Explanation:

 At the global scale, the key greenhouse gases emitted by human activities are:

• Carbon dioxide (CO2): Fossil fuel use is the primary source of CO2. CO2 an also be emitted from direct human-induced impacts on forestry and other land use, such as through deforestation, land clearing for agriculture, and degradation of soils. Likewise, land can also remove CO2 from the atmosphere through reforestation, improvement of soils, and other activities.
• Methane (CH4): Agricultural activities, waste management, energy use, and biomass burning all contribute to CH4 emissions.
• Nitrous oxide (N2O): Agricultural activities, such as fertilizer use, are the primary source of N2O emissions. Fossil fuel combustion also generates N2O.
• ?Fluorinated gases (F-gases): Industrial processes, refrigeration, and the use of a variety of consumer products contribute to emissions of F-gases, which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), chlorofluorocarbons (CFC).