Greenhouse gases are long-lasting atmospheric molecules that absorb and keep the sun’s heat close to Earth rather than allowing it to escape into space. They are mainly produced by human activities such as the burning of fossil fuels and natural processes like respiration and decomposition. Their concentrations in the atmosphere are much higher now than in pre-industrial times. The most important greenhouse gases are carbon dioxide, methane and nitrous oxide.
Their impact on global warming depends on their radiative efficiency and atmospheric residence time. A key measure is their ‘Global Warming Potential’ (GWP), which expresses the warming effect of 1 ton of a particular gas in terms of the equivalent amount of CO2. Gases with lower GWPs have less impact per ton emitted, while those with higher GWPs have more impact per ton emitted.
While carbon dioxide has a lower GWP than several other major greenhouse gases, its increasing atmospheric concentrations since the start of the Industrial Revolution have caused most of the observed global warming. Methane and nitrous oxide have slightly lower GWPs than carbon dioxide but their accumulation in the atmosphere has also been very rapid, accounting for more than half of all human-caused warming over the last 20 years.
Methane is a natural gas, produced in many different ways in nature such as from wetlands, tundra and ocean bottom sediments and from the breakdown of dead plants by microbes. In addition, the main human sources of methane are landfills, rice farming, livestock breeding and fossil fuel extraction and use.
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