Waste involves the collection, treatment, and disposal of solid and liquid waste. For example, decomposition of waste in landfills releases methane (a potent greenhouse gas). What is Mt CO₂-e? Emissions are a combination of gases. Most are Carbon Dioxide (CO₂). The other gases are measured as a Carbon Dioxide equivalent (CO₂-e) based on the amount of heat they trap in the atmosphere. They’re measured in Million Tonnes (Mt).