Is the relative contribution to global warming of greenhouse gasses settled science?

Submitted by BrndNwAccnt t3_10y6emc in askscience

When I look up the global warming potential of methane, most quickly-accessible answers are 25x CO2. The IPCC reports it as 84. This is a huge discrepancy, and would greatly change the viability of mitigation approaches. Is one right and one wrong? If they both have merit, in which situations would it be appropriate to consider one over the other?

Edit: I’m asking with regards to the application/assessment of carbon credits, which regulators will evaluate based on (a degree of) consensus about which multiplier is appropriate. The two numbers used most commonly are 25 and 84, and I am trying to gain an understanding of which is most likely to stand up to scrutiny/be most accepted in calculation.

Edit2: some great answers, thanks! I do have to wonder how many came from ChatGPT…

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stdio-lib t1_j7y8nr3 wrote

> most quickly-accessible answers are 25x CO2. The IPCC reports it as 84. This is a huge discrepancy

You're probably comparing apples to oranges. Their effects are not time invariant: methane has a stronger effect in the short term but breaks down sooner, whereas enough CO2 to cause the same amount of warming in that period would have a far more disastrous effect in the long term.

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UnamedStreamNumber9 t1_j7yxz5x wrote

Methane breaks down to CO2. The discrepancy is mostly like due to one method looking at the “instantaneous” warming effect vs the warming for total time in atmosphere. Methane warm at the greater rate for 3-5 years before breaking down to CO2. CO2 in the atmosphere has a residence time on the order of a century. So, methane has the much higher warming potential (84x) by integrating its 3-5 years at 25x CO2 and then another century at same as CO2

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Any-Broccoli-3911 t1_j7z0pwq wrote

If you average 5 years of 25x and 100 years of 1x, compared to 100 years of 1x (CO2), that's 2.25x in average.

The long term average will always be smaller than the short term one.

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BrndNwAccnt OP t1_j7y94dh wrote

Thank you. If you are still interested the edit on my post may help clarify what I’m looking for.

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cheses t1_j803mk8 wrote

Just look at wikipedia: https://en.wikipedia.org/wiki/Global_warming_potential

It is always important to look at the time scale. Methane has a lifetime of 12 years. This means for a a longer time frame (most of the time we are looking at a 100 year time scale), the effect of methane shrinks as it does nothing for 88 years, but a lot in 12 years.

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VolkerRacho t1_j809nk4 wrote

What becomes of the methane after the its lifetime?

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ivanthekur t1_j80dge0 wrote

I believe it mostly oxidizes turning into water vapor and carbon dioxide.

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UnamedStreamNumber9 t1_j856sz7 wrote

Indeed it does, both of which are greenhouse gasses in their own. The water has a relatively quick recycle but the CO2 has a longer one, and needs to be treated as warming attributed to the original methane emission as it is CO2 which would otherwise not be in the atmosphere

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Sub0ptimalPrime t1_j7zprl9 wrote

Just a side point that I think is relevant to this discussion: It's almost impossible to accurately measure the massive amounts of GHGs from any single source, much less from ALL of them. This is not to say that they shouldn't be approximated and they could end up being pretty accurate percentage-wise, but the exact measurements are assuredly wrong just because the system we are trying to measure is so HUGE and dynamic. Science is never settled, but this is currently the best guess based on other observed principles.

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[deleted] t1_j7y8c03 wrote

[removed]

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OvershootDieOff t1_j7yf7yp wrote

Actually climate models are doing pretty well. As was predicted by the models decades ago glaciers have retreated, sea ice declined, sea level increased, diurnal temperature ranges have decreased, increased periods of droughts, more intense rainfall, heat records being broken, altered ocean currents etc etc

All were forecast and all have now been observed. Most people saying climate science isn’t robust are motivated by political views.

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Pablo-on-35-meter t1_j7yl5zt wrote

And thus, we keep discussion about a few percent here or there instead of accepting that we never can be accurate in our predictions but should be looking at the overall trend. Ofcourse, we are in a bad way, the overall signs are clear. What do you do when you are driving a car.and.you see a child playing with a ball? You slow down and anticipate the worst, that the kid suddenly will cross the street. So, why do we.continue full blast with our emissions? Let's sort out the 25 issue later, just accept that it is bad and reduce. But.... We are building coal fired powerplants like crazy, we burn more wood, we keep flying long distance to our holidays and do not seem to be willing to take a serious step back... No, let's discuss about the percentages. And then what? Just continue because maybe it is 24.8? Get real, accept that we will never know exactly how it works and just like the weather predictions: live with the uncertainty but anticipate the worst.

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BrndNwAccnt OP t1_j7y8yhe wrote

I’m asking with regards to the application/assessment of carbon credits, which regulators will decide based on (a degree of) consensus about which multiplier is appropriate. The two numbers used most commonly are 25 and 84, and I am trying to gain an understanding of which is most likely to stand up to scrutiny/be most accepted in calculation.

1