Global Warming Potential
The emissions of different greenhouse gases can be aggregated by converting them to carbon dioxide equivalents (CO2-e). It is analogous to converting several different currencies to a common denomination. The greenhouse gases are converted by multiplying the mass of emissions by the appropriate “global warming potentials” (GWPs). GWPs represent the relative warming effect of a unit mass of the gas when compared with the same mass of CO2 over a specific period.
A 20-year GWP for methane (CH4) may be more valid than the 100-year figure used by most reporting bodies. That’s because methane, a critical factor in livestock’s climate change impacts, generally breaks down in the atmosphere to a significant extent in 9-12 years. Accordingly, a 100-year GWP (which shows the average impact over a period of 100 years) greatly understates its shorter term impact. The issue is critical when considering the impact of climate change tipping points, with potentially catastrophic and irreversible consequences.
Methane’s relatively rapid breakdown is demonstrated in the following image.
Figure 1: Breakdown of Methane (CH4) and Carbon Dioxide (CO2) 
For methane, the GWPs used by the UN’s Intergovernmental Panel on Climate Change (IPCC) are 34 for 100 years and 86 for 20 years after allowing for climate-carbon feedbacks. Without those feedbacks, the figures are 28 and 84 respectively. 
The IPCC says, “There is no scientific argument for selecting 100 years compared with other choices (Fuglestvedt et al., 2003; Shine, 2009). The choice of time horizon is a value judgement because it depends on the relative weight assigned to effects at different times.” 
NASA’s Goddard Institute for Space Studies estimates GWPs for methane of up to 33 for 100 years  and up to 105 for 20 years . NASA’s figures take into account the effects of aerosol responses. Aerosols are airborne particulates such as sulphates, nitrates, and dust from smoke and manufacturing.
Nitrous oxide (N2O) is another prominent greenhouse gas. The IPCC estimates its GWPs to be 298 for 100 years and 268 for 20 years after allowing for climate-carbon feedbacks. Without those feedbacks, the figures are 265 and 264 respectively. 
 Image: Smith, K., University of California – Berkeley, cited in World Preservation Foundation, “Reducing Shorter-Lived Climate Forcers through Dietary Change: Our best chance for preserving global food security and protecting nations vulnerable to climate change” (undated),
 Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: “Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group 1 to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change” , Table 8.7, p. 714 [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, http://www.ipcc.ch/report/ar5/wg1/
 ibid., pp. 711-712.
 Sanderson, K, “Aerosols make methane more potent”, Nature, Published online 29 October 2009, doi:10.1038/news.2009.1049; http://www.nature.com/news/2009/091029/full/news.2009.1049.html
 Shindell, D.T.; Faluvegi, G.; Koch, D.M.; Schmidt, G.A.; Unger, N.; Bauer, S.E. “Improved Attribution of Climate Forcing to Emissions”, Science, 30 October 2009; Vol. 326 no. 5953 pp. 716-718; DOI: 10.1126/science.1174760, http://www.sciencemag.org/content/326/5953/716.figures-only
14th June, 2013 (at terrastendo.net)
15th March, 2015
NASA/JPL, “Earth – Antarctica Mosaic“, ID: PIA00116