What is a “Sustainable Diet” in the UK? – Part 1/3: GHG Emissions
Updated: Jul 21, 2021
By A. Clare
There is a continuous debate regarding what constitutes a ‘sustainable diet’. The ‘Farm to Fork’ strategy is at the heart of the European Green Deal, which sets out a plan to make Europe the first climate-neutral continent by 2050. A plant-based or no-meat diet forms a fundamental part of many ‘sustainable diet’ definitions (Twine, 2017), with the number of vegans in the UK currently represented by 3% of the population (D’Angelo et al., 2020).
Animal agriculture rapidly expanded during the twentieth century (Twine, 2017), which coincides with the fastest doubling of the world population between 1950-1987 (Hirschman, 2005). The effects of agriculture on land use, deforestation, biodiversity, GHG (greenhouse gas) emissions and water scarcity; are well documented in many studies (see: Bajželj et al., 2014; Popp et al., 2010; Springmann et al., 2016; Stehfest et al., 2009; Tilman et al., 2014; Twine, 2017). The damage seen to the environment following animal agriculture expansion has led to the consensus that eating a diet containing meat is environmentally damaging. The climatic effects of activities within the ‘farm gate’ are well documented; however, many studies disregard the impacts of food systems during the pre-and post-production stages (Mbow et al., 2019; Scarborough et al., 2014; Wakeland et al., 2012). A sustainable plant-based diet is often promoted, but the location-specific impact of this diet along the entire supply chain is rarely discussed.
Within Farm Gate Agriculture Emissions:
Emissions from Crops and Soils:
GHGs emitted from crops and soils are mainly in the form of nitrous oxide (N2O); however, small amounts of methane (CH4) and carbon dioxide (CO2) are also produced (Mbow et al., 2019). Most agriculture GHGs result from fertiliser use, which emits N2O from denitrification by microbes and some CH4 from anaerobic decomposition (Walling & Vaneeckhaute, 2020). Fertilisers are essential for increasing plant productivity and yields to sustain population growth (Walling & Vaneeckhaute, 2020). In 2003, synthetic (inorganic) fertilisers contributed to at least 3% of global GHG emissions (Jenssen & Kongshaug, 2003). Between 1970-2010, synthetic fertiliser use increased by 200-300% (FAOSTAT, 2013; Smith et al., 2014; Walling & Vaneeckhaute, 2020) and, it is currently estimated that 50% of the world’s population depends on synthetic fertilisers for food security (Walling & Vaneeckhaute, 2020).
Organic fertilisers, such as manures, composts and digestates, can also be used to promote plant growth. Although organic fertilisers also generate GHG emissions, in 2016, these natural fertilisers were only responsible for 13% of agriculture N2O emissions, whereas 42% resulted from inorganic fertiliser application (Committee on Climate Change, 2018). Furthermore, during this time 65% of UK farms were using organic fertilisers on their crops (DEFRA, 2017), suggesting that the use of natural fertilisers is far less polluting than using synthetics. Organic fertilisers are also beneficial as they provide plants with a range of compounds, such as humic acids and carbon, which are lacking in synthetic alternatives (Walling & Vaneeckhaute, 2020).
Emissions from Livestock:
Livestock emissions include those from manure and enteric fermentation in ruminant animals. All global estimates agree that cattle are the primary source of livestock emissions (approximately 65-77%) (Mbow et al., 2019) and, the production of animal products is recognised as generating greater emissions per unit weight (Scarborough et al., 2014). A study by Springmann et al. (2016) reported that “if everyone ate a plant-based diet, this would result in a 49% reduction in GHG emissions from food production”. This statement is likely accurate; however, the specific mention of food ‘production’ (i.e., the manufacturing or growing of a product) is often misinterpreted. Many studies on dietary carbon impacts focus on the generation of emissions within the farm gate but neglect to include those generated during the upstream and downstream processes.
