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| Fig. 1: Cattle Grazing in Brunswick, ME. (Source: Wikimedia Commons) |
Over 55 billion pounds of red meat was produced in the US in 2020. [1] The massive scale of livestock farming releases significant green-house-gases (GHGs). In 2006, the industry globally generated 7.1 × 109 tonnes of CO2 equivalent (CO2-eq) representing 14.5% of anthropogenic emissions. [2] These numbers have been publicized by the media, environmentalists, and vegan crusaders alike; however, are GHGs from livestock really a culprit in global warming?
Unlike fossil fuels, the carbon released in rearing livestock doesn't necessarily come from deep underground. Animals eat grass, grain, hay, and other animal feeds and release CO2 and methane. Since plants sequester carbon from the atmosphere, this cycle is a closed loop. The more animals you raise, the more organic matter (feed) you need to grow and therefore the carbon sources and sinks offset one another. So is meat production causing climate change or merely along for the ride?
Understanding the full picture of livestock emissions begins with disaggregating the 7.1 × 109 tonnes of CO2-eq into its constituent components. Namely CO2, CH4, and NO2. [2] It's not immediately clear whether 1 tonne of methane emissions are better or worse for the environment than 1 tonne of CO2. To provide clarity on this issue, the Intergovernmental Panel on Climate Change (IPCC) calculated the relative green-house-gas potencies of various emissions. [3] Officially known as global warming potential (GWP), the metric is a function of two factors, radiative efficiency and lifetime. The former measures how much energy can be absorbed by a given molecule and the latter corresponds to how long a particle remains in the atmosphere. The measure is standardized by CO2 which has a GWP of 1. Methane's three C-H bonds are stretchy and can absorb more energy than C=O bonds resulting in a high GWP of 21. [3,4] NO2 can also absorb more energy and stays in the atmosphere longer, resulting in a GWP of 290. [3]
The implications of these conversions can be interpreted in several ways. It is easy to argue that CH4 and NO2 are drastically worse for the environment given their high GWP and thus animal farming is detrimental to society. On the other hand, efficiency improvements aimed at reducing CH4 and NO2 emissions could drastically improve the sustainability of farming. For example, incorporating 0.1% seaweed into cattle feed has been shown to reduce methane emissions by 40%. [5] Given that CH4 has a GWP of 21, eliminating 1 kg of CH4 emissions would reduce CO2-eq by 21 kg.
Finding the root source for livestock emissions is the second critical part in understanding the true impact of animal rearing. If emissions are truly part of a short-term carbon cycle then maybe the scale of meat and dairy production isn't problematic. If instead, emissions come from fossil fuels or other non-renewable sources, the picture looks less rosy. Previous research has concluded that CO2 emissions are predominantly driven by feed production and land-use-change, CH4 emissions result from ruminant enteric processes, and NO2 emissions result from manure and fertilizer nitrification processes. [4] Discussing each of these streams will help unearth the true cost of meat and dairy production.
Methane is naturally produced in the gut of many animals, notably cattle, with a single cow belching over 100 kg of CH4 per year. [6] Unlike CO2, most plants don't readily absorb CH4, thus breaking the closed cycled carbon cycle discussed previously. Additionally, since CH4 can also absorb significantly more energy than CO2 per molecule, it is a more potent GHG. However, it also only has an average atmospheric lifetime of 8-12 years. [7] At this point it reacts with OH- to form CO2 and H2O and the carbon cycle can march forward. Thus, methane is a transitory problem that is perpetuated by the continuous production of meat and dairy. While problematic, the effects may be mitigated quickly if lab-grown meat or other alternatives take hold.
CO2 is released in the livestock supply chain through several processes. Notably, crops need to be fertilized by ammonia. Ninety-five percent of ammonia is produced through the Haber-Bosch process which uses fossil fuel resources and releasees sequestered CO2 which deviates from the idealistic carbon cycle presented earlier. [8] While alternative, renewable strategies have been developed, none have yet to achieve cost parity with Haber-Bosch. [8] Land-use change also has long-term climate implications. In order to create grazing pasture for cattle such as the field shown in Fig. 1, dense carbon capturing forests are burned or otherwise removed. Each year 13 billion hectares of forest area are lost due to land conversion every year. [9] This land-use change leads to less biomass growth and thus less CO2 sequestration. While reforestation can theoretically make this process reversible, excessive use of the land can cause soil degradation which can make regeneration challenging. [4]
Fertilizer is also responsible for a majority of NO2 emissions caused in the meat production process. With a GWP of 290, it is significantly worse for the environment than both CO2 and CH4. [3] Nitrous oxide is prominently released in the nitrification and denitrification of soil and manure. [10] Nitrous oxide also has an average atmospheric lifetime of 116 years meaning the release of emissions today will impact generations to come. [11]
Industrial meat and dairy production releases significant GHG emissions, many of of which are not renewable; however, some of the sources of these emissions are not tied to animal rearing but rather to industrialized farming. While it cannot be understated that replacing meat and dairy with plant based alternatives would decease farm and glazing land and methane emissions, the need to innovate on supply chain, fertilizer, and land management practices are all factors in limiting green-house-gas emissions. Going vegan is one way to help reduce GHGs in the short term, but other innovations need to be developed in order to cope with climate change.
© Jake Hoffman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Livestock Slaughter, 2020 Summary," U. S. Department of Agriculture, April 2021.
[2] "Tackling Climate Change Through Livestock: A Global Assessment of Emissions and Mitigation Opportunities," Food and Agriculture Organization of the United Nations, 2013.
[3] Climate Change: The IPCC Scientific Assessment (Cambridge University Press, 1990).
[4] "Livestock's Long Shadow," Food and Agriculture Organization of the United Nations, 2006.
[5] R. D. Kinley et al., "Mitigating the Carbon Footprint and Improving Productivity of Ruminant Livestock Agriculture Using a Red Seaweed," J. Clean. Prod. 259, 120836 (2020).
[6] W. Jentsch et al., "Quantitative Results For Methane Production of Cattle in Germany," Archiv Tierzucht 52, 587 (2009).
[7] M. Wahlen, "The Global Methane Cycle," Annu. Rev. Earth Planet. Sci. 21, 407 (1993).
[8] C. Smith, A. K. Hill, and L. Torrente-Murciano, "Current and Future Role of Haber-Bosch Ammonia in a Carbon-Free Energy Landscape," Energy Environ. Sci. 13, 331 (2020).
[9] R. Joy, Unsustainable: The Urgent Need to Transform Society and Reverse Climate Change (Bristol university Press, 2021).
[10] P. J. Gerber et al., "Technical Options For the Mitigation of Direct Methane and Nitrous Oxide Emissions From Livestock: A Review," Animal. 7, 220 (2013).
[11] M. J. Prather et al., "Measuring and Modeling the Lifetime of Nitrous Oxide Including Its Variability," J. Geophys. Res. Atmos. 120, 5693 (2015).
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