The study presents a quantification of greenhouse gas mitigation from waste management through the implementation of a separate collection scheme and subsequent treatment of biowaste at a global scale. In a first step the global biowaste amount is determined which could be collected separately (biowaste potential). Over 600 Million Mg of the 2 billion Mg waste generated annually can be kept away from the “global landfill”. With this assumption, the global greenhouse gas inventory of current biowaste management using a greenhouse gas calculator tool is estimated. Results show that the greatest release of greenhouse gases is attributable to the open landfilling of food and green waste, which is responsible for 332 million Mg CO2 equivalents per year. Based on this, for the first time on a global perspective, the possible GHG reduction potential through separate collection is analyzed using four different scenarios for the treatment of separately collected biowaste. Compared to the current status, 320 Million Mg CO2 equivalents per year can be saved by a complete composting of food and green waste, which equals the overall emission inventory of a mid-size country like Poland. In a realistic scenario with partly separate biowaste separation 55 Million Mg CO2 equivalents can be saved. The estimation aims to close the data gap of mitigation potential from optimized biowaste treatment on a global perspective, especially to emphasize the role of source separation for the planet. We acknowledge that a calculation with global data is subject to high uncertainties, but should provide an initial investigation to be further developed. Additionally, costs for the implementation of global composting of biowaste are calculated allowing to derive CO2 avoidance costs for this reduction measure. We state, it is among the first global analyses to explicitly couple biowaste separate-collection scenarios with a transparent cost assessment for composting.