole-executive agreements are constructed based upon these options: a modified pledge and review system and a form of variable geometry composed of number of opt-out, voting based protocols on specific issues accompanied with bilateral agreements on mitigation commitments with other major emitters through the use of S115 and sole-executive agreements under the Montreal Protocol and Chicago Convention (Critical Mass Governance). While there is no single solution, Critical Mass Governance appears to provide the optimum combination of tools to effectively allow for US legal participation whilst ensuring an effective treaty.