With dietary choices increasingly highlighted as a major contributor to climate change, it may be tempting to argue in favour of certain forms of meat consumption over others.
That’s a key element of the so-called “climatarian” diet. Here’s how the New York Times defines it: 
“A diet whose primary goal is to reverse climate change. This includes eating locally produced food (to reduce energy spent in transportation), choosing pork and poultry instead of beef and lamb (to limit gas emissions), and using every part of ingredients (apple cores, cheese rinds, etc.) to limit food waste.”
But can such choices realistically achieve what may be hoped for?
This article focuses on greenhouse gas emissions, but firstly a word on the issue of eating locally.
“Post-farm” emissions, including those from transportation, only account for 0.5 per cent of beef’s emissions, so there’s not much benefit in purchasing the locally produced product.  For lower-emissions products, transportation’s share of emissions is higher; Nijdam, et al. have reported an average contribution across all food types of around 11 per cent. 
Many life cycle assessment (LCA) studies have shown that meat from ruminant animals, such as cows and sheep, is far more emissions intensive than that from pigs, chickens or fish, while emissions from plant-based foods are lower still. Ruminants emit large amounts of methane, a potent greenhouse gas, and often graze widely, with implications for CO2 emissions through land clearing and soil carbon losses.
The LCA figures are generally based on a greenhouse gas “global warming potential” (GWP) calculated over a 100-year time horizon. 
The adverse impact is even more pronounced when a 20-year time horizon is used, primarily because most of the methane breaks down in the atmosphere before that point. As a result, the 100-year measure (showing the average impact of a gas over the longer period) understates methane’s shorter-term impacts, as it would be almost non-existent over the final eighty years.
Its significant impact in the early stages can be critical when considering feedback mechanisms that contribute to accelerating, potentially irreversible changes in our climate system.
Comparative emissions intensities of different food products, relative to their protein content, are outlined in Figure 1. [Footnotes 1 and 2] The chart shows figures with 20-year and 100-year GWPs. The 100-year livestock figures, other than fish, are based on global average estimates from the Food and Agriculture Organization of the United Nations.  The figures for fish and other products are from a 2014 paper by Oxford University researchers, who drew on the work of the Food Climate Research Network and the World Wildlife Fund  . Where relevant, they have been adjusted to a 20-year basis utilising GWP estimates from the Intergovernmental Panel on Climate Change’s (IPCC’s) 2013 Fifth Assessment Report.
The figures for beef represent meat from the specialised beef herd, rather than meat from the dairy herd. Dairy beef’s emissions are relatively low, as the herd’s emissions are also attributed to dairy products, such as milk and cheese.
The FAO reports were based on LCAs using its Global Livestock Environmental Assessment Model (GLEAM). The model, like the LCA assessment utilised by Oxford, took into account emissions along the supply chain to the retail point. For meat, they are based on carcass weight.
The figures for animal-based foods, in particular, vary significantly by region, and are influenced by factors such as feed digestibility, livestock management practices, reproduction performance and land use.
The figures take into account protein estimates from the US Department of Agriculture’s National Nutrient Database for Standard Reference. 
Figure 1: kg CO2-e greenhouse gas / kg protein based on GWP100 and GWP20 (global average figures)
The twenty-year figures for beef, sheep meat, pig meat and cows’ milk are influenced by the high proportion of methane emissions, ranging from 25.8 per cent (pigs) to 56.9 per cent (sheep). Most of pigs’ methane emissions, representing 19.2 per cent of their total emissions, come from manure management.
Is it okay to eat other animal products?
Even using the conservative 100-year time horizon, chicken meat, pig meat, fish and eggs are more than 3 times as emissions intensive as soybeans. Based on the 20-year period, pig meat is 5 times, and eggs are nearly 6 times. (The time period does not affect the emissions intensity of chicken meat and fish, as methane is not a significant factor in their emissions.)
If climate change impacts were considered to be a cost in their own right, those figures could be expressed as chicken meat being 200 per cent more “expensive”, pig meat being 400 per cent more “expensive”, and eggs being 500 per cent more “expensive”, than soybeans.
Inefficiencies on that scale would not normally be tolerated in government or private sector businesses, where discrepancies of 5 – 10 per cent can mean life or death to any project or program. Why should such levels of inefficiency be tolerated when they relate to greenhouse gas emissions, particularly when our current position in relation to climate change is so precarious?
A climate emergency with no buffer
As poorly as pig meat, chicken meat, fish and eggs compare to plant-based options on the basis of emissions intensity, that measure is only part of the story.
We face an emergency in which we are effectively sitting on the edge of a precipice, with little room to move before we lose any ability to favourably influence our climate system.   In such a dangerous position, we need to select those dietary choices with the best chance of allowing us to move to a position of relative safety.
Due to the rapid expansion of soybean plantations for animal feed, consumption of pig and chicken meat, farmed fish, eggs and dairy products plays a critical role in the destruction of the Amazon rainforest and other carbon-rich ecosystems, such as the Cerrado region further south. 
With rising global temperatures and excessive forest fragmentation, we may be pushing the rainforest toward a dangerous threshold. Such fragmentation can lead to general drying and an increased propensity for fires and other causes of loss. Studies published in late 2014 and early 2015 documented the extremely adverse long-term effects of forest fragmentation, including carbon losses far in excess of what was previously believed. Much of the fragmentation arises from agriculture, including livestock feed crops.  
