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| Fig. 1:Fig. 1: Global monthly mean CH4 concentration. [4] (Courtesy of NOAA) |
Enteric fermentation is a fermentation process that takes place in the digestive tract of ruminant animals such as cattle, sheep, goats, buffalo, and camels to aid in their digestion. The process goes as follows: the animal, in most cases it is cattle, eats plant material that most other animals are not able to such as grass. The animal has a microbe-rich stomach and is able to break down the plant unlike most animals through the process of fermentation. A product of this reaction is methane which is then mostly belched up. [1] The aim of this report is to understand how much methane comes from enteric fermentation of a single cow and how it relates to the concentration of methane in the atmosphere.
In 2015, there were about 1.4 billion cows on the planet. [2] A single cow produces about 250 to 500 liters of methane each day. [3] Since the density of methane at STP is about .72 g/L, this means the average cow produces about 180 to 360 grams of methane every day. Multiplying by 365 days, the average cow produces about 65.7 to 131.4 kilograms of methane every year. Multiplying by 1.4 billion cows, we obtain a total amount of methane produced by living cattle each year of about 9.1 × 1010 kg. This number is a conservative estimate, so it acts as a lower limit of the amount of methane actually produced. If we use 131.4 kg as a liberal estimate, the amount of methane produced comes out to be 1.8 × 1011 kg a year.
To find the total amount of methane in the atmosphere, we need to know the concentration and the total mass of the atmosphere. The concentration of methane in 2021 was measured at 1908 parts per billion (Fig. 1). For the total mass of the atmosphere M, we have
| M | = | 4πR2 p g |
where p is pressure and g is the acceleration due to gravity. Pressure at sea level is p = 1.01 × 105 Pascals, the acceleration due to gravity is g = 9.8 m s-2, and the radius of the earth is R = 6.38 × 106 m. Plugging these values in, the mass of the atmosphere is M = 5.27 × 1018 kg. The molar weight of methane is 16 while the average molar weight of the atmosphere is 29. So, to find the amount of methane in the atmosphere m we multiply the mass of the atmosphere by the concentration of methane and the molar ratio of the two.
| m | = | 5.27 × 1018 kg × 1.908 × 10-6 × ( | 16 29 |
) |
| = | 5.55 × 1012 kg | |||
From Fig. 1 we can see an average yearly increase in methane concentration of 10ppb in the atmosphere from 2010 to 2020. This corresponds to a methane mass increase per year of
| dm dt |
= | 5.27 × 1018 kg × 1.0 × 10-7 y-1 × ( | 16 29 |
) |
| = | 2.92 × 1010 kg y-1 | |||
This is very close to the conservative estimate calculated above, although lower. This is interesting because cattle constitute only one source of methane. It is arguably a coincidence because methane sinks, things that take methane out of or convert it to carbon dioxide, in the atmosphere make these calculations only rough estimates. However, it is still noteworthy.
The amount of methane produced yearly by cattle is almost as much as the total mass of methane in the atmosphere when considering it as a single source. This is because atmospheric methane breaks down into carbon dioxide and water vapor. So, the atmospheric methane concentration is determined by the rate at which it breaks down and the rate at which it is produced, not just the amount produced. This is good news in the fight against climate change because methane is a much more powerful greenhouse gas than carbon dioxide. However, this does not negate the fact that methane in the atmosphere is a serious issue and that as the cattle industry grows, the amount of methane will continue to increase. There are other sources of methane, but if human kind is to stop climate change and keep the planet's ecosystems from being destroyed, measures must be taken to lower the methane production from cattle.
© Garin Gross. 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] S. Vijn et al., "Key Considerations for the Use of Seaweed to Reduce Enteric Methane Emissions From Cattle," Front. Vet. Sci. 7, 597430 (2020).
[2] T. Robinson et al., "Mapping the Global Distribution of Livestock," PLOS One 9, e96084 (2014).
[3] K. A. Johnson and D. E. Johnson , "Methane Emissions from Cattle," J. Anim. Sci. 73, 2483 (1995).
[4] E. J. Dlugokencky et al., "The Growth Rate and Distribution of Atmospheric Methane," J. Geophys. Res. 99, 17021 (1994).