Methane degrades to C02 in 1-1 proportion. So at any time, if more atmospheric carbon is concentrated as methane than as C02 then the greenhouse effect will be stronger at that time (because methane is a stronger GHG). If agriculture removes 1 unit of C02 from the atmosphere (from feedstock) and emits 1 unit of methane (from cow) then total atmospheric carbon is unchanged but the proportion of methane, and therefore the total GHG effect, is increased at that time. Summing over time, and you get an increase in overall heat that has been trapped.
It's not like mammalian herbivores are a recent invention. If a constant amount of mammalian herbivores in the world would cause a constant acceleration of greenhouse effect then the greenhouse effect would have increased exponentially for millions of years already. That has not been the case.
If you increase the number of mammalian herbivores in the world then the greenhouse effect will increase for about a decade because of increasing levels of methane. After a decade, the methane level (and therefore the greenhouse effect) will have found a new (higher) equilibrium and no longer increase.
And, also, we kinda quadrupled the cheptel size in 50 years ? https://ourworldindata.org/meat-production#global-meat-produ...
(To be fair, I was expecting more, and I was again astonished by the relative numbers of cattle vs poultry)
Factory style livestock farming is a recent invention. There's no way a similar amount of herbivores existed a million years ago in the wild compared to what we can currently cage and feed corn product to.
In contrast, using fossile sources of carbon (oil etc) will increase the greenhouse effect every year even if the usage is constant.
This is not correct. We are already in a steady state. Methane levels due to cattle are already at their maximum (given a fixed amount of cattle).
At any point in time, gras will absorb some CO2 from the atmosphere, cattle will convert some gras / carbon into methane and emit the latter into the atmosphere, and some methane in the atmosphere will decompose into CO2. This is a continuous process, that's happening every second, and if the amount of cattle is kept constant, there is a maximum amount of methane that you can reach in the atmosphere as well as a maximum amount of CO2. After all, the total amount of carbon involved in the process is constant – energy conservation dictates that mass doesn't appear out of nowhere.
In fact, since we've been doing agriculture for a time much longer than the time scale in which grass grows, a cow digests grass or methane decomposes into CO2, this whole closed loop has already settled into a steady state: At any point in time, the amounts of methane and CO2 in the atmosphere due to cattle are constant. The resulting radiative forcing will thus stay constant, too.
> ...and if the amount of cattle is kept constant...
My point (unclear) is that this is not a constant, but a variable (in the sense that it is a societal choice). Animal agriculture shifts more of the distribution of atmospheric carbon at any time toward methane, which accelerates warming.
> ...this whole closed loop has already settled into a steady state: At any point in time, the amounts of methane and CO2 in the atmosphere due to cattle are constant.
The premise that the amount of animal agriculture is constant is incorrect. It has been increasing year over year for some time as the human population increases. The theoretical concentration of methane has been increasing over that same time. It's also worth noting that industrial animal agriculture is not a closed loop because it currently relies on mined hydrocarbons for feedstock fertilizer and transport.
> The premise that the amount of animal agriculture is constant is incorrect.
That certainly is true – I just had a look at the exact numbers: Cattle has seen a 15.6% increase between 2000 and 2020[0]. However, even if you double the numbers in my calculation in my other comment[1], you will see that agricultural greenhouse gases in the atmosphere are still basically nothing compared to those caused by fossil emissions.
> because it currently relies on mined hydrocarbons for feedstock fertilizer and transport.
Many types of food require fertilizer for production and logistics. I'm not trying to say that animal agriculture might not be comparatively bad here but we certainly won't get rid of fertilizer- and logistics-related emissions over night by overcoming animal agriculture. In fact, it could be that we'd just replace one emission source with another. So, before prescribing vegan diets for everyone, I'd really like to see some numbers that suggest that such a move would actually change anything.
[0]: https://www.destatis.de/EN/Themes/Countries-Regions/Internat...
Source? I understood that the long-term radiative forcing of methane was lower than CO2, which is incompatible with this claim.
Although atmospheric losses to space and capture into oceans and subsequent precipitation into hydrates cannot be excluded as potential atmospheric methane sinks, I expect those to be only marginal at the grand scheme of things.
Indeed: https://en.m.wikipedia.org/wiki/Atmospheric_escape#Earth
