California Air Resources Board

Greenhouse Gases

California Health and Safety Code section 38505 identifies seven greenhouse gases (GHG) that CARB is responsible for monitoring and regulating in order to reduce GHG emissions: carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), sulfur hexafluoride (SF₆), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and nitrogen trifluoride (NF₃). SF₆, HFCs, PFCs, and NF₃ are collectively referred to as "high global warming potential (GWP) gases" in California's Assembly Bill 32 Greenhouse Gas Emissions Inventory (Inventory).

A list of all GHGs included in the Inventory along with their GWP values can be found under Global Warming Potentials.

Carbon Dioxide (CO₂)

CO₂ is a naturally occurring gas, and also a by-product of burning fossil fuels and biomass, as well as land-use changes and other industrial processes. CO₂ is the principal anthropogenic GHG that affects the Earth's radiative balance and is the primary GHG emitted in California.

Transportation is the single largest source of CO₂ in California; which is primarily comprised of on-road travel. The industrial, electricity generation, and residential sectors also make important contributions to CO₂ emissions in California.

Methane (CH₄)

CH₄ is produced through anaerobic (without oxygen) decomposition of waste in landfills, animal digestion, decomposition of animal wastes, production and distribution of natural gas and petroleum, coal production, and incomplete fossil fuel combustion. CH₄ is the second highest emitted GHG in California.

Agriculture accounts for the majority of California's CH₄ emissions, primarily from livestock enteric fermentation and manure management. Landfills, transmission and distribution of natural gas, and wastewater treatment are also important sources of CH₄ emissions in California. 

Nitrous Oxide (N₂O)

Globally, major sources of N₂O include soil cultivation practices, especially the use of commercial and organic fertilizers, manure management, fossil fuel combustion, nitric acid production, and biomass burning. 

Agriculture accounts for the majority of California's N₂O emissions, primarily from management of agricultural soils and manure. On-road transportation, commercial and residential use of nitrogen fertilizer, and wastewater treatment are also important sources of N₂O in California.

High Global Warming Potential Gases (High-GWP)

Unlike many other GHGs, high global warming potential (High-GWP) gases (SF₆, HFCs, PFCs, and NF₃) have no significant natural sources and are generated almost entirely from human-related activities. It is important to track these compounds due to their high GWPs. PFCs and HFCs are used as substitutes for chlorofluorocarbons (CFCs) which destroy stratospheric ozone. SF₆ is used in electricity transmission and distribution and in semiconductor manufacturing. Semiconductor manufacturing also emits a small amount of NF₃.

The use of HFCs and PFCs as substitutes for ozone depleting substances accounts for nearly all of California's high-GWP gas emissions. Semiconductor manufacturing is the second largest source of high-GWP gases while electricity transmission and distribution is the largest source of SF₆ emissions in California.