Note from author

Subsequent to posting this article, I messaged and emailed One Green Planet. As a result, they have amended their article and headline. Global time differences resulted in the changes occurring on 5th January. I had offered to remove this article, but that was not required.

My article

This is a brief post, which attempts to correct a report commenting on the recent article “Warning: Your festive meal could be more damaging than a long-haul flight” by Guardian columnist, George Monbiot.

The headline to the article of concern, from One Green Planet, declared that “consuming 2 pounds of grass-fed meat is worse for the planet than flying from New York City to London”. That statement was reinforced by the article itself, which contained the words, “. . . if two pounds of meat is equivalent to the carbon footprint of a six-hour flight . . .”.

I tried to post in the comments section of the article, but was required to log in via Facebook, Google or Twitter. I declined when told they would require access to various items of information. I then considered commenting on One Green Planet’s Facebook page, but saw that the relevant post had been shared 299 times, and liked 835. I felt that such coverage required a response that was more significant than a Facebook comment (although I am not suggesting this article will be significantly more prominent than the other).

What’s the problem with One Green Planet’s article?

Monbiot wrote, “a kilogramme of beef protein reared on a British hill farm can generate the equivalent of 643kg of carbon dioxide”. One Green Planet has incorrectly interpreted Monbiot’s reference to “beef protein” as “beef”, and similarly “lamb protein” as “lamb”.

The authors of the study cited by Monbiot (corresponding author Durk Nijdam) had assumed beef and lamb each contain 20 per cent protein. That wasn’t clear from Monbiot’s Guardian article, but was mentioned in notes following the corresponding article on his own website, with the headline “Sacrifice“. (Based on those notes, one passenger’s emissions on the trans-Atlantic flight would be 614 kg.)

The result is that for any given quantity of protein in beef or lamb, the meat itself weighs five times as much.

In any event, Monbiot referred to kilograms, rather than pounds. 1 kilogram is roughly equal to 2.2 pounds rather than 2, so the weight of the meat he was referring to was actually 5 kilograms, or around 11 pounds.

Based on those numbers, One Green Planet should have declared “consuming 11 pounds of grass-fed meat is worse for the planet than flying from New York City to London“.

It’s still remarkable that such an amount of beef (representing around 20 regular servings of beef steak) could result in more emissions than one passenger’s share of a trans-Atlantic flight.

Other issues

Attribution of emissions

I cited the Nijdam study in a March, 2015 article, in which I commented on a paper by Chatham House. (Monbiot also referred to the Chatham House paper in his article.)

A key point from my article was that the emissions of dairy products and beef from the dairy herd are generally low relative to emissions of meat from the specialised beef herd. The reason is that the dairy herd’s emissions are attributed to a wider range of products (including milk, cheese and meat) than are the emissions of cows bred specifically for beef.

The relevance of that point to Monbiot’s emissions intensity figures (kilograms of greenhouse gas per kilogram of protein) is that his figures have been grossed up from live weight to retail weight. In other words, all emissions relating to the animal have been attributed to the meat on the plate.

Consistent with the approach applied to the dairy herd, it could be argued that emissions should also be attributed to other products generated from the animal, such as liver, kidneys, tripe, tongue, gelatin and leather, thereby reducing the emissions attributed to the retail cut of meat.

The Food and Agriculture Organization of the United Nations (FAO) and others have reported emissions by carcass weight, which falls between live weight and retail weight. I have used carcass weight and retail weight in various articles, noting the issues involved.

What percentage protein?

As mentioned earlier, Nijdam and his co-authors assumed that beef and lamb are 20 per cent protein. The United States Department of Agriculture (USDA) assumes a figure of 27 per cent. Using that figure, the emissions figures determined for the Nijdam paper would have reduced to 476 kg for beef and 555 kg for lamb.

As noted by Monbiot, the figures are at the top end of a wide range of emissions intensity figures for beef and lamb, from numerous studies. Livestock-related emissions vary significantly by region and production system, and are influenced by factors such as feed digestibility, livestock management practices, reproduction performance and land use. I have often quoted global average figures, such as those in Figure 1 from the FAO (for livestock products) and Nijdam, et al. (for plant-based products).

Figure 1: Emissions intensity of various products (kg CO2-e/kg protein)


The emissions intensity of food products is often calculated as kilograms of greenhouse gas per kilogram of end product, rather than per kilogram of protein. The latter produces higher figures, as protein is one component of many. Both measures were used in the Nijdam paper.

Global warming potential

Although the quoted emissions intensity figures are based on a 100-year time horizon, it is also important to consider a 20-year period. The reason is that methane, which is prominent in emissions of ruminant animals such as cows and sheep, breaks down in the atmosphere to a large extent within that time frame. The 100-year measure (showing the average impact of a gas over the longer period) understates methane’s shorter term impacts, as the gas would almost be non-existent over the final eighty years.

Those impacts will be critical as we try to avoid near-term acceleration of climate change, influenced by significant feedback mechanisms, potentially causing us to lose any ability to influence the climate system in favourable ways.

The multiplier used to convert a gas’s warming impact to a “CO2-equivalent” (CO2-e) figure is known as the “global warming potential” or “GWP”.

Figure 2 shows the FAO and Nijdam, et al. figures, adjusted to a 20-year time horizon.

Figure 2: Emissions intensity of various products (kg CO2-e/kg protein with GWP20)


Based on those figures, the emissions of specialised, non-dairy beef are only slightly below those from one passenger’s share of a trans-Atlantic flight. As methane is not a significant factor in aviation emissions, the 20-year time horizon would only marginally affect the results. Nitrous oxide is also not significant in aviation emissions. Unlike methane, its potency as a greenhouse gas is slightly lower over a 20-year time horizon than over the 100-year period.


The impact of livestock production on global warming and climate change is increasingly prominent in the media. The issues can be reasonably complex, and oversights such as the one that appears to have occurred in One Green Planet’s article should perhaps not come as a surprise, particularly when end notes that appeared in a corresponding article were absent from the article they reviewed. I hope this article enhances the general understanding of the issues, to assist us in addressing a factor that will be critical in our efforts to overcome climate change.


Paul Mahony (also on Twitter, Facebook, Scribd, Slideshare, New Matilda, Rabble and Viva la Vegan)


A full reference list will follow.


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