1. Carbon dioxide (CO₂)

There are both natural and human sources of CO₂ emissions.

 Natural sources (CO₂-NS)

• Soil respiration and decomposition: (28.56%),
• Ocean-atmosphere exchange: (48.2%),
• plant and animal respiration: (28.56%),
• volcano eruptions: (0.03%).

Human sources (CO₂-HS):

• industrial processes (cement production): (4%),
• deforestation: (9%),
• burning of fossil fuels (coal, natural gas and oil): (87%).

Observe that: CO₂-HS are much smaller than CO₂-NS. However, CO₂-HS disturbed the CO₂ steady levels existed before the burning of fossil fuels and deforestation (which are the primary cause of the increased CO₂ concentrations in the atmosphere). Clearly, CO₂-HS can be controlled.

2. Methane (CH₄)

There are both natural (wetlands, termites and the oceans) and human (landfills, livestock farming sources, and the production, transportation and use of fossil fuels) sources of CH₄ emissions. However, humans create the majority of the total emissions.

Natural sources (CH₄-NS):

• wetlands (78%),
• termites (12%) and
• oceans (10%).

Human sources (CH₄-HS):

• fossil fuel production and intensive livestock farming (60%),
• landfills and waste (16%),
• biomass burning (11%),
• rice agriculture (9%),
• biofuels (4%).

Observe that the human caused emissions increased since the fossil fuel production and intensive livestock farming started (these two sources contribute 60% of all human CH₄ emissions) and disturbed the steady levels of CH₄, which existed before the fossil fuel production and intensive livestock farming. Clearly, this caused the increased of CH₄ concentrations in the atmosphere.

3. Nitrous oxide(N₂O)

There are both natural (soils under natural vegetation and the oceans) and human (agriculture, fossil fuel combustion and industrial processes) sources of N₂O emissions. However, humans create the majority of the total emissions.

Natural sources (N₂O-NS):

• soils under natural vegetation: (60%),
• oceans (35%) and
• atmospheric chemical reactions (5%).

Human sources (N₂O-HS):

• agriculture, fossil fuel combustion and industrial processes: (77%),
• biomass burning (10%),
• atmospheric deposition (9%) and
• human sewage (3%).

As in the cases of CO₂ and CH₄ human N₂O sources (mainly agriculture, fossil fuel combustion and industrial processes) disturb the natural N₂O levels and increase N₂O concentrations in the atmosphere.

4. Fluorinated gases (HFCs, PFCs, SF6, and CF4)

Almost all of them are created by humans and are used mainly in industrial processes, with the exception of PFC-14 (CF4).

• HFCs: (91%), used inside of refrigerators, air-conditioners, foams and aerosol cans (i.e. hair spray).
• PFCs: (6%), created during the production processes of aluminum and semiconductors.
• PFC-14: (CF4), small amounts of CF4 are found in fluorite, granite and natural gas deposits. Geochemical reactions in the lithosphere cause these emissions.
• SF6: (3%), used by the electric power industry as an insulator and arc interrupter. The other important source of SF6 emissions is from its use as a cover gas in magnesium production.

In addition,

• Ozone is a greenhouse gas because it has an effect on global temperature. At higher elevations in the atmosphere (stratosphere), where it occurs naturally, it is needed to block harmful UV light. At lower elevations of the atmosphere (troposphere) it is harmful to human health and is a pollutant regulated independently of its warming effects.
• Water vapor is the most abundant greenhouse gas, but most scientists believe that water vapor produced directly by human activity contributes very little to the amount of water vapor in the atmosphere.