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It is always told that eating meat is bad for our climate. This is most often explained by mentioning following reasons (Source):

  • Methane is produced when digesting the food and also the faeces of cows produces greenhouse gasses afterwards
  • Fertilizers produce greenhouse gases
  • The processing of food with machines produces CO2
  • The farming of land with tractors etc. produces CO2

The last three points I can understand. But I have some problems understanding the first. My questions in this regard are:

  1. Cows emit methane. But the methane is produced by digesting plants. The plants themselves got the carbon from the atmosphere. So at least this is a closed cycle and no net greenhouse gases are emitted. Isn’t livestock therefore climate neutral?
  2. It may be that especially for factory farming, forests are cut down. This produces a lot of CO2. But I also heard that the plants that are grown may store more carbon then the trees that were there before. Is this true? If no, is this really the only reason meat is not carbon neutral?
  3. How is the carbon footprint of livestock even determined? Is it really determined by the forest which was cut down in order to produce food or is just the methane exhaust of the livestock measured?

Own research

In this source it is stated that grass feed cows may be carbon neutral in the long run (under optimal conditions) and may even increase carbon capture in the short run. Nevertheless, nothing is stated why this is not true for crop feed cows (how do crops and grass differ in this regard?)

On this site it is stated that “[Only] In a stable climate, trees store more carbon than grasslands”. But what’s about crops like wheat? They have more biomass and deeper roots than simple grasses. Also there may be several wheat plant on the area one tree takes. I found nothing regarding this on the web.

Ps. I am not a biology professional so please forgive me if I misuse some terminology and do not cite scientific papers.

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3 Answers 3

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Short answer
Two factors playing a role in livestock's contribution to the greenhouse effect have to do with methane being a more potent greenhouse gas and the fact that the conversion of plant materials into meat comes with low efficiency.

Background
Up front - I'm not a climatologist. My 2 cents worth here are two points:

  1. Methane is a potent greenhouse gas, and way more potent than CO2. Quoted from UN Environment Programme's website:

Methane is [...] a powerful greenhouse gas. Over a 20-year period, it is 80 times more potent at warming than carbon dioxide. Methane has accounted for roughly 30 per cent of global warming since pre-industrial times and is proliferating faster than at any other time since record keeping began in the 1980s.

and

Livestock emissions [...] account for roughly 32 per cent of human-caused methane emissions.

  1. Another issue is the food pyramid, which consists of different layers: organisms higher up in the food chain consume biomatter from lower layers. Every layer dissipates heat, because of efficiency being less than 100% (Fig. 1). Often live stock is fed on crops specifically produced for the meat sector. If the production of all this animal fodder would instead have put to use to grow veggies for human consumption, a lot more food would have been available than when it is inefficiently cycled through live stock first to turn it into meat (and methane).

food pyramid
Fig. 1. Transfer of energy through an ecosystem. At each trophic level only a small proportion of energy (approximately 10 percent) is transferred to the next level. source: Encyclopædia Britannica

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  • $\begingroup$ To illustrate trophic level loss for the OP: What would feed you longer? One rabbit? Or the vegetables you fed that rabbit over its lifetime to raise it? You feed a lot more people with less resources eating vegetables than eating rabbits. $\endgroup$
    – DKNguyen
    Jan 6, 2023 at 20:06
  • $\begingroup$ @DKNguyen that's the point, yes $\endgroup$
    – AliceD
    Jan 7, 2023 at 15:22
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Short Answer

I'll address the first two of your three sub-questions:

  1. Livestock are not climate neutral because different processes of the carbon cycle create different carbon-based molecules at different rates and that linger in various "reservoirs" throughout the globe for different lengths of time. Given these details and the fact that the underlying chemistry of the different C-based molecules varies, each carbon-based molecule has varying degrees of effect on the climate.

  2. Croplands have drastically lower carbon storage ability than forests, natural grasslands, and wetlands. You can compare global measures of aboveground and belowground biomass to determine this.

