Methane (CH4) is claimed to be a potent ‘Greenhouse Gas’, having an effect 20 – 100 times greater than Carbon Dioxide according to media sources. The IPCC 6th Assessment Report attributes a value of 81.2 [1]. These values are based on on a ‘per molecule’ or ‘per part per million by volume (ppmv)’ comparison which, although providing a technically interesting result, does not reflect the contribution of the gas at it’s known concentration to actual atmospheric absorption. A more useful ‘per doubling’ value is proposed and a value of 0.13 is determined.
Absorption of energy by Methane is well established Science; The characteristics have been calculated, confirmed by experiment and are catalogued in many places (NIST, HITRAN etc). The following graph shows the absorption by methane at two concentrations, 1.7ppmv and 3.4ppmv, obtained from the NASA Planetary Spectrum Generator [2]. The choice of concentrations represents the doubling favoured as a metric in climate science. The Y axis scale is the range 0-1, where 0 is no absorption and 1 is total absorption.
Radiation from the surface of the Earth is described using Planck’s equation, which is dependent on temperature. When this is considered it is seen that it is only part of the total absorption is effective:
The additional absorption resulting from the doubling of atmospheric Methane in isolation, represented by the orange region, is 2.19 Watts /m2 [3].
When the absorption of the other atmospheric constituents (Water vapour, CO2 , N2O and trace gases) are included, the effect of a doubling of Methane concentration is more difficult to observe:
It is necessary to ‘zoom in’ to see the absorption:
The increase in absorption due to the specified doubling of Methane concentration in the presence of water vapour at 80%RH is 0.42 W/m2.
The same is true of CO2; The absorption of an atmosphere containing CO2 without water vapour and other ‘greenhouse gases’ is as follows:
The increase in absorption for a doubling of CO2 concentration from 400 to 800ppmv is 7.13 W/m2, but in a ‘real’ atmosphere as described above, the actual increase in absorption is 3.29 W/m2:
Using this data, the ‘per doubling’ potency of Methane relative to Carbon Dioxide is determined as 0.42/3.29 = 0.13. The ‘per ppmv’ potency, for comparison, is (0.42/1.7) / (3.29/400) = 30.
The above was repeated at 46%RH, the value closest to the water vapour content specified in the US Standard Atmosphere, the values obtained are 0.55 W/m2 for CH4 and 3.74 W/m2 for CO2. The ‘per doubling’ potency is then 0.15 and the ‘per ppmv’ potency is 34.6.
Data and calculations used to prepare the above are contained in the spreadsheet [3].
It is also informative to consider the likeley rate of increase of CH4 and CO2; According to NOAA data [5,6], the concentration of methane has increased from 1577.6 parts per billion by volume (ppbv) in 1979 to 1895.6 ppbv in 2021 as shown in this graph:
For the purpose of comparison with Carbon Dioxide, if an exponential growth is assumed for both gases:
then the ratio of the two suggests that the CH4 doubling rate is three quarters that of CO2.
Conclusion
It is clear from the above that despite the significant ‘per ppm’ relative potency of Methane, data shows that the CH4 ‘per doubling’ potency of 0.13, which reflects realistic changes in atmospheric concentration, is a fraction of that of CO2 rather than a multiple of it. Furthermore, NOAA data suggests that the rate of increase of CH4 is notably slower than that of CO2.
Given the above, it is difficult to understand why Methane is considered such a significant ‘Greenhouse gas’.
Footnote: In calculating the above ratios, the use of ‘atmospheric absorption’ values rather than ‘radiative forcing’ values is justified on the grounds that the two are related by a constant. ‘Radiative forcing’ is the fraction of ‘atmospheric absorption’ that does not escape to Space and is determined by multiplying absorption by a constant k which has a value between 0 and 1. Simple maths tells that (a x k) / (b x k) is the same as a / b.
References:
1. IPCC AR6 Chapter 7 supplementary material: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07_SM.pdf
2. NASA Planetary Spectrum Generator: https://psg.gsfc.nasa.gov/index.php
3. Spreadsheet (.ods format) containing data and graphs: https://cw50b.github.io/CH4.ods
4. A sample configuration file for the NASA PSG: https://cw50b.github.io/psg_cfg80_400_NS_2xCH4.txt
5. “The NOAA Annual Greenhouse Gas Index”: https://gml.noaa.gov/aggi/aggi.html
6. Spreadsheet (.ods format) containing data and graphs: https://cw50b.github.io/NOAA_MoleFractions_2022.ods
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