Climate change and a vegetarian diet

Climate change and a vegetarian diet
Science of Life


Mary Robinson, Head of the Mary Robinson Foundation for Climate Justice, recently advocated that we should all reduce our consumption of meat in order to ameliorate global warming, saying: “We need to eat less meat, maybe become vegetarian, or even vegan.”

The underlying idea is to eliminate emissions of warming gases from raising livestock for meat. If we all became vegetarian (eating no animal meat) or vegan (eating neither meat nor any animal product, like eggs) it would be a dramatic step indeed, but what effect would it actually have on global warming?

In an attempt to answer this question, two researchers, Robin White and Mary Beth Hall, studied what would happen in the US if everyone there went vegan right now (Proceedings of the National Academy of Sciences, November 13, 2017), thereby removing animals altogether from agriculture. The study found that, although this would reduce warming greenhouse gas (GHG) emissions from agriculture by about 57%, it would only reduce GHG emissions from all sources by 2.3% and would also create a plant-only food supply incapable of supporting the nutritional requirements of the population.

The Earth is surrounded by a blanket of gases called the atmosphere.  Visible radiant energy from the sun, Earth’s only external source of energy, beams down upon the Earth. The atmosphere is transparent to this radiant energy, which passes freely through and warms the Earth. Heat energy now rises from Earth into the atmosphere in the form of infrared radiation. The atmosphere traps some of this infrared heat radiation thereby keeping the Earth warm.

The atmosphere acts like the glass in a greenhouse, allowing the Earth to warm up and preventing all the heat from leaking away – the greenhouse effect.  This greenhouse effect keeps the temperature of the Earth suitable for life. The average temperature of Earth is 14°C. Without the greenhouse effect Earth’s temperature would drop as low as -18°C.


Dry air contains 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide and small amounts of other gases such as methane and nitrous oxide.  Air also contains variable amount of water vapour – about 0.4% of the entire atmosphere.

The main atmospheric gases responsible for the greenhouse effect are carbon dioxide, methane, nitrous oxide and water vapour. These gases cycle naturally between the Earth and the atmosphere. Animals and plants continuously emit carbon dioxide to the atmosphere as they break down food to extract energy.

On the other hand, green plants suck carbon dioxide out of the atmosphere and use it to build up their complex structures. Water evaporates from oceans and rivers to form clouds and falls back again as rain. Methane is emitted from marshy ground and nitrous oxide from microbial activity in soils. Under conditions where human activity interferes very little, the composition of the atmosphere remains constant over very long periods as does the strength of the greenhouse effect.

The problem is that, since the advent of the industrial revolution in the 1800s, human activities have been adding copious amounts of greenhouse gases, principally carbon dioxide, to the atmosphere, thereby strengthening the greenhouse effect. Carbon dioxide is released when fossil fuels (coal, oil, natural gas and peat) are burned. Carbon dioxide concentration in the atmosphere was 290 parts per million (ppm) in 1880 but has gradually risen since to a concentration of 405 ppm today.

Methane (0.84 ppm 1880, 1.86 ppm today) a  powerful greenhouse gas, is also emitted by human activities, principally from cattle raised for meat. The addition of water vapour to the atmosphere cannot be attributed to human activities, for the most part.

The enhanced greenhouse effect resulting from human activity is over-warming the Earth, but human emissions of greenhouse gases continue.

The mean surface temperature of the Earth has risen by at least 0.8 degrees C since 1880. It is predicted that the global average temperature is likely to exceed 1°C above pre-industrial levels by 2022 and could even exceed the 1.5°C aspiration set at the 2015 Paris Climate Change Agreement.

A temperature increase of 1.5°C was set as a limit because warming beyond this would have disastrous worldwide consequences –  extreme weather conditions, widespread flooding of low-lying areas, food insecurities and mass migrations. The only way to avoid this scenario is to drastically reduce overall emissions of greenhouse gases, including emissions from livestock agriculture.

Agriculture is responsible for 24% of all human-generated emission of greenhouse gases. The study quoted before by White and Hall examined the effect on American greenhouse gas emissions if everyone in the USA went vegan. Animal agriculture presently accounts for 49% of all agricultural greenhouse gas emissions in the USA. White and Hall determined that, if everyone turned vegan, agricultural greenhouse gas emissions would drop by 28%  –  a significant reduction but far less than the 49% total.

There are several reasons why a universal switch to a plant-only diet would have such a modest effect on greenhouse gas emissions. Currently livestock eat much human-inedible agricultural waste, such as corn stalks, potato waste and other waste.

In the absence of livestock, this waste would have to be burned, releasing carbon dioxide. Also, land previously devoted to livestock would be switched to crops, increasing demand for fertiliser and manufacturing more artificial fertiliser to meet this demand would also release carbon dioxide to the atmosphere.

But, the authors also calculate that a plant-only diet would be unable to meet the US population’s nutritional requirements for calcium, Vitamins A and B12 and several key fatty acids. While an individual vegetarian diet can be balanced to meet these nutritional requirements, this cannot be done on the scale necessary if the entire population went vegan because the types of foods necessary for balance are not currently produced in sufficient quantities.

In summary, while eliminating meat from the US diet would reduce greenhouse gas emissions from the agricultural sector by 28%, total greenhouse gas emissions would reduce by only 2.3% and a universal plant-only diet would be nutritionally inadequate. A lot of work for very little pay-off. Mary Robinson could continue to eat the odd hamburger without fear of significantly warming the world.

William Reville is an Emeritus Professor of Biochemistry at UCC.