This paper proposes an integrated framework for Scope 3-greenhouse gas (GHG) emission accounting by aligning the Greenhouse Gas Protocol (GHGP) with Activity-Based Costing (ABC) methods. Scope 3-GHG emissions, which occur outside organizational boundaries, are often difficult to allocate accurately, leading to inconsistencies between corporate and product-level carbon reporting - a problem known as the ‘divergence problem.’ By conceptualizing all 15 Scope 3 categories within a Unified Modeling Language (UML) activity diagram, this paper offers a methodology to connect emissions with resource consumption (e.g., materials, energy, labor, equipment). The framework distinguishes between attributable and non-attributable emissions to provide compliance with the GHGP and introduces a 2-tiered accounting system: one for product-level reporting and the other for corporate reporting and internal decision-making. This system improves transparency, supports decision-making, and lays the groundwork for integration into enterprise resource planning (ERP) systems. The findings highlight the potential of activity-based approaches for aligning emissions and cost data, thereby enhancing both environmental performance and regulatory compliance.