The IV IPCC Assessment Report estimates the degree of warming caused by increasing CO2 as about a factor of 3.6 greater than by increasing CH4, on a molecular basis.

You can utilize the MODTRAN calculation model (Univ. Chicago), which uses IR absorption data, to display graphically the relatively IR frequency effects when comparable amounts of CH4 and CO2 are added. When I did this last year, I found that CO2 produces about four times the warming of CH4, on a molecular basis.

The variable and quite high CH4/CO2 warming ratios one sees (I have seen ratios up to 80) depend on how the comparison is made. That is how yo pose the question varies the answer. CH4 occurs in much lower atmospheric concentrations than CO2, and the relative effect of the two depends on the concentration levels. Probably the best way to compare warming potential between CH4 and CO2 is to compare a future atmospheric increase similar to that which has occurred in recent decades. Over 1984 to 2019, atmospheric CO2 increased by 20%, and atmospheric CH4 increased by 14%. If we assume a future increase of 20 % for each gas, giving future CO2 =486 ppm and CH4 = 2.16 ppm, then the MONTRAN model indicates an outgoing IR decrease for CO2 of 0.88 w/m2 and a decrease for CH4 of 0.22 w/m2. Those values mean that similar fractional increases in atmospheric CO2 and CH4 would produce four times more warming from CO2 than from CH4.

The relative warming contribution from H2O is more difficult to estimate. MODTRAN estimates H2O warms about 2.5 times as much as CO2. That would imply CO2 produces ~25% of greenhouse warming and current CH4 ~6-7%.

Thanks a lot for your answer Donald. The value of 104 times the warming potential of a CH4 molecule compared to a CO2 molecule over ten years i had through personal conversation with a climate scientist. Never found one in the literature. But it seemed likely because it should be substantially more than the ~80 over 20 years because of the lifetime of methane in the atmosphere being 8-10 years.

So i can not really use this number to understand the direct effect on the warming of our planet. Anyway, the result seemed much too high for me. Therefore, better to ask before making such a rough, wrong and frightening calculation.

But even the IV IPCC number is frightening enough but it is from 2007 (much lower increase then) so i will read the paragraph where this is mentioned to see over which time frame it is calculated.

And thanks a lot for the answer - hope that i will find one day a study on this topic where the direct forcing on the annual warming by rising methane levels is thematized, because it is, as i thought, much more complicated than just using these number of the direct climate forcing over time.

But crazy that 25M tons of methane more or less with total emissions being ~550 M tons can have such a huge impact in terms of atmospheric levels halting on a plateau or showing a steep increase.

Cheers and a good new year!

Jan

Jan

There are different ways to compare relative warming potential of CH4 and CO2, which is why one sees CH4/CO2 warming ratios varying from a few to almost 100. Much of the confusion occurs because atmospheric CH4 is less than 2 ppm, whereas CO2 is over 400 ppm. How then does one compare -- on an additional ppm basis, on a proportional addition basis., on past actual increases, or ???

If 1 ppm additional CO2 were added to the atmosphere, the MODTRAN model indicates essentially no change in IR flux. If 1 ppm additional CH4 were added (an ~50% increase), the outgoing IR flux is reduced by 0.9 W/m^2. However, if one started with an atmosphere containing no CH4 or CO2 and added both proportionally, then CO2 produces greater relative warming. Also, If existing atmospheric CO2 and CH4 concentrations were each increased by 1% (4.1 ppm for CO2 and 0.018 ppm for CH4), then the CO2 increase produces the greater warming.

Thus, given current CH4 and CO2 concentrations, as CH4 grows, its warming effect relative to CO2 decreases.

As I said above, using past increases in CH4 and CO2 as a guide (1984 to 2019) and assuming future increases or 20% for each, MODTRAN indicates an outgoing IR decrease of 0.88 W/m^2 for CO2 and 0.22 W/m^2 for CH4, for a CO2 warming ratio of ~4.

Further, atmospheric CH4 reacts with O2 to produce CO2 and H2O with a survival time of only a decade or so.

Here is the MODTRAN model site. You can test effects of various gas mixtures on IR loss rates yourself.

http://climatemodels.uchicago.edu/modtran/

Hi Donald, just wanted to say that i play around with MODTRAN.

Thanks again!

Jan