In the agricultural sector, especially dairy farming, attention to climate change is growing. The sector is actively engaged with various initiatives in which farms play a key role.
But what exactly is the climate problem? And what does the term carbon footprint mean? Why does this topic get so much attention? What is the role of agriculture in this? And are there any side notes on the impact of dairy farming?
In this article, we answer these questions step by step, starting with the basics.
Greenhouse effect and global temperature
We talk about a product's carbon footprint in the context of global warming - the steady increase in the earth's average temperature.
The temperature on our planet is largely determined by the natural greenhouse effect. This creates a balance between the solar energy the Earth absorbs and the heat radiated back, some of which is retained in the atmosphere by greenhouse gases. This natural process makes the earth livable with an average temperature of 15°C. Without the greenhouse effect, the average temperature on our planet would be around -19°C.
The main greenhouse gases are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and water vapor (H₂O). Together they affect the Earth's temperature, in addition to other factors such as solar activity, for example.
Since the creation of our planet about 4.5 billion years ago, natural processes have fluctuated the concentrations of these greenhouse gases in the atmosphere. This has resulted in alternating ice ages and warmer periods, often over tens of thousands to millions of years.
Looking at the past 2500 years, the CO₂ concentration remained relatively stable for a long time. This changed dramatically after the industrial revolution, when James Watt's more efficient steam engine (1760-1770) enabled the large-scale use of fossil fuels, such as coal. This greatly increased CO₂ emissions in a short period of time, resulting in a rapid rise in CO₂ concentration in the atmosphere. In other words, the historical balance was upset.
It has been scientifically proven that the rapid rise of CO₂ in the atmosphere since 1850 is entirely due to human activities. This includes methane and nitrous oxide. Since then, the Earth's average temperature has increased by about 1.5°C. This warming can be explained solely by the increase of greenhouse gases in the atmosphere and not by natural factors such as solar activity.
If no action is taken, climate models predict that average temperatures could increase by 5°C or more by 2100 in the worst-case scenario. The consequences of this are radical: more frequent extreme weather events, crop failures, water shortages and mass migrations.
To combat climate change, the international community has taken action in recent decades. The Kyoto Protocol was a first step. The UN Climate Conference in Dubai (COP28, 2024) reaffirmed the objective of the Paris Agreement (COP21, 2015): limit warming to well below 2°C, with a goal of no more than 1.5°C above pre-industrial levels.
Impact of agriculture
Like other sectors in the global economy, agriculture also contributes to climate change. In the EU, agriculture is responsible for 10,8% of total human greenhouse gas emissions, expressed in CO₂ equivalents (more on this below). The largest impact comes from CO₂ emissions from burning fossil fuels for energy, transportation and industry.
Looking at livestock production in the EU, beef, cow's milk and pork have the largest contribution in absolute figures . Emissions from beef production and cow milk production each amount to about 29%, pork production accounts for 25%, all other animal products together account for 17% of total emissions from livestock [1].
Carbon footprint of milk
Three greenhouse gases are important in dairy farming: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). A major contribution comes from enteric fermentation, the process by which microorganisms in the rumen of ruminants break down nutrients releasing methane. In addition to methane from rumen fermentation, there are also methane emissions from volatilization from manure, nitrous oxide emissions from manure storage and fertilization, carbon dioxide emissions from fuel use, and carbon dioxide emissions from land use change. You can find more explanation of these different emission sources, for example, in our previous publication.
Each greenhouse gas has a different ability to retain heat in the atmosphere. One could therefore report emissions of the different gases separately. However, to allow comparison with other industries that emit only CO₂, for example, emissions are converted into CO₂ equivalents. A CO₂ equivalent of a certain greenhouse gas indicates how much heat it retains relative to the same amount of CO₂ over a specific time period. The international community generally uses a period of 100 years, giving each greenhouse gas a GWP100 factor (Global Warming Potential .
Biogenic methane (the term "biogenic" will be explained later) has a GWP100 factor of 27, meaning that 1 particle of methane in 100 years contributes 27 times more to global warming than 1 particle of CO₂. Nitrous oxide has a GWP100 factor of 273, meaning it contributes an even much larger amount.
To calculate the carbon footprint per unit of milk , the total emissions released from milk production are divided by the amount of milk produced. To make a comparison possible here as well, FPCM (Fat and Protein Corrected Milk) is used. In this way, one obtains a carbon footprint expressed in grams of CO₂ equivalents per kilogram of FPCM (g CO₂-eq/kg FPCM).
The average carbon footprint of milk in Europe varies, but is typically between 1000 and 1500 g CO₂-eq/kg FPCM [2]. The final value can vary greatly between farms and depends on, for example, productivity, cropping plan, fertilization and ration composition. There are several strategies to reduce the carbon footprint, each with a techno-economic impact, as already discussed in our previous publication.
Side notes
It has been scientifically proven that climate change is a real problem and that humans play a significant role in it. Denying or dismissing these facts does not help or bring solutions. However, it is crucial to approach the problem as scientifically as possible and consider the full picture. From that perspective, three observations can be made.
Side note 1: short-lived biogenic methane versus long-lived CO₂
Cattle are capable of digesting high-fiber roughage such as grass - foods that humans cannot digest. They convert these energy sources into milk and meat. During the digestion process by the microorganisms, the rumen also produces methane, which is emitted through the animal's mouth.