Dieback of the Amazon rainforest represents a potential tipping point, where a small change in human activity can lead to abrupt and significant changes in earth systems, with catastrophic and irreversible impacts.  Even in the absence of clear tipping points, climate feedback mechanisms create accelerating, potentially irreversible changes.
It could be argued that any agricultural plantation in the Amazon basin and elsewhere represents an environmental problem. That is true, but the problem is magnified in regard to animal feed, due to the gross and inherent inefficiency of animals as a food source. In converting soybeans to pig and chicken meat for example, we lose around 80 per cent of the plant-based protein used in the production process.  That means the land area required is around five times the area required if we obtained the protein directly from plants.
Feed conversion ratios of various livestock production systems are shown in Figure 2, which can also be seen in the article Chickens, pigs and the Amazon tipping point. The researchers determined the figures by analysing between twenty-nine and eighty-three studies per item.
Figure 2: Feed conversion ratios (kg feed protein required per kg of animal protein produced)
Although soybean meal for livestock feed was once considered a by-product of soybean oil production, it is the requirement for livestock feed that now drives the international soybean trade. 
China’s livestock sector is the major global consumer of traded soy products. However, the trade is global, and demand pressure from any country contributes to an increase in overall supply, thereby increasing pressure on critical ecosystems in soy-producing regions.
In the absence of an overall global shift away from ruminant meat such as beef and lamb (the opposite trend is occurring in many developing nations), any increase in the consumption of pig meat, chicken meat, fish, eggs and dairy products will almost certainly cause soybean plantations to expand, rather than contract, with the potential loss of the massive carbon sink that the Amazon basin and Cerrado region represent. On the other hand, a general move away from those products may allow vast areas of cleared land to regenerate to something approaching their natural state.
Corn is also a major component of animal feed production. The crop is far more water and nutrient intensive than soy, so its use has major implications for producing nations, including those in South America. 
Other overlooked climate change impacts of consuming fish and other sea creatures
I recently commented on a paper that had appeared in Nature Climate Change, which had helped to highlight some of the impact of industrial and non-industrial fishing on our climate system.   The problem arises largely from the fact that fishing disturbs food webs, changing the way ecosystems function, and altering the ecological balance of the oceans in dangerous ways. The paper focused on the phenomenon of “trophic downgrading”, the disproportionate loss of species high in the food chain, and its impact on vegetated coastal habitats consisting of seagrass meadows, mangroves and salt marshes.
The loss of predators such as large carnivorous fish, sharks, crabs, lobsters, seals and sea lions, and the corresponding population increase of herbivores and bioturbators (creatures that disturb ocean sediment, including certain crabs) causes loss of carbon from the vegetation and sediment. The ocean predators are either caught intentionally by fishing fleets, or as by-catch when other species are targeted.
The affected oceanic habitats are estimated to store up to 25 billion tonnes of carbon, making them the most carbon-rich ecosystems in the world. They sequester carbon 40 times faster than tropical rainforests and contribute 50 per cent of the total carbon buried in ocean sediment.
Estimates of the areas affected are unavailable, but if only 1 per cent of vegetated coastal habitats were affected to a depth of 1 metre in a year, around 460 million tonnes of CO2 could be released. That is around the level of emissions from all motor vehicles in Britain, France and Spain combined, or a little under Australia’s current annual emissions. If 10 per cent of such habitats were affected to the same depth, it would be equivalent to emissions from all motor vehicles in the top nine vehicle-owning nations (USA, China, India, Japan, Indonesia, Brazil, Italy, Germany, and Russia), whose share of global vehicle numbers is 61 per cent. It would also equate to around eight times Australia’s emissions.
Loss of ongoing carbon sequestration is the other problem. If sequestration capability was reduced by 20 per cent in only 10 per cent of vegetated coastal habitats, it would equate to a loss of forested area the size of Belgium.
These impacts only relate to vegetated coastal habitats, and do not allow for loss of predators on kelp forests, coral reefs or open oceans, or the direct impact on habitat of destructive fishing techniques such as trawling. They are not accounted for in the emissions intensity figures referred to earlier, or in national greenhouse gas inventories.
The argument of those who encourage increased consumption of pig meat, chicken meat, fish and eggs at the expense of beef and lamb is essentially one of “getting the biggest bang for the buck”, as reflected in the relative emissions intensity of different products. However, consumption of the supposedly more favourable animal-based foods has adverse impacts that are unaccounted for in most forms of climate change reporting, which should cause them to sit alongside ruminant meat in terms of campaigning efforts.
- The “GWP 20” figures are based on the global average percentage split of the various factors contributing to the relevant products’ emissions intensity, and are intended to be approximations only.
- Pulses comprise chickpeas, lentils, dried beans and dried peas. Along with soybeans, peanuts, fresh beans and fresh peas, they are members of the “legume” food group.
- This article focuses on climate change, but other critical environmental impacts arise from animal-based food production, such as contamination of land and waterways from animal waste, largely related to the inherent inefficiency of animals as a food source.
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Figure 2 added on 25th October 2016