Which molecules are considered greenhouse gases?
The absorption of infrared radiation depends on molecular structure. Symmetrical diatomic molecules such as O₂ and N₂ absorb very weakly in the thermal infrared range (roughly 4 to 40 μm, corresponding to much of the Earth’s infrared emission spectrum). By contrast, triatomic or non-symmetrical molecules such as H₂O, CO₂, and CH₄ absorb infrared radiation much more efficiently. Some greenhouse gases are particularly important because of both their abundance and the position of their absorption bands. Carbon dioxide, for example, has a strong absorption band near 15 μm, which lies close to the range where the Earth emits a large share of its infrared radiation [1].
This absorption capacity is directly linked to the spectroscopic properties of molecules, and therefore to their structure and vibrational modes. Diatomic molecules such as nitrogen (N₂) and oxygen (O₂) do not significantly absorb terrestrial infrared radiation because their molecular vibrations do not produce the dipole moment changes required for efficient infrared absorption. As a result, they do not contribute significantly to the greenhouse effect [1].
How are greenhouse gases quantified?
A carbon dioxide equivalent (CO₂e) emission is the quantity of carbon dioxide that would produce the same integrated radiative forcing, over a given time horizon, as a given quantity of one greenhouse gas or a mixture of greenhouse gases. CO₂e emissions are calculated by multiplying the mass of a greenhouse gas by its Global Warming Potential (GWP) over the selected time horizon [2].
The IPCC, which introduced this metric, commonly uses a 100-year time horizon for aggregated greenhouse gas accounting (Table 1). Carbon dioxide is used as the reference gas and is therefore assigned a GWP of 1. Over a 100-year horizon, methane has a GWP of 25, nitrous oxide a GWP of 298, and fluorinated gases GWPs ranging from 7,390 to 22,200, depending on the compound considered. In other words, for the same emitted mass, methane has a warming impact 25 times greater than that of carbon dioxide over 100 years [3].
Table 1: Summary table of the main greenhouse gases, their atmospheric lifetime, and their 100-year GWP. Under the Kyoto Protocol, the Conference of the Parties decided that the GWP values from the IPCC Second Assessment Report should be used to convert greenhouse gas emissions into comparable CO₂-equivalent units for global accounting of sources and sinks.
It should nevertheless be noted that GWP is, by definition, an approximate metric (Formula 1). Its estimation depends on assumptions regarding radiative forcing, atmospheric lifetimes, background atmospheric composition, and future emissions of other radiatively active species. In addition, some greenhouse gases have overlapping absorption bands. For example, methane and nitrous oxide absorb part of the same infrared spectrum, meaning that the additional effect of one gas is not fully independent of the concentration of the others already present in the atmosphere [4].
Formula 1: Global Warming Potential equation. x denotes the gas being compared with the reference gas r, here carbon dioxide. ax(t) represents the radiative efficiency of the gas, that is, its capacity to absorb and emit radiation, while [x(t)] describes its decay in the atmosphere. The integrals are evaluated between 0 and TH, where TH is the chosen time horizon.
Bibliography
[1] Legras, B., Dufresne, J.-L., & Megie, G. (2000, June 15). Les propriétés communes des gaz à effet de serre — Planet-Terre. Planet-Terre.ens-Lyon.fr. https://planet-terre.ens-lyon.fr/ressource/gaz-effet-serre.xml
[2] IPCC. (2014). Climate change 2014 : synthesis report : contribution of working groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Ipcc, , Cop.
[3] Solomon, S. (2008b). Climate change 2007 : the physical science basis : contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. p. 212 : “Table 2.14. Lifetimes, radiative efficiencies and direct (except for CH4) GWPs relative to CO2.”
[4] Jancovici, J.-M. (2001, August 1). Quels sont les gaz à effet de serre ? – Jean-Marc Jancovici. Jancovici.com. https://jancovici.com/changement-climatique/gaz-a-effet-de-serre-et-cycle-du-carbone/quels-sont-les-gaz-a-effet-de-serre-quels-sont-leurs-contribution-a-leffet-de-serre/