In the context of sub-global participation in greenhouse gas mitigation efforts, this paper investigates the effectiveness of a Canada-U.S. emissions bubble under their existing regional trade agreement. It also explores the potential economic impact of carbon tariff harmonization through the implementation of a common Canada-U.S. external border tariff adjustment as a mean to address competitiveness issues. Using a multi-region, multi-sector computable general equilibrium model, the paper finds that the creation of an emission bubble between the two countries could improve efficiency. The findings also suggest that a carbon tariff harmonization policy could give rise to distributional issues among Annex I regions and could fail to mitigate the negative competitiveness impacts of carbon abatement policies.