Downstream Processes – Transportation:
Transport is the largest contributor to global warming in many developed countries (Wakeland et al., 2012). In the UK, transport contributed to 27% of total GHG emissions in 2019 (DEFRA, 2020a). The effects of transport can be significant if food travels long distances to reach the consumer. The total distance a food item is transported from Farm to Fork is termed ‘food miles’, which can be reduced through local food production and local consumption.
UK Food Imports:
Importing food drastically increases a diet’s carbon footprint. Seasonal fluctuations and climatic limitations within the UK limit the growth of many fruit and vegetables (Table 1). In 2019, 84% of fruit and 47.3% of veg was imported (DEFRA, 2020b). Alternatively, only 29% of meat was imported (DEFRA, 2020b) and, the UK was 80-86% self-sufficient in beef production (Committee on Climate Change, 2020; Reeve, 2020). Furthermore, 81% of beef sold in the UK is marketed under a British-produced logo and, many supermarkets (including Aldi, Budgens, Co-op, Lidl, M&S, Morrison’s and Waitrose) all use 100% British beef (National Beef Association, n.d).
In countries that are net food-importers, such as the UK, emissions outside the farm gate are much greater than in countries that are self-sufficient in food production (Green et al., 2015; Mbow et al., 2019). GHG emissions derived from transportation are significant and; therefore, it is critical to consider these when determining the carbon footprint of UK diets (Macdiarmid, 2012).
Transport by air has a much greater carbon footprint than transport by road or shipping (Table 2) (LDA, 2006; Wakeland et al., 2012); however, it is the fastest-growing transportation mode (Wakeland et al., 2012). The European Commission’s Farm to Fork strategy states: “they (society) want food that is fresh, less processed and sustainably sourced”. The high demand for fresher foods is likely the cause of increased airfreight, as these foods are highly perishable and, therefore, must arrive quickly to maintain quality and shelf-life (Ritchie & Roser, 2020). Other foods that contribute to increased UK GHG emissions from transportation include prepared (i.e., trimmed or chopped) produce and seasonal produce (Clonan & Holdsworth, 2012).
Common air-freighted foods include exotic fruits, green beans, berries and asparagus (Ritchie & Roser, 2020). Asparagus is imported into the UK by air from Peru, resulting in a carbon footprint of 5.3 kg CO2e per kg of asparagus produced (Frankowska et al., 2019). Although produce labels often state the country of origin, it is difficult to identify how the food is imported because there is limited transparency in fruit and vegetable supply chains (ETA, n.d; Ritchie & Roser, 2020). An extensive literature review by Chai et al. (2019) demonstrated that a plant-based diet based on high consumption of imported foods can have a carbon footprint equal to a moderate meat eater.
Refrigerated trucks are the most common mode for transporting food within and around the UK. Although these trucks have a lower carbon footprint than airfreight, energy is also required for maintaining temperature to ensure the quality and shelf-life of products (Stellingwerf, 2018). GHG energy emissions from refrigerated trucks may be as high as 40% of the vehicle’s emissions (Stellingwerf, 2018; Tassou et al., 2009) and, 15% of the world’s fossil fuel-derived energy is used in refrigeration during food transport (Adekomaya et al., 2016; Stellingwerf, 2018). On a global scale, 40% of the greenhouse effect is down to emissions from food transportation (Adekoymaya et al., 2016; Stellingwerf, 2018), thereby emphasizing the importance of local consumption when lowering the carbon footprint of your diet.
Agriculture vs. Transport GHG Emissions:
Contribution to Total UK Emissions:
A breakdown of the contribution of different sectors to total GHG emissions in the UK can be seen in Figure 1. In 2019, approximately 27% of GHG emissions in the UK were attributable to CO2 from transport (DEFRA, 2020a; Mbow et al., 2019) with transportation of food responsible for approximately 4% (17 Mt CO2e). The amount of food imported into the UK is evident by the greater amount of emissions produced during external food transportation (10 Mt CO2e) relative to internal (7 Mt CO2e).
