3.2 What are the main GHG contributions to agricultural emissions?
3.2.1 Livestock is by far the main contributor of GHGs from farming.
The largest source of direct GHGs are enteric fermentation emissions from ruminants
Smith P., et al. (2014).
Global emissions from agriculture are mainly in the form of methane and nitrous oxide arising from ruminant enteric fermentation.
Synthetic and organic fertiliser applications (e.g. manure) are also significant contributors.
GHG contributions from livestock
Gerber, et al. (2013).
Livestock contribute 14.5% of human-made GHG emissions. Of this 14.5%:
Within the livestock sector, almost all GHG emission contributions come from enteric fermentation (methane emissions), manure (both methane and nitrous oxide), animal feed production (carbon dioxide and some methane), and from land-use change (carbon dioxide emissions from land clearing).
The percentage contribution from post-production emissions is very small for livestock – the CO₂ that is emitted results from the processing, transport of livestock products between the production and retail point as well as from cold storage.
Livestock contribute significantly to GHGs, as already discussed. But impacts of livestock are not limited only to GHGs.
Livestock & animal products: a convergence of issues
Source: FCRN. See side panel for references.
Livestock are an important driver of deforestation, and as such are implicated in biodiversity loss.
Much of the world’s grain production is to feed animals, with up to 1/3 of arable land dedicated to producing feed for animals. Since food energy is lost during the metabolic process, the use of cereals to feed animals is argued by some to be inefficient. Critics say that the land used to produce livestock feed could be better used for other purposes – such as to produce crops to be eaten by humans, or allowed to return to non-agricultural land use.
Livestock rearing also requires large volumes of water for feed production and to a lesser extent for consumption directly by animals, washing and so forth – although most of this is ‘green water’ – water that falls on grazing land or on unirrigated crops. Livestock manures are a major cause of water pollution, largely through poor manure management which pollute water systems .
Many human diseases are zoonotic in nature, meaning they care transmitted from animals to humans.
At the same time, animal products can be important sources of nutrition and livestock keeping is central to the livelihoods of some of the world’s poorest people.
3.2.2 Livestock emissions vary depending by species.
Focus on livestock – global emissions by species
Gerber, et al. (2013)
Globally, beef and dairy cattle contribute most to the GHG total from livestock, since they are ruminants and farmed in high numbers. Other ruminants (for example buffalo, sheep and goats) contribute less overall because fewer of them are reared.
Non-ruminants such as pigs and chickens contribute less than cattle mainly because they do not emit as much methane from enteric fermentation, and their feed conversion efficiency is higher. As such GHG emissions from pork and poultry production are lower, despite their being farmed in large numbers.
3.2.3 Livestock systems can also generate benefits to the environment.
|Nutrition||Excellent for protein, calcium, iron, vit. B12||Excessive fat; protein can be more than needed||Animal foods not essential; plants can substitute|
|Non food benefits||Leather, wool, manure, rendered products||Manure can be a pollutant||Quantities needed? Plant alternatives?|
|Substitution cost||Eating will always produce an impact||Generally plant foods have lower GHG profile|
|Carbon storage||Pasture land stores carbon – can sequester in right conditions if moving from bad to good grazing||Excessive grazing & land use change releases carbon and reduces soil quality||Land use change from pasture to crops will generate CO2. Huge uncertainties around the sequestration potential of grazing systems – more work needed|
|Resource efficiency||Livestock can consume grass & byproducts (some limited by legislation)||Supplemented with grains & cereals in intensive systems||Byproducts can be used directly as energy source in AD systems|
|Geography||Some land not suitable for cropping||Arable land used for livestock when it could for example be allowed to re-wild.||Intensified systems are arable hungry|
Livestock can make positive contributions to sustainable food systems.
Animal products can supply many important nutritional benefits (see Chapter 8); livestock farming can also generate byproducts such as leather and wool; under the right conditions grazed pastures can store increased levels of carbon. Some research suggests that under the right conditions good grazing management can even lead to soil carbon sequestration, although the evidence base is uncertain and this is still a contested and under-researched issue. Animals can eat grass and byproducts not suitable for humans; and they can be produced on land not suited to crop production. Well managed livestock can also contribute to biodiversity and the aesthetic value of landscapes.
However, if livestock production continues to expand, or if livestock are poorly managed, the negatives can outweigh the positives.
