Greenhouse gases (GHGs) are atmospheric gases that trap heat from the sun, creating a warming effect similar to a greenhouse. While this natural greenhouse effect makes Earth habitable, human activities have dramatically increased GHG concentrations, causing climate change. The primary GHGs include carbon dioxide (CO2, from burning fossil fuels and deforestation), methane (CH4, from agriculture, landfills, and natural gas), nitrous oxide (N2O, from agriculture and industry), and fluorinated gases (from industrial processes). Canada measures and reports national GHG emissions, tracking progress toward reduction targets under the Paris Agreement. Municipalities are significant GHG sources through buildings, transportation, waste, and operations, and increasingly develop climate action plans with emission reduction targets. GHG inventories—accounting of emissions by source—help identify reduction opportunities. Reducing GHG emissions requires changes in energy systems, transportation, buildings, industry, agriculture, and waste management.