Agriculture is estimated to be responsible for 10% of UK emissions, with cattle farming representing only 3% (14 Mt CO2e). These figures demonstrate the climatic impact of cattle farming is 3 Mt lower than that of food transportation. Although there will be a small amount of GHG emissions due to the transport of beef within the UK, the emissions produced from inorganic fertiliser application (5.3 Mt CO2e) are more likely associated with crop-based farming.
Figure 1 - Different sectors and how they contribute to total GHG emissions in the UK. Units are presented as MtCO2e.
The Committee on Climate Change (2020) estimated the GHG intensity from British beef to be approximately 48 kg CO2e per kg of meat. Efficient production processes within UK cattle farming have considerably lowered this value relative to the global average (99 kg CO2e/kg) (The Committee on Climate Change, 2020). The large difference between the UK and global cattle farming emissions demonstrates the importance of location when evaluating the climatic impacts of diets.
Trends in Agriculture Emissions:
Reducing agricultural CH4 and N2O emissions is much more complicated than minimising CO2 emissions. Agriculture emissions are difficult to measure as they are derived from a complex set of interactions between microbes and the soil (NFU, 2019a). In the UK, agricultural emissions have been in decline for many years (The Parliamentary Office of Science and Technology, 2015), with a reduction by 18% since 1990 due to fewer animal numbers (Mbow et al., 2019; NFU, 2019a), more efficient feeding (Hopkins & Lobley, 2009) and a reduction in synthetic fertiliser use (NFU 2019a). High-quality feed and adaptions in feeding techniques have improved animal feed efficiency and reduced CH4 generation during digestion (PIRD, 2020). Furthermore, global GHG emissions have declined by 60% since the 1960s and, this is predominantly due to improvements in cattle farming productivity (Davis et al., 2015; FAOSTAT, 2018; Mbow et al., 2019).
Atmospheric Impacts and Global Warming Potential (GWP):
The atmospheric impacts of GHGs depend on the length of time they remain in the atmosphere (their “lifetime”) and their ability to absorb energy (their “radiative efficiency”) (EPA, n.d; IEA, 2020). GWP combines the radiative efficiency and lifetime of each gas; to enable comparison of the global warming impacts of different GHGs over 100 years, GWP100 (EPA, n.d). GWP100 allows one tonne of different GHG emitted to be expressed in CO2 equivalent terms (IEA, 2020); therefore, the GWP for CO2 is always 1. Methane is more potent than CO2; however, it remains in the atmosphere for a shorter time (approximately ten years, relative to hundreds-thousands of years for CO2). The GWP of 1 tonne of methane over 20 years is estimated to be 84-87 (IEA, 2020), which equates to a GWP100 of 28-36 tonnes (EPA, n.d; IEA, 2020).
GWP has often been criticised by many scientists because it does not consider the contrasting impacts between long- and short-lived gases (Allen et al., 2016; Lynch et al., 2020). The impact determined by GWP100 assumes a GHG is only present in the atmosphere for 100-years; however, CO2 can persist for millennia. On the other hand, CH4 only remains in the atmosphere for 10-years on average (IEA, 2020). If all GHG emissions were to cease, the presence of CH4 would dissipate after a few decades, whereas CO2 would remain at a fixed level and continue warming the planet for hundreds of years (Lynch et al., 2020).
Utilising GWP100 in climate mitigation strategies may be unrealistic, as it does not consider the short-lived impacts some GHGs have. For example, the employment of GWP100 by the European Commission in the Zero Emissions Target will have a distinct impact when determining the climatic effect of different gases. A modification of GWP100, named GWP* has been proposed to overcome these problems. GWP* considers the cumulative effect long-lived gases have on global warming (Lynch, 2019). Many studies have worked on improving the accuracy of GWP* (see: Allen et al., 2016; Allen et al., 2018; Cain et al., 2019; Lynch et al., 2020); however, it is still yet to be adopted as a globally recognised metric.
Downstream Processes – Food Processing, Packaging and Waste:
Food supply chains outside the farm gate are energy-intensive; however, this is rarely discussed in studies. Food processing emits GHGs from the intensive use of electricity, natural gas, coal, diesel and other energy sources (Mbow et al., 2019). CO2 is emitted by cookers, boilers and furnaces, whereas wastewater streams from factories emit large amounts of CH4 and N2O (Mbow et al., 2019). Approximately 11 Mt CO2e is generated from food processing every year (Goodall, 2007), with the wet milling of maize and the processing of sugar and oils being some of the most energy-intensive processes (Mbow et al., 2019). Furthermore, the food processing stage often contributes to a large proportion of the total carbon footprint of vegan diets when animal-based products are replaced by industrial, highly processed meat and dairy alternatives (Chai et al., 2019; Sabate & Soret, 2014).
Food loss and waste (FLW) is defined as “food and, or associated inedible parts removed from the food supply chain” (Birney et al., 2017; Hanson et al., 2016). Wasting food also wastes the resources used to grow, harvest, transport and package the food. The manufacturing of food packaging alone generates approximately 10 Mt CO2e every year and, even biodegradable packaging releases CO2 and CH4 (equating to; 4 MT CO2e per year) during its decomposition at landfill sites (Goodall, 2007). FLW occurs at every stage in the food supply chain; during production, handling, storage (e.g., due to pests or disease), processing and packaging (e.g., from damaged products), distribution (e.g., when food passes its ‘best by date’) and consumption (Birney et al., 2017; Lipinski et al., 2013).
Consumer choices can most directly influence how much food is wasted. In the UK, food waste produces more than 25 Mt CO2e annually, with 70% of this derived from households (WRAP, 2020). The majority of household waste is due to fruit and veg (approximately 28%), whereas meat and fish only contributing to 6% of the total (WRAP, 2020), likely due to their higher price and ability for preservation.
Mitigating Climate Impacts:
Plants remove CO2 from the atmosphere (i.e., carbon sequestration) during photosynthesis as part of the biological cycle. Plants absorb CO2 and store it in different tissues. Forests and grasslands are referred to as ‘carbon sinks' because they can store large amounts of carbon in their roots and vegetation (Schahczenski & Hill, 2009). Livestock grazing plays a critical role in carbon sequestration by causing moderate disturbance which increases grassland production, enabling the capture and storage of CO2 in soil (NFU, 2019b). Furthermore, organic matter in the form of manure enriches the soils microbial life. Complex interactions between the livestock, soil microbes and plant roots result in net carbon sequestration (Dignac et al., 2017; NFU, 2019b). Livestock rotation can prevent overgrazing, promoting optimal forage growth and enabling the regeneration of land. Optimally managed herds have the potential to lower the carbon impact of beef production.
Soil carbon storage also increases soil water holding capacity, retaining nutrients that are beneficial to plant growth and improving yields and productivity (Bot & Benites, 2005). Grasslands are essential for agriculture in Europe and; therefore, they have great potential to act as carbon sinks (European Commission, 2017).
Sustainable food systems form a core part of the European Green Deal, which aims to tackle the climate and environmental challenges currently faced by our planet. Any product produced on an industrial scale and; therefore, every type of food production has an environmental impact - with GHG emissions produced during every stage of the food supply chain. Our food systems continue to be under high stress due to our rapidly growing population. Achieving sustainable food systems depends on; local production, seasonal consumption and a reduction in the amount of food waste.
Many studies promoting plant-based diets focus on the within ‘farm gate’ impacts on climate change but do not consider the effect of the complete supply chain. Studies often quote the emissions of GHGs concerning food production and not those produced from farm-to-fork. Although agriculture is a net contributor to GHG emissions globally, diets can also have unexpected consequences without careful considerations of where food has come from and how it was produced or processed. In the UK, the increasing population is coupled with an increased demand for many foods that cannot be grown in this country and; therefore, the UK will never be self-sufficient in food production.
However, the environmental impacts of diets extend beyond the generation of GHG emissions. Take a look at part 2 of this article, which covers the impacts UK dietary choice can have on deforestation, land use, biodiversity and water footprints. Part 3 is also available, which further discusses the concept of sustainability within food supply chains.
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