A source is any natural or human-induced process or activity that releases a greenhouse gas (GHG) or aerosol into the atmosphere. Alternatively, a sink is any process, activity, or mechanism that removes a GHG, an aerosol, or a precursor of a GHG or aerosol (e.g., volatile organic compounds, nitrogen oxides) from the atmosphere. Primary examples of sources and sinks for each major GHG (carbon dioxide, methane, and nitrous oxide) include:
Carbon dioxide (CO2)
- Sources (natural): Microbial decomposition of organic material in the presence of oxygen, forest fires, and volcanoes.
- Sources (human-induced): Burning of fossil fuels, deforestation, and the burning of solid waste.
- Sinks: Photosynthetic vegetation, reforestation, and soils.
- Sources (natural): Microbial decomposition of organic material in the absence of oxygen (e.g., wetlands, permafrost, oceans, freshwater bodies), termites, and wildfires.
- Sources (human-induced): Livestock via enteric fermentation; manure management; rice cultivation; waste management (i.e., landfills and burning biomass); and energy production from natural gas, coal, and petroleum.
- Sinks: Upland soils contain methane-oxidizing bacteria (methanotrophs) and oxidation within the troposphere by hydroxyl radicals.
Nitrous oxide (N2O)
- Sources (natural): Soils and the ocean, primarily via denitrification.
- Sources (human-induced): Production and use of nitrogen fertilizers, combustion of fossil fuels, burning of biomass, and manure management.
- Sinks: Destruction in the stratosphere via photolysis.
The sidebar sections go into the detail of agricultural sources and sinks of GHG emissions and recommended mitigation strategies and case studies.