Facts and figures on the greenhouse gases methane and nitrous oxide
Methane and nitrous oxide are formed through biochemical processes which occur in natural ecosystems. Agriculture is also a source of these greenhouse gases and can lead to a significant increase of emissions. Here we ask what their effects are, how they arise, and how we can prevent them.
According to the National Greenhouse Gas Inventory, the agricultural sector is responsible for approx. 14 per cent of Switzerland’s greenhouse gas emissions (year 2022; FOEN 2024). These figures refer to the scope of the agricultural sector as defined by the Intergovernmental Panel on Climate Change (IPCC) for the National Greenhouse Gas Inventories (IPCC 2006). Slightly different (usually higher) values may be obtained if the system boundaries are set differently.
According to the National Greenhouse Gas Inventory, agriculture is responsible for 86 per cent of methane (CH4) emissions and 64 per cent of nitrous oxide (N2O) emissions in Switzerland (FOEN 2024).
The parties to the Paris Agreement have agreed on a standard metric to quantify the climate impact of different greenhouse gases – the global warming potential (GWP, United Nations Framework Convention on Climate Change, UNFCCC, 2018). GWP100 values from the Fifth Assessment Report of the IPCC (IPCC, 2013) have been adopted for reporting purposes under the UNFCCC. The GWP100 for methane is 28, which means that, over a period of one hundred years, one kilogramme of methane has a 28 times greater global warming impact than one kilogramme of carbon dioxide (CO₂). In the Swiss agricultural sector, methane is emitted mainly in the dairy and livestock sector, during the digestion of fibrous feed in the rumen of ruminants (cattle, sheep and goats) and during the storage of farmyard manure in liquid form. The global warming potential of nitrous oxide is approx. 265 times higher than that of carbon dioxide. Nitrous oxide is produced wherever nitrogen is converted, e.g. during the storage of organic fertilisers and farmyard manure, and in particular during the application of nitrogenous fertilisers.
From 1990 to 2004, methane emissions fell by 8.5 per cent (FOEN 204). After 2004, methane emissions rose briefly, after which the trend reversed again. In 2022, methane emissions were around 10 per cent below 1990 levels. Nitrous oxide emissions show a similar trend, although they fell considerably more sharply – by 20 per cent – up to 2004. Since 2004, nitrous oxide emissions have barely declined at all and were 23 per cent below 1990 levels in 2022.
The reduction in methane emissions is primarily due to breeding progress and the intensification of production. This mainly led to an increase in the energy density of the feed ration and a more balanced nutrient ratio, and thus to a corresponding increase in feed efficiency. As a result, livestock numbers fell whilst production remained constant or even increased slightly. Nowadays, the average cow produces around 48 per cent more milk than thirty years ago. As a consequence, it emits 21 per cent more methane and excretes 11 per cent more nitrogen. Yet, overall, emissions per litre of milk have fallen by almost one-fifth (FOEN 2024).
The introduction of the Proof of Ecological Performance in the early 1990s led to a reduction in the amounts of nitrogen used (farmyard manure and commercial fertilisers), and hence to significantly lower nitrous oxide emissions. Nitrogen use efficiency could thus be increased substantially.
Both milk production efficiency and nitrogen use efficiency are subject to biophysical limits. Although further progress is still possible, it will become increasingly difficult to achieve. Accordingly, only minor advances have been made over the last 10 years.
Although there are hardly any practical measures with a major reduction effect of over 20 per cent, substantial reductions can be achieved through a combination of several smaller measures (Bretscher et al., 2018). These include, for example:
Breeding for efficient and site-adapted livestock production with low methane emissions
Optimisation of feeding and use of feed additives
Healthy, productive and long-lived cows
Herd management (fertility, calving interval, proportion of dairy and beef breeds)
Low-emission storage of farmyard manure (covering of slurry tanks, biogas plants)
Implementing many of these measures on commercial Swiss farms is often a challenge. Organisational and structural constraints, high costs and insufficient knowhow can make it difficult, if not impossible, to achieve the theoretically possible reduction potentials in practice (e.g. Zosso et al. 2024).
A feed ration containing plants rich in polyphenols, such as tannins, can reduce methane emissions. However, the quantity of such plants required is usually far greater than is realistic under practical conditions.
In addition, ruminants could be fed with extruded linseeds or other feedstuffs with a high fat content, which have the potential to lower ruminal methane production. However, the fairly large quantities required to achieve a significant reduction often make these measures unattractive for economic reasons. In addition, the cultivation of linseed as animal feed creates a conflict of interest with regard to land requirements for food and feed production.
Various feed additives based on essential oils and other plant extracts can also lower methane emissions and/or further increase feed efficiency. To date, however, a persistent, significant reduction has only been documented in individual cases.
Methane emissions can be significantly reduced with the aid of synthetic feed additives (e.g. 3-Nitrooxypropanol, abbreviated to 3-NOP). Numerous studies have confirmed the effectiveness of 3-NOP at reducing methane emissions. However, reliable long-term studies are also lacking in this area. Furthermore, there are concerns about the transferability of the research findings to breeds and production systems typical for Switzerland. To achieve the expected results, 3-NOP can currently be administered only to housed livestock through a total mixed-feed-ration.
A balanced protein/energy ratio in the feed ration can prevent the excretion of excess nitrogen in farmyard manures. This can contribute to higher nitrogen use efficiency and lower nitrous oxide emissions (Schrade et al., 2023). The urea content of the milk is a suitable and readily accessible indicator of nitrogen excretion. A carefully planned feed regime is recommended.
Some feed additives may also adversely affect or inhibit the beneficial rumen microorganisms that are responsible for a ruminant’s optimal utilisation of their natural feed resources, particularly fibre in grass and hay. This can impair production, which means that there is no overall reduction in greenhouse gases emitted per litre of milk or kilogramme of meat. Normally these aspects are rigorously tested in feed trials before a feed additive is recommended for commercial use. However, careful dosing of feed additives and ongoing monitoring of livestock performance indicators are still essential.
Ruminants naturally produce methane. The global rise in the number of ruminants, especially over the last 200 years, has contributed to a significant increase in atmospheric methane levels and thus to a global rise in temperature (Reisinger and Clark, 2018). If methane emissions worldwide remain constant, the global temperature will increase only slightly due to the short lifespan of methane. At the same time, it will not be possible to exploit an effective and rapid ‘cooling effect’ unless methane emissions are reduced.
Furthermore, the intensive production of feed concentrates and the use of nitrogen fertilisers contribute significantly to carbon dioxide and nitrous oxide emissions.
How much do direct livestock emissions actually contribute to global warming?
Modelled atmospheric temperature anomalies from 1850 to 2015 for all anthropogenic emissions (bold black line). The contribution of direct livestock emissions of CH4, N2O and CO2 from the conversion of grassland to total global emissions is shown in blue, red and green respectively (Figure 1, Reisinger and Clark, 2018).
The scale and form of current livestock farming is a consequence of the historical development of the global agriculture and food system. Hence it is virtually impossible to assign responsibilities to individual stakeholders. At the same time, all stakeholders have an individual responsibility to help solve the climate crisis.
Essentially, ruminants should primarily graze on permanent grassland that is unsuitable for growing arable crops. This roughage-based diet can be supplemented with by-products of food processing. In this way, ruminants would consume resources that only they can utilise. Arable land should ideally be reserved for producing products for direct human consumption (Swiss Federal Council, 2022). Alternative uses (e.g. forage production) should only be allowed as part of a crop rotation strategy aimed at maintaining soil fertility (Swiss Federal Council, 2022). In this context, consideration must be given to the important role played by leys (temporary grasslands) in conserving the humus content, i.e. carbon stocks in agricultural topsoils (Guillaume et al, 2022). Overall, this strategy requires ruminant numbers throughout Switzerland to be reduced compared with current levels. As a result, overall emissions would be lower due to lower stocking levels (e.g. Schader et al., 2015), although methane emissions per litre of milk would be slightly higher. In addition, the relatively high carbon stocks in permanent grassland soils can be maintained. Using farmyard manure in a way that is compatible with the humus balance and converting the by-products of arable cropping will still require integration of ruminant and arable crop production.
Different foods have very different greenhouse gas footprints (Poore and Nemecek, 2021). Greenhouse gas emissions from the production of animal-based foods, in particular ruminant meat, are generally many times higher than those from plant products.
Hence, there is also scope for action by consumers, who can substantially influence the footprint of their food consumption via dietary choice. By eating according to the recommendations for a healthy and balanced diet (food pyramid), the average food-related greenhouse-gas footprint in Switzerland could be reduced by approximately one half (Zimmermann et al., 2017).
Environmental impact of greenhouse gas potential (reference= 100%, LMP = scenario food pyramid) (Figure 6, Zimmermann et al. 2017)
Reduction of food waste would be a further major contribution to climate protection within the context of the food system.
The consumption of seasonal and local products should also be encouraged in order to avoid long transport routes (including storage and refrigeration) and reduce greenhouse gas-intensive production of foods in other regions of the world.
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