It has been due to the methane gas has heat trapping efficiency is about 20 times the carbon dioxide.

Methane is a stronger green house gas than CO2 because it has much higher heat trapping ability and on a weight basis 21 times more potential of CO2 for global worming.

Dear Tamru Tesseme and J. C. Tarafdar ,

Thank you for your replies. However, what I really want to know is how these values: 20, 21, 87 are determined. I have edited the title of my question to reflect that.

Regards, Alastair.

You can find it in many places in the scientific litterature. A pretty clear source is however IPCCs third assessment report, chapter 6. Look at page 358 (page 10 of the chapter 6 pdf). In table 6.2 you find the expression to calculate the radiative efficiency (RE) of a unit mass GHG for several GHGs, amongst them CO2 and CH4.

What you do is to calculate the RE of CH4 and CO2 plug it into the GWP formula.

The GWP formula is also everywhere, but you can look at page 710-711 of IPCCs fifth assessment report (the physical science basis) (page 52 and 53 of chapter 8). What it does is to take into account the different atmospheric lifetimes of different GHGs.

CH4 is actually more than 100 times stronger, when it comes to RF, than CO2. It is however removed from the atmosphere more quickly. CH4 is oxidised. CO2 is a stable end-product of CH4 and other GHGs containing C. It is only removed through natural uptake by sinks, such as forests and oceans, which makes it more persistent in the atmosphere and more of a long term problem then CH4.

But to calculate the values you are asking for, study table 6.2 in AR3 and the GWP equation. GWP is really sensitive to the time horizon chosen (TH in the GWP formula) when comparing long-lived and short-lived GHGs.

hope that helped a little

some links to the AR chapters i mentioned :

https://archive.ipcc.ch/ipccreports/tar/wg1/pdf/TAR-06.PDF

https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter08_FINAL.pdf

Niclas Ericsson

Thank you for your full reply.

I now realise that the 86 factor was for the Global Warming Potential (GWP) over 10 years and the 20 and 21 factors mentioned above were for GWP over 100 years. See Table 6.7 https://archive.ipcc.ch/ipccreports/tar/wg1/pdf/TAR-06.PDF

Table 6.2 shows how Radiative Forcing (RF) can be calculated, but Table 6.1 shows CO2 RF as 1.46 W m-2 and CH4 RF as 0.48 W m-2 a factor of~ 3 less.

Thanks again for those links.

My bad. I wrote radiative forcing above instead of radiative efficiency. That probably confused you a little, even if the two concepts describe the same thing. Table 6.2 shows the total radiative forcing of the increased concentration of each gas in year 1988, relative to 1750. Since CO2 has increased much more than CH4 the effect from CO2 is 3 times greater, although CH4 is >100 times stronger (radiative efficiency).

Might be good for you to know that the reason the GWP values of CH4 have changed between different IPCC reports is from including more indirect effects from CH4 in later reports, and even more important, including the carbon-climate feedback in the latest report.

Finally, the GWP of all GHGs change as the concentration of CO2 increases in the atmosphere. This is due to the effect of CO2 on the CO2 lifetime in the atmosphere itself, which affects the climate impact of CO2. This is a direct consequence of using CO2 as the reference gas: the GWP of all other GHGs change as the climate impact of only CO2 changes (which it does constantly due to fossil fuels). Just remember, the climate impact of other gases have not changed the way it appears if you stare at the evolution of GWPs over the years.

I have not heard the term radiative efficiency before, so I was not confused by its non-use :-)

I Googled it and found this paper:

Article Global warming potentials and radiative efficiencies of halo...

Section 2 'Background Concepts' has a good description of how it is determined from the "fundamental spectroscopic properties of a molecule." That was what I wanted to know, so you have been very helpful. Thanks again.

The GCMs, besides failing to consider ocean surface temperature cycles or solar cycles, erroneously failed to consider local measurements of water vapor.

Water vapor has been increasing in parallel with the increase in CO2. Both are contributed to by human activity. Both are active absorbers of IR in the wavelength range associated with earth temperatures, i.e. they are both greenhouse gases.

An early guess, still clung to by many, was that the warming was caused by CO2 increase amplified by feedback from water vapor. Deeper penetration into science using output from Hitran (data derived using Quantum Mechanics) determines that each water vapor molecule is about 1.37 times more effective at warming (absorb/emit of thermal radiation) than a CO2 molecule.

Both CO2 and water vapor (WV) have been accurately measured worldwide for more than 30 years; CO2, which is evenly distributed (±2%) over the world, at Mauna Loa Hawaii and WV by NASA/RSS using satellite sensors. The molecule increases in parts per million have been 441 for WV and 59 for CO2. Therefore, WV increase has been 441/59*1.37≈ 10 times more effective at increasing ground level temperature than the CO2 increase.

An independent corroboration of the importance of the increase of water vapor is obtained using average global water vapor (TPW) measurements and average global temperature measurements. This shows that WV has been increasing faster than POSSIBLE from feedback. The irony is that reducing the use of fossil fuels, which is agenda of many, will have no effect on climate.

https://watervaporandwarming.blogspot.com

The small increase in ground level warming from added CO2 is countered by increased cooling in the stratosphere from the added CO2 with the end result being that CO2 does not now, never has and never will have a significant effect on climate. The lack of influence of CO2 on climate is corroborated by multiple compelling evidence listed in Section 2 of http://globalclimatedrivers2.blogspot.com