How much warming in the middle #Miocene 15million years ago came from methane?
Methane is not constrained at all for the Miocene.
But I did the maths –yet I also warn you: I am maths dyslexic. 😁

tldr: with assumed 10 times more wetlands than today and all of the remaining landmass assumed to be like today's tiny "wild rest",
CH4 emissions were 2124 Mt per year.
Which amounted to 6608 ppb CH4 in the atmosphere which in itself caused +2.1°C .

CO2 in 15Ma is not well constrained either. (see below)
I calculate 560ppm to have contributed 3°C (current science working theory for ECS ±1).

So methane 2.1°C and CO2 3°C on their own, omitting all other climate factors, caused +5.1°C in the Miocene.

The breakdown of the numbers follows. With links.

# CO2:

Hoenisch et al 2023 published meticulously revised CO2 values from global #d13C proxies https://paleo-co2.org , their considered-best proxies are all oceanic in origin.

The chart #1 of 1milion years 15 million years ago, shows #Hoenisch ' s CO2 proxies as the horizontal lines. I chose to fill the gaps with repeated values between the rare data points. So each line segment really is only 1 data point at its right-most end.

560 ppm CO2 seems an okay guess, no?

#CH4 #methane

@Peters_Glen did a cool chart, more intuitive than the one in #AR6, I think. See pic 2 or his tweet where he plots the various greenhouse gases with their warming contribution 2010-2019: https://x.com/Peters_Glen/status/1431873249449680901

The average CH4 concentration in the decade 2010-2019 was 1840ppb (NOAA) and caused +0.51°C as per Glen's chart.

From Glen's chart follows my secret methane formula 😁
1 Mt methane <=> 3.111 ppb <=> 0.001 ºC

If emissions in 15Ma were 2124 Mt CH4 (see #landmass below), it resulted in 2.12°C at a concentration of 6608 ppb.

#Landmass

According to the Global Methane Budget by #GlobalCarbonProject : https://essd.copernicus.org/articles/12/1561/2020/

emissions from the "wild rest" 2008-2017 were 222 Mt CH4 annually . See picture 3.

The wild rest today is 54mio km2, according to #OurWorldInData https://ourworldindata.org/global-land-for-agriculture

Wild rest: 222 Mt CH4 from 54mio km2 = 4.1 t CH4 / km2.

Emissions from wetlands 2008-2017 were 180Mt CH4 (Tg=Mt) .
They cover 4.37% of the total land mass: https://ourworldindata.org/grapher/coverage-of-wetlands?tab=chart&country=~OWID_WRL
4.37% of 141mio km2 total is:
Wetlands 6.2mio km2.

Wetlands: 180 Mt from 6.2 km2 = 30 t CH4 / km2.

In 15Ma Miocene, 10 times more wetlands would have been
62 mio km2.
And
wild rest 79 mio km2.

wetlands 62mio km2 times 30t CH4 = 1800 Mt CH4
wild rest 79 mio km2 times 4.1t CH4 = 324 Mt CH4.

Wetlands plus wild rest:
1800 Mt + 324 Mt = 2124 Mt CH4

secret methane formula:
1 Mt methane <=> 3.111 ppb <=> 0.001 ºC

2124 Mt <=> 6608 ppb <=> 2.12°C

Why do I assume that wetlands were 10x more than today, tho? Why not 15, 20 or 5 times more?

Dunno. Well, humans have unwetted lotsa wetlands since the invention of agriculture in the #Holocene. (Btw, the area of today's dried peatland alone emits 2Gt CO2 per year. See table on dried wetland areas and their emissions GHG:
https://www.nature.com/articles/s41467-020-15499-z/tables/2 #Günther et al 2020, based on IPCC guidelines for wetlands https://www.ipcc.ch/publication/2013-supplement-to-the-2006-ipcc-guidelines-for-national-greenhouse-gas-inventories-wetlands/ )

Hard to tell what area was covered by wetlands in the previous interglacial 126thsd years ago.

And in the middle Miocene, 15Ma?

My thinking goes like this:
The #Sahara was still forested 15Ma. As was the Gobi Desert probably. The prairies in the US were still forested, even #Greenland and #Antarctica. Northern #Russia had much more land mass back then, too.

Some of the different vegetation compared to pre-Holocene was due to different topography: the Rockies and Alpes were much, much lower, the high mountain ranges in East Asia didn't exist. #Australia was 15° further South. See also #Steinthordottir et al 2021 in "Miocene The Future Of The Past https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020PA004037
And the whole special Miocene issue:
https://agupubs.onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)2572-4525.Miocene1

All land area had gap-less biomes growing. Mostly forests. What do forests do? Away from the coast, within the continents, forests control the hydrological cycle, how much evaporates and how much it rains. All biomes do, but forests most.
The more forests there are, the more it rains. Uninterrupted plant cover with its propagating rain cycle hinders deserts from forming in the heart of the continents, too.

Also, air holds 7% more water per 1°C warming, raising the potential rain amount.

Now, if it rains a lot, and depending on the topography, land is inundated temporary, seasonally or permanently, methane-producing microbes in the soil get to work presto, eat carbon and fart CH4.
The warmer it is, the more the microbes work.

But why 10x more wetlands?
Why not 7 or 15x?
Dunno. 10 feels right. And 6608ppb is nicely close to a guesstimate of mine that mid Miocene CH4 concentration cd have been 7000 ppb.
Maybe 400ppb came from huge animals, happily roaming among giant trees.
Brazil's Giant Sloth? The "wild rest" in the Miocene was HUGE! And cute.
#FridaysForFuture
#anloCH4