High levels of meat consumption may generate health problems (see Chapter 8); mismanaged byproducts (such as manure) can be polluting; excessive or poorly managed grazing can cause land degradation and associated soil carbon loss while forest clearance to make way for grazing animals or feed crops causes CO2 release and biodiversity loss. While livestock can consume byproducts, livestock in many systems (and increasingly so) are not fed solely on grass or byproducts but on dedicated feedcrops; and 40% of arable-quality land is used for livestock.
3.2.4 Grazing livestock and soil carbon sequestration: a contested issue.
Grazing livestock and soil carbon sequestration: a contested issue
- Advocates of grass-fed beef systems argue that well managed grazing livestock can help sequester carbon in soils
- It is claimed that this sequestration can partly or entirely outweigh the methane and nitrous oxide the animals emit; potentially grazing livestock systems can even be ‘emission negative.
If sequestration is assumed the carbon footprint of beef can shift from very high to very low
Changes in management strategy have been argued to lead to sequestration
Graph produced from data presented in: Wang, et al (2015)
But caution is needed…
- This is still an under researched area and the evidence base is still uncertain.
- Important to note that the benefits of avoided methane and nitrous oxide emissions are permanent, while carbon sinks are temporary.
- The extent to which sequestration occurs depends on: the status of the soil carbon levels before any management change, the baseline soil type and conditions, specific management techniques, climate, rainfall etc. – and many of these factors can change
- Also need to consider: reversibility (grassland can be ploughed up) and saturation (after some decades soils approach carbon equilibrium; methane and nitrous oxide emissions will then always outweigh the sequestration); impacts on biodiversity can be mixed and may be negative
- What is clear is that grasslands are major carbon stores – so it is important not to plough them up
- Some grasslands are home to unique flora and fauna and grazing livestock may have historically contributed to this. But other grazing lands contain very little biodiversity.
- Poor grazing management can contribute to soil carbon losses while grazing livestock have historically (although less so now) been an important driver of deforestation.
3.2.5 Emissions from cropping and horticultural production are on average lower than those from livestock.
Fruit & vegetables
Lower GHGs, but this depends on energy use and later stage inputs.
Grown with additional heating or protection
Pre-prepared, trimmed or chopped
Fragile or highly perishable
Unseasonal products (where airfreighted or grown with heated/protected environment)
Seasonal field grown, robust produce
grown without additional heating or protection
Overseas produce field grown, robust produce grown without additional heating or protection & transported short distances by sea or road
Domestic is not always better
Fruit and vegetables contribute fewer GHGs than livestock on average, measured in terms of kg CO2 eq./kg product – although other functional units may include emissions per unit of calories or protein, and the differences between products may narrow or widen.
Lower impact produce are those that are field grown (for example not grown in greenhouses), transported shorter distances by road or ship (not air) and those that are more robust – i.e. less requiring of rapid transport modes such as air and less perishable – thus less prone to being spoiled or wasted.
More emissions-intensive fruit and vegetables are typically those grown in heated greenhouses, where much higher energy inputs are required. Airfreighting is extremely GHG intensive and so any produce transported in this way has a very high carbon footprint. Highly fragile produce may be more easily spoiled and wasted, meaning a waste of embedded emissions.
3.2.6 Produce grown in heated greenhouse systems can have high emissions (UK example).
UK example – heated greenhouse impacts
UK greenhouse grown tomatoes 3x more energy intensive than Spanish field grown
But, need to consider:
A study by the UK Department for Environment, Food and Rural Affairs (Defra) measured the GHG emissions from heated tomato production in the UK, compared to imported field grown (non-heated) tomatoes from Spain. The UK tomatoes were found to be three times more energy intensive than Spanish, due to the fossil-fuel energy requirements of heated food production in the UK. The warmer climate in Spain meant that additional heating was not required. See Section 3.3 for more on this.
However, this was not a full study of all the impacts, including water use, while important factors such as energy use in refrigeration during transport were not included. Additionally, it is important to make sure that like-for-like product comparisons are made – for example cherry tomatoes have different production and storage requirements to standard tomatoes.
A further study (Antón et al. 2005) compared the impacts of permanent glass greenhouses in the UK with plastic (short-lived) tunnels in Spain, finding that the energy used to produce the plastic tunnels was a significant contribution to the overall impact.
The point is that heated production uses energy and has GHG emissions associated with it, but impacts from transport, material production, and storage as well as environmental issues other than GHGs need to be considered.