In the frame of the European project titled “Zero Waste” (1G-MED08-533), a tool has been developed called CO2ZW for estimating the GHG emissions for the management of MSW at the municipal, regional or national levels with small amounts of input data. The objective of this paper is to demonstrate that the CO2ZW tool allows us to inventory and monitor GHG emissions from MSW following the IPCC guidelines for national inventories and the principles of life cycle assessment (LCA). The CO2ZW tool includes the key stages and parameters for calculating GHG emissions and includes several advantages regarding the implementation of the default values of the Mediterranean European countries, an improvement in accessibility (online free access) and two approaches for calculating GHG emissions from landfills. The results of this paper show that for countries with medium and high rates of deposition, implementation of the European policies limiting waste in landfills can contribute to mitigate climate change in a remarkable way. With the CO2ZW tool, it is possible to evaluate waste management choices depending on waste management infrastructures and waste policies, along with the quantification of GHG emissions from MSW management, which is essential to guide waste policy options and climate change solutions.