Methane is a potent greenhouse gas that contributes to climate change. At the same time, methane is a short-lived gas: it remains in the atmosphere for about ten years before it is broken down to mainly CO₂. This released CO₂ is then reabsorbed by plants through photosynthesis, which grows new forage. This circle - of methane from cattle, to the atmosphere, to plants and back to cattle - is known as the biogenic carbon cycle. If as much methane is formed as is reabsorbed by plants (roughage) after decomposition to CO₂, the balance is maintained.
This contrasts with CO₂ or methane released from the combustion of fossil fuels. These gases come from carbon stocks stored in the earth for millions of years. When burned, they add additional greenhouse gases to the atmosphere without natural reabsorption.
Emission reports are often based on GWP100 (the 100-year impact), equating one ton of methane to 27 tons of CO₂. However, GWP100 does not take into account the fundamentally different behavior of methane and CO₂. Nitrous oxide, the other major greenhouse gas in agriculture, behaves much more like CO₂ and is therefore well represented by conventional GWP measures.
The much shorter lifetime of methane compared to CO₂ makes comparison of climate impact difficult. Nevertheless, a comparison of the global warming impact of the various greenhouse gases is necessary to make policy choices. To better reflect the true impact of methane emissions, the GWP* (Global Warming Potential Star) was developed as a complement to the traditional GWP100 method [4]. GWP* more accurately describes the actual warming caused by methane (CH₄).
The diagram below compares the impact of long-lived, cumulative CO₂ (red) with that of a short-lived methane (blue). Three scenarios are shown: a steady increase in emissions, a constant amount of emissions, and a decrease in emissions to zero, in all cases over several decades. The bottom panels show the temperature change due to these emissions.
- When emissions increase, both CO₂ and methane cause warming.
- With constant CO₂ emissions, the temperature continues to rise because CO₂ continues to accumulate in the atmosphere. In contrast, constant biogenic methane emissions lead to stable methane concentrations in the atmosphere, so temperatures continue to rise only very slowly.
- As CO₂ emissions fall, temperatures continue to rise as long as they are not zero. In contrast, a decrease in methane emissions by more than about 3% per decade leads to a decrease in temperature.
Summary: Methane requires a differentiated approach when evaluating climate impacts. For example, using GWP100 overestimates the impact of constant methane emissions on temperature. The alternative GWP* provides a more accurate picture of the actual climate effects of methane. and a targeted reduction of methane emissions can be a powerful strategy to combat further warming.
Side note 2: emissions versus nutritional value
The traditional method of comparing food sustainability focuses on the carbon footprint per kilogram of product. However, this approach does not take into account the nutritional value. For example, a liter of milk offers significantly more nutrients than a liter of cola. From this perspective, it is interesting to include nutrients (proteins, vitamins and minerals) and their absorption in the human body.
When comparing animal to "plant" milk, we find that the carbon footprint of plant-based variants - expressed per kilogram of product - is lower than that of semi-skimmed milk. However, when the footprint is assessed based on nutrient content, semi-skimmed milk generally has a lower footprint, with the exception of soy drinks (as shown in the figure below). To improve nutritional value, plant-based beverages are often fortified with additional nutrients. For the fortified variety (on the right side of the figure and marked with an *), the footprint based on nutrient content for soy beverages, oat and almond drinks is lower than semi-skimmed milk, while coconut and rice drinks continue to have higher footprints [6].
Summary: dairy has naturally high nutritional value. If the dairy sector succeeds in reducing its footprint, dairy will remain competitive with plant-based varieties in terms of sustainability as well.
Side note 3: circularity in the food system
The global food system is under pressure from several factors, including the effects of climate change, depletion of natural resources and even political conflicts. At the same time, the world's population continues to grow. A key question, therefore, is: how to ensure healthy food for all while protecting the planet? The answer is complex, as the food system is a dynamic and interconnected whole.
One promising approach that is gaining increasing attention is a circular food system. Circular food systems utilize agricultural land primarily for crops for human consumption, while livestock are fed on low-value biomass, such as grassland, food industry by-products (e.g., brewer's grains and beet press pulp) and food waste, taking into account the importance of crop rotations primarily for soil fertility.
A recent study investigated the potential to redesign the European food system according to circular principles, with the aim of ensuring food security and reducing climate and environmental impact [8]. The current European food system was taken as the basis, with associated cropping plans, yields and fertilization.
The best-performing circular food system would produce food for an additional 767 million people with virtually the same land use, while the CO2-emissions per person decrease by 38%. In this modified system, there is a significant reduction in beef cattle, pigs, broilers and laying hens, while fish and dairy cattle increase by more than 100%. In the corresponding diet, there is a change in the ratio of animal to vegetable protein from 60:40 to 34:66.
Summary: a circular food system, with an essential role for dairy cattle, has the potential to significantly improve both human and planetary health.
References
- Leip, A., Weiss, F., Wassenaar, T., Perez, I., Fellmann, T., Loudjani, P., Tubiello, F., Grandgirard, D., Monni, S., & Biala, K. (2010). Evaluation of the livestock sector’s contribution to the EU greenhouse gas emissions.
- Mazzetto, A. M., Falconer, S., & Ledgard, S. (2022). Mapping the carbon footprint of milk production from cattle: A systematic review. Journal of Dairy Science, 105(12), 9713–9725.
- Vellinga, T., & