GHGs are the gaseous constituents that trap heat in the atmosphere. They are released through natural processes (e.g. decomposition of biomass) and as a result of human activity (e.g. the burning of fossil fuels). Some gases are naturally occurring (e.g. carbon dioxide) while others are human-made (e.g. the halocarbons). Carbon dioxide (CO2) is the largest single contributor to climate change. The United Nations Framework Convention on Climate Change covers the following GHGs:
CO2 is naturally occurring but is also a by-product of burning fossil fuels, of burning biomass, of land-use changes and of industrial processes.
CH4 is the major component of natural gas and it is associated with all hydrocarbon fuels. Significant emissions also occur as a result of animal husbandry, waste management and agriculture.
The main anthropogenic source of N2O is agriculture, in addition to sewage treatment, fossil fuel combustion, and chemical industrial processes. N2O is also produced naturally e.g. through microbial action in wet tropical forests.
F-gases include sulphur hexafluoride (man-made chemical primarily used in electrical transmission and distribution systems, and in electronics), hydrofluorocarbons and perfluorocarbons (alternatives to ozone depleting substances, these by-products of industrial processes are powerful GHGs).
The three main systems capable of storing carbon and nitrogen, known as “stocks” or “pools”, include the land ecosystems, the ocean and the Earth’s crust.
Carbon and nitrogen not stored in these pools resides in the atmosphere as a component of greenhouse gases.
Land ecosystems (such as forests and peatlands): Plants absorb carbon through photosynthesis. The carbon they capture is stored in vegetation or integrated into soils when plants die. The breakdown of plant material and soil by microorganisms leads to emissions.
The Earth’s deep mantle sequesters carbon through sedimentation and other geological formations, on geological timescales (many millennia). Carbon is released into the atmosphere through the extraction and combustion of fossil fuels.
Atmospheric CO2 dissolves into the ocean, and phytoplankton also sequester carbon by photosynthesis, while deep cold waters absorb carbon.
What is released or cannot be stored by other carbon stocks accumulates into the atmosphere.
For example, forests are the largest terrestrial sink - globally, their net removal of carbon is equivalent to 5.7 billion metric tonnes of carbon dioxide (GtCO2) a year. This represents 45% of carbon dioxide sequestration from the land sink.
This is already happening in forest areas across the tropical belt…
This map shows the net carbon sinks (green) and sources (red) from forests across the period 2001-19 (MtCO2e). The largest sinks are found in tropical forests. The largest sources are found in disturbed tropical forests.