Nothing will benefit health and increase chances for survival of life on Earth as much as the evolution to a vegetarian diet. ...Albert Einstein (quoted)
Greenhouse gases (GHG - mostly carbon dioxide CO2, methane CH4 and nitrous oxide N2O) in the atmosphere trap heat radiated from the Earth and keep it at a comfortable temperature of 14C . Without GHG the Earth will be about 32 degrees colder with an average temperature of -18C - life would be impossible. During an ice age, the average global temperature only falls by about 7 to 10 degrees.
Since the first industrial revolution, humans have been putting more CO2 and other GHGs in the atmosphere with a sharp increase in the 20th century. Population numbers have gone up too; the following slides show the steep rise in GHG over the past 100 years. This has resulted in the Earth warming by about 1C over this period.
In 2017, total CO2 emissions from fossil fuel and industry were 36.8 billion tons; CO2 stays in the atmosphere for over 100 years.
Total annual methane emissions had reached 623 million tons in 2016.
The residency time of methane in the atmosphere is about 12 years.
(Click on a slide to see its full page image, press ESC to return to text
Methane is a potent GHG. Over a 20 year period, methane has 86 times greater global warming potential than carbon dioxide. Many publications quote a global warming potential of methane as 26 over a 100 year period.
We normally attribute the rise in GHGs to increased industrial, transport and agricultural activity based around the burning of fossil fuels (coal, oil and natural gas). Most publications do not discuss GHG emissions due to metabolic activity in humans and their livestock, both have increased fourfolds in the past 100 years.
I shall estimate such metabolic contributions using 'back of the envelope' calculations. The simple calculations give surprisingly realistic results and help the non-specialist general population to understand this aspect of the science behind global warming.
GHG Emission by Humans and Animals:
Are we heating the planet as we breathe?
Many news articles mention the role of cattle in emission of the greenhouse gas methane, but it is not widely appreciated that humans and animals also emit significant amounts of CO2 which contributes to the GHG budget and to global warming. I shall estimate these numbers in the following:
Carbon dioxide emission by humans: Humans inhale atmospheric air containing CO2 at 0.0004% concentration but exhale air with CO2 at 5% concentration. (I shall use rounded numbers for ease of calculation).
The following slide provides more details:
CO2 is a product of metabolic activity (respiration). While it is possible to calculate the amount of CO2 emitted by using volume of exhaled air and breathing rate (I have described the calculation at the end of this blog), it is easier (and probably more accurate) to work out the amount of CO2 produced per day by the aerobic respiration reaction in which glucose uses oxygen to produce energy, water and CO2.
For our purpose, it suffices to say that carbohydrates in the food are broken down to glucose molecules and the reaction may be written as
Glucose + Oxygen -> Water + Carbon dioxide + 2886 kJ Energy
C6H12O6 + 6 O2 → 6 H2O + 6 CO2 + heat
Essentially, one molecule of glucose uses 6 molecules of oxygen to produce 6 molecules of CO2. The molar weight of glucose is (calculated from its formula) 180 and that of CO2 is 44.
This means that 180 gram of glucose burns to produce 6 x 44 = 264 gram of CO2.
Assuming that we consume 2000 Calories per day and carbohydrates produce 4 Calories per gram, we are burning equivalent of 500 gm of glucose per day in aerobic respiration.
This will produce 500 x 264 /180 = 733 grams of CO2 per day or 268 kg/year.
Each of us produces this amount of CO2 per year. The world population is over 7 billion and will produce 1.87 Billion Tonnes of CO2 per year. This is 5.2% of the total CO2 put into the atmosphere by human activity.
What about Animals? The main population of animals is comprised of those reared for human consumption - cattle, sheep, pigs, poultry etc. Their numbers have sharply increased in response to increasing human population and higher demand of meat over the past 50 years or so.
Additionally, there were about 23 billion poultry in the world in 2014.
Livestock animals exhale CO2 generated in the respiration process - most amount is exhaled by cattle which are much bigger in size than other livestock animals.
The metabolic energy requirements of a mammal vary as 0.75 power of the body mass, we can calculate for a given body mass how much CO2 per day an animal will produce. The table shows the amount of CO2 exhaled by livestock animals. The emissionn of methane is also given.
In conclusion, humans and livestock release
1.87 + 6.65 = 8.52 billion tons of CO2 in the atmosphere per year. This number has increased fourfold in the past 100 years. Only China emits more CO2 than humans and animals exhale (2015 figures in billion tons per year: China 10.6 and USA 5.2)!
Methane Exhaled by Livestock: The table above
gives methane emisssions from various livestock animals. Meat cattle are the main source of methane. Exhalation represents about 96% of total methane put into the atmosphere by the cattle - there is a common misunderstanding that cattle emit methane by farting but that only contributes a tiny %. Most exhaled methane is produced in rumination.
Wiki explains: Ruminants are mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The process, which takes place in the front part of the digestive system and therefore is called foregut fermentation, typically requires the fermented ingesta (known as cud) to be regurgitated and chewed again. The process of rechewing the cud to further break down plant matter and stimulate digestion is called rumination.
Currently, we are putting in 623 million tons of methane in the atmosphere every year. Of this 110.7 million tons ( ~ 18% is emitted by livestock rumination - enteric fermentation or EF). These numbers have been revised upwards by 11% in 2017 by Wolf et al. In 2010, EF accounted for 43% of all GHG emission from agricutural activity in the world.
Remember that biggest increase in methane emissions is due to meat cows and pigs (their numbers have gone up fivefold over the last century); more people are consuming meat that will only make the situation worse.
Manure Treatment is a serious source of Methane too:
It is not only that livestock exhale methane (product of the rumination process in cattle) but the storage and treatment of manure from the livestock is also a serious source of methane.
Manure is stored in tanks, lagoons where microbial activity causes its deacy. Anaerobic breakdown produces greater amount of methane and a switch to aerobic decay will reduce the emission of methane.
Final Word: In this blog, I have used the population numbers and emission of carbon di-oxide and methane by individual animals to calculate the total contribution to GHG emossions that humans and their livestock make. Additional emission will also come from other animals that I have not considered.
The warming of the world due to GHG emissions is predicted to reach at least 2 degrees centigrade by the end of the century. Human and Livestock emissions can be most effectively reduced by population control - while the human population is supposed to stabilise around the 10 billion mark in the next 50 years, livestock population can only decrease if we change our emphasis to eating meat - this is a trend that has a lot of mementum and is unlikely to change in the near future.
Better manure management (using aerobic storage tanks) will help but that is not the major contribution. Pasture grazed cattle emit less methane but again it is not a practical solution if we all insist on eating meat-rich diet.
Thanks for reading...