    • The model-driven analysis of the paper your link cites is taken out of context and its comments on resiliency are not generalizable in understanding absolute carbon storage capacity. The paper comes with many caveats.

Long answer

  1. You're inappropriately assuming that all carbon molecules are the same.
  • There is a finite amount of matter in the universe, including carbon. You probably know this from the Law of conservation of matter. However, one carbon-based molecule does not necessarily (and in fact does not) have the same physical and chemical characteristics of other dissimilar carbon-based molecules.

    • Importantly, not all carbon-based molecules are greenhouse gasses, which have unique reactivity and physical structure to function as re-emitters of infrared radiation. See here for an explanation and here for a simplified YouTube video cartoon explanation by "Minute Earth".

    • Carbon returns to the atmosphere typically via combustion or decomposition. (See comments about the carbon cycle below). Much of the carbon released by these processes are either CO2 or CH4.

      • The carbon cycle reflects the law of conservation of matter in the recognition that carbon transfers from one object to another via biogeochemical cycling; however, how it transfers varies both in the chemical and physical processes occurring, in the time spent in one "place" (i.e., a nutrient/chemical "reservoir") vs another, and in how readily it "comes"/"goes" from one place to another (i.e., its flux rate).

        • The timing of residence times and flux rates (i.e., movement from one place to another) dictate whether a reservoir acts as a source or sink.

          enter image description here

      • CO2 (carbon dioxide) and CH4 (methane) are quite different in the reactions from which they are generated (see Would fewer cows mean less methane emission?), the rate at which they are generated, and ultimately in their chemistry. This last point has drastic impacts on residence times, flux rates, and ultimately their their climate warming impacts:

        • Both molecules differ in their residence times in the atmosphere as well as their magnitude of radiative forcing. From MIT:

          methane immediately begins to trap a lot of heat—at least 100 times as much as the CO2. But the methane starts to break down and leave the atmosphere relatively quickly. As more time goes by, and as more of that original ton of methane disappears, the steady warming effect of the CO2 slowly closes the gap. Over 20 years, the methane would trap about 80 times as much heat as the CO2. Over 100 years, that original ton of methane would trap about 25 times as much heat as the ton of CO2.

        • Note, also, that some of the atmospheric methane will eventually be chemically converted to carbon dioxide. From here.

          Methane enters the atmosphere and eventually combines with oxygen (oxidizes) to form more CO2. Methane converts to CO2 by this simple chemical reaction.

Regarding methane production of cows and rates of release of greenhouse gasses, please see my other recently-answered BIO.SE post: Would fewer cows mean less methane emission?

  1. Regarding carbon storage:

    As stated above, understanding the carbon cycle requires both an understanding of residence times and flux rates. To determine the carbon storage ability of one biome or environment vs another, we typically measure the biomass of the organic matter (with interest in determining the C:N ratio) as well as flux rates in/out of our target carbon reservoir (e.g., plants such as crops or trees). Regarding flux rates, of particular interest are rates of carbon sequestration.

    Ultimately, any community of plants that is capable of sequestering more carbon into large amounts of high-C:N biomass will result in larger carbon storage. The longer-lived those plants (i.e., the less labile their carbon), the longer that community of plants acts as a carbon sink.

    So two questions of interest are:

    • which plant communities store the greatest amount of carbon-rich (i.e., woody) biomass in the smallest area (or volume) of space for the longest amount of time?

    • Which plant communities have the least disturbed (least labile) organic matter sinks?

    We can approach this in two ways: per area or across the globe:

    • Across the globe: see here, here, and here for depictions and explanations. Many have tried to quantify this, so various values show up. Below are some examples:

      enter image description here

      Carbon (Gt C) stored in ecosystems (based on Scharlemann et al., 2014); source: U.S. Forest Service

      enter image description here

      Source: IPCC

    • Per area:

      enter image description here

      Source: FAO

      If we take the carbon mass numbers from the IPCC table in the section above and divide by total area, we can get estimates of total biomass. visual Capitalist did so conveniently, which I copy below:

      enter image description here

      Source: Visual Capitalist; original data from IPCC 2000

      Do note, however, that the per-area storage of carbon of any one of these biomes (especially forests) would be greatly impacted by the age of a given patch of vegetation. As such, the numbers in the above table only reflect current biome statuses vs maximum storage of these plant communities under ideal (i.e., non-anthropogenically degraded) conditions. Of particular note, is that temperate forests vary greatly in their biomass based on the age of the stand: old-growth forests have much greater biomass per hectare than secondary forests or managed forests. However, the majority of temperate forest land (i.e., the numbers captured in the table above) currently is managed or secondary in nature and therefore does not fully capture the carbon storage ability of less-anthropogenically-modified versions of temperate forests. This explains why these numbers are much lower than other forest types.

      • Some estimate that <10% of forests in the US (as low as 1%, see Davis 1996's Old growth in the East: a survey), are old-growth in status. (You can ignore references (e.g., here) to studies like DellaSala et al. 2022 which show much higher numbers because they are defining mature forests as 80 years or older -- hundreds of years younger than is typically used to define old-growth status and certainly nowhere near even the transition point of many temperate forests).

      • Also note, that total carbon currently sequestered vs. rate of sequestration are different. Large woody biomass is a huge sink of carbon but not necessarily quicker at sequestering new carbon. For example, see Heinrich et al 2021 ("Tropical secondary forests sequester carbon up to 20 times faster than old-growth forests").

Croplands (especially those managed by modern high-throughput industrialized agricultural practices) are constantly losing organic matter in soil due to tilling, erosion, etc. Undisturbed forests essentially have little to know organic matter loss (though this depends on climate and local hydrologic cycling).

Your reference to grasslands being better carbon sinks vs. forests is taken out of context (or at least inappropriately generalized). The study (Dass et al., 2018) out of UC Davis -- influenced by the propensity for large California wildfires due to years of fuel buildup negating their typical long-term storage ability -- used a set of models to predict future change. I'll leave it to you to read the actual paper, but at the core of their claims is that "extreme heat-waves, drought and wildfire have increased tree mortality" in California while grasslands are more resilient to these issues given their propensity to store most of their carbon underground. they built their models assuming these risks continue, and thus claim that grasslands are more resilient carbon sinks in the state of California.

  • From their conclusion: "Our study demonstrates that, in the absence of aggressive mitigation of global greenhouse gases, forest management strategies to reduce fire risks, or both, grasslands will store more C for a longer period of time than forests in California." they also note that their results reflect "dryland" forests, but as we can see above, most of the carbon is stored in tropical and boreal forests as well as wetlands. They also note: "our results indicate the potential direction of change as opposed to predictions that consider the full ensemble of ecological, physiological and management factors that can alter pathways and responses of ecosystems to climate change."

  • In other words, their model-based prediction study is not meant to be a generalizable nor understood-to-be-factual study, but another piece of thought for California policy-makers...

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I have found an answer for your first question. The Global Warming Potential of methane is much higher than that of carbon dioxide. Because methane stays in the air for a shorter period of time than carbon dioxide, the GWP is higher for a shorter period of time: over 20 years it is approximately a factor 70. In 100 years the factor is approximately 30. And in 500 years, methane has still 7 times higher warming potential.

Note that the GWP compares the warming potential of one tonne. If we assume that one mole of carbon dioxide is converted to one mole of methane (since both contain one C atom), then it follows that 1 tonne becomes 0.36 tonnes. This follows from comparing the molar mass of carbon dioxide and methane. Taking this into account, it would still be better if carbon dioxide is not converted to methane.

Therefore, a closed cycle which converts carbon dioxide to methane contributes to the greenhouse effect.

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