Climate change is the issue of the century and, according to Agenda 21, local actions are essential to impact global mitigation of greenhouse gases (GHG) emissions (“think globally, act locally”). However, in order to plan and implement effective, sustainable actions, local authorities need detailed information on their GHG emissions and their sources. This paper presents the work that led to the development of a GIS-based tool for local GHG accounting, which provides data for local decision-makers in an innovative manner different from traditional GHG inventories. The difference is that all emissions (carbon sources) and removals (carbon sinks) are geo-referenced. The paper also describes the development of a methodology for the environmental impact assessment of a major cultural event, which is an activity that can significantly contribute to local GHG emission. The main results of the application of the tool to an Italian municipality are presented below.