The actual warming effect of Methane

 

We often hear about the relative green house effect of Methane vs Carbon dioxide on a 20 year basis or 100 year basis.  This is a perfectly legitimate way of looking at the situation since methane is oxidized relatively rapidly in the atmosphere while Carbon dioxide remains much longer.  Carbon dioxide is taken up by various processes and notably by photosynthesis.  The half life of Methane in the atmosphere is about 7 years since it is oxidized to carbon dioxide, while the half life of Carbon dioxide is estimated at about 100 years.

 

What I'm talking about here is the relative green house gas forcing of the amount of Methane and Carbon dioxide in the atmosphere at a given moment. 

 

In this chart from1998 you can see  that the total radiative forcing of 365ppmv (parts per million by volume) of Carbon dioxide is 1.46 Watts per square meter of the earths surface.

The radiative forcing of 1.75ppmv of Methane is 0.48W per square meter.

So doing a simplistic calculation*, if Methane was 365ppmv it would have a forcing of 355/1.75 X 0.48 = 100.1Watts per square meter.

Since the forcing of the 365ppmv of Carbon dioxide is 1.46W/m2, Methane is 

100/1.46 = 68.6 times as powerful a green house gas as Carbon dioxide.

*The calculation is only a first approximation.  This has to do with the fact that the effect of green  house gasses increases a given amount for a doubling of the concentration.  It is not a linear function.  In other words the increase in the warming effect  would be the same from 100ppm to 200ppm as from 200ppm to 400ppm.

However, at present (April 2022) the concentration of Methane in the atmosphere is 1.91ppm while the concentration of Carbon dioxide is 418ppm.  The effect of Methane is equal to 1.91 X 68 = 130 ppm of Carbon dioxide.  It is, as if we had 418 + 130 = 548ppm of Carbon dioxide in the atmosphere.

What is the importance of this.  It looks as if we are destabilizing huge reservoirs of methane clatherates, both on the ocean bottom and below permafrost.  If we have major blow outs of methane, the effect on our climate could be truly catastrophic.  There will be no controlling the acceleration in warming.  The rate of rise of methane in the atmosphere is accelerating, suggesting that we have already begun to have such an effect.

Another worrying factor is that with a huge output of methane, we might overcome the mechanism in the atmosphere that is responsible for the oxidation of Methane into Carbon dioxide.  That would truly be catastrophic.