5 ms·

Methane, molecule per molecule, has an infrared-cross section about 20 times that of carbon dioxide. However, the current ratio of methane (1.9 ppm) to CO2 (42

Methane, molecule per molecule, has an infrared-cross section about 20 times that of carbon dioxide. However, the current ratio of methane (1.9 ppm) to CO2 (420 ppm) is only ~ 1/220, so the total methane forcing is about 10% that of CO2, very roughly. (Pre-industrial methane was only 0.6 ppm as per ice core data) Note also Arctic permafrost is going to be a steady source of methane as it thaws, at a relatively slow rate, currently only 1% of the total atmospheric methane budget. Shallow marine sediments are another likely source, although that methane might get gobbled up by marine bacteria before it exits to the atmosphere, to some extent. In any case the stated goal of 'a rapid reduction in methane emissions to 30% below 2020 levels' would have little noticeable effect on warming rates over the next century. Just look at the global trend rate as well, methane's at record levels and the rate is steepening, not flattening. https://www.noaa.gov/news-release/increase-in-atmospheric-methane-set-another-record-during-2021 https://www.noaa.gov/news-release/increase-in-atmospheric-me...

scrozart 4y ago
Thanks for actually using data and thoughtful comment. This thread is a microcosm for HN's decline - loads of flip comments and even more supposition without any sources.
onlyrealcuzzo 4y ago
> In any case the stated goal of 'a rapid reduction in methane emissions to 30% below 2020 levels' would have little noticeable effect on warming rates over the next century. > Just look at the global trend rate as well, methane's at record levels and the rate is steepening, not flattening. ^ This is the problem! Methane is 25x worse than CO2. Exponential growth instead of a 30% reduction (and then flat or still declining) makes a big difference! Over the span of 3 years - not really. Over the span of 30 years, yes!
at_compile_time 4y ago
>Methane, molecule per molecule, has an infrared-cross section about 20 times that of carbon dioxide. How do you square that with the IPCC estimate that methane is 84 times more potent than CO2 over a 20-year period?
- MagnumOpus 4y ago
  Methane decomposes into CO2 over centuries. So CO2's atmospheric forcing is a constant over time, while methane forcing starts 100s of times worse but decays over time. Over 100 years the forcing is on average about 30x higher, over 20 years on average about 84x higher[1]. [1] https://unece.org/challenge https://unece.org/challenge
- mzs 4y ago
  so about 40% instead of 10%
- sampo 4y ago
  > How do you square that with the IPCC estimate that methane is 84 times more potent than CO2 over a 20-year period? Maybe: https://en.wikipedia.org/wiki/Global_warming_potential#Calculating_the_global_warming_potential https://en.wikipedia.org/wiki/Global_warming_potential#Calcu...
- photochemsyn 4y ago
  I think that's mostly the effect of methane oxidation in the stratosphere delivering water vapor to the stratosphere, plus reduction of the concentration of OH radical in the atmosphere, and some effects on stratospheric ozone: > "Methane enhances its own lifetime through changes in the OH concentration, it leads to changes in tropospheric ozone, enhances stratospheric water vapour levels, and produces CO2. The GWP given in Table 2.14 includes the first three of these effects. The lifetime effect is included by adopting a perturbation lifetime of 12 years." - IPCC Report (That report indicates the radiative factor is more like 26:1 for CH4:CO2)
yread 4y ago
methane was 0.6ppb and is 1.9ppm? did we really increase its concentration 3000 times?
gmuslera 4y ago
Not just permafrost. Due to increased temperatures wetlands are emitting more methane (https://cires.colorado.edu/news/new-analysis-wetlands-other-microbial-sources-fueling-rise-atmospheric-methane https://cires.colorado.edu/news/new-analysis-wetlands-other-...). That is another positive feedback loop, besides the permafrost one, that is increasing the amount of methane in the atmosphere.

scrozart 4y ago

Thanks for actually using data and thoughtful comment. This thread is a microcosm for HN's decline - loads of flip comments and even more supposition without any sources.

onlyrealcuzzo 4y ago

> In any case the stated goal of 'a rapid reduction in methane emissions to 30% below 2020 levels' would have little noticeable effect on warming rates over the next century. > Just look at the global trend rate as well, methane's at record levels and the rate is steepening, not flattening. ^ This is the problem! Methane is 25x worse than CO2. Exponential growth instead of a 30% reduction (and then flat or still declining) makes a big difference! Over the span of 3 years - not really. Over the span of 30 years, yes!

at_compile_time 4y ago

>Methane, molecule per molecule, has an infrared-cross section about 20 times that of carbon dioxide. How do you square that with the IPCC estimate that methane is 84 times more potent than CO2 over a 20-year period?

MagnumOpus 4y ago
Methane decomposes into CO2 over centuries. So CO2's atmospheric forcing is a constant over time, while methane forcing starts 100s of times worse but decays over time. Over 100 years the forcing is on average about 30x higher, over 20 years on average about 84x higher[1]. [1] https://unece.org/challenge https://unece.org/challenge
mzs 4y ago
so about 40% instead of 10%
sampo 4y ago
> How do you square that with the IPCC estimate that methane is 84 times more potent than CO2 over a 20-year period? Maybe: https://en.wikipedia.org/wiki/Global_warming_potential#Calculating_the_global_warming_potential https://en.wikipedia.org/wiki/Global_warming_potential#Calcu...
photochemsyn 4y ago
I think that's mostly the effect of methane oxidation in the stratosphere delivering water vapor to the stratosphere, plus reduction of the concentration of OH radical in the atmosphere, and some effects on stratospheric ozone: > "Methane enhances its own lifetime through changes in the OH concentration, it leads to changes in tropospheric ozone, enhances stratospheric water vapour levels, and produces CO2. The GWP given in Table 2.14 includes the first three of these effects. The lifetime effect is included by adopting a perturbation lifetime of 12 years." - IPCC Report (That report indicates the radiative factor is more like 26:1 for CH4:CO2)

yread 4y ago

methane was 0.6ppb and is 1.9ppm? did we really increase its concentration 3000 times?

gmuslera 4y ago

Not just permafrost. Due to increased temperatures wetlands are emitting more methane (https://cires.colorado.edu/news/new-analysis-wetlands-other-microbial-sources-fueling-rise-atmospheric-methane https://cires.colorado.edu/news/new-analysis-wetlands-other-...). That is another positive feedback loop, besides the permafrost one, that is increasing the amount of methane in the atmosphere.