Sources of the main greenhouse gases

 

The main greenhouse gases (GHGs) have both natural sources and anthropogenic sources — from human activity — with the exception of the group of GHGs referred to as halocarbons, which are human-made. Since anthropogenic sources add emissions to the atmosphere at a rate greater than natural processes can remove them from the atmosphere, atmospheric levels of GHGs are building up.

 

Carbon dioxide

Carbon dioxide (CO2), along with methane (CH4), is part of Earth’s carbon cycle, which involves the movement of carbon among the atmosphere, the land, the ocean, and living things. CO2 enters the atmosphere from a variety of natural sources, most notably as a result of plant and animal respiration, and is removed from the atmosphere through the photosynthesis of plants and uptake by the ocean. The main anthropogenic sources of CO2 are the burning of carbon-containing fossil fuels (coal, oil, and natural gas) and deforestation / land clearing. Land clearing can involve either burning trees and other vegetation, which releases CO2 immediately, or allowing cut vegetation to decay, which releases CO2 slowly. The manufacture of cement is another important source, as it involves heating limestone (calcium carbonate), the main component of cement, in a process that releases CO2

 

Methane

The main sources of CH4 — a carbon-containing GHG — are from decomposition of organic matter by micro-organisms under low-oxygen conditions. Wetlands are by far the largest natural source of CH4. Anthropogenic sources include rice paddies, landfills, and sewage; fermentation in the gut of ruminant animals; and artificial wetlands. Along with other pollutants, CH4 is also produced when fossil fuels and trees are burned with insufficient oxygen for combustion to be complete. It also leaks or is vented to the atmosphere from geological sources, mainly during the extraction, processing, and transportation of fossil fuels, although natural leaks also occur.

 

Nitrous oxide

Nitrous oxide (N2O) is part of Earth’s nitrogen cycle. Anthropogenic sources are mainly related to the use of nitrogen-based synthetic fertilizers and manure to improve crop productivity, and the cultivation of certain crops that enhance biological nitrogen fixation. These sources have added significant amounts of reactive nitrogen to Earth’s ecosystems, some of which is converted to N2O and released to the atmosphere. Some N2O is also released to the atmosphere during the combustion of fossil fuels and biomass (e.g., trees or woodbased fuels) and from some industrial sources.

 

Halocarbons

Halocarbons are a group of synthetic chemicals containing a halogen (e.g., fluorine, chlorine, and bromine) and carbon. There are a variety of industrial sources.

 

Water vapour

Water vapour is the most important naturally occurring GHG. Human activities do not directly influence the amount of water vapour in the atmosphere to any significant degree. However, the amount of water vapour in the atmosphere changes with temperature, and changes in water vapour are considered a feedback in the climate system.