The goal of the SEMANCO project is to develop an ontology-based energy information system and associated tools that help stakeholders involved in urban planning to make informed decisions about how to reduce CO2 emissions in cities. An ontology system is to be developed from the requirement analysis performed at the case studies. This approach enables data, services and stakeholders to be taken into account in the process of building ontologies. To create the ontology-based system we have adopted a decentralized approach according towhich modular ontologies from diverse domains become interlinked through a semantic framework. In this paper we summarize the project vision and discuss some research issues currently being developed in the project concerning ontologies for energy information at the urban level, multi-scale analysis of carbon reduction problems and integration of GIS with linked data.