Technological and societal changes today are accompanied by a growing need to address the challenges posed by rising greenhouse gas (GHG) emissions. While national and international efforts are made to improve accountability for these emissions and to ensure accurate carbon footprint calculations, these efforts require substantial intervention of experts and the use of highly heterogeneous data sources and methodological standards. In this paper, we present the WISER ecosystem for the integration, exchange, computation, and elaboration of GHG assessments, based on ontologies and Knowledge Graphs to model not only GHG data sources but also the assessment standards that guide their use and application. Specifically, the WISER ecosystem has been applied to multiple real use-cases, of which we focus on: GHG assessments of industrial/production sites, GHG-informed procurement for ICT in cities, and decarbonization of data centers. These use cases have been developed and validated in collaboration with external industry partners as well as the ICT departments of Swiss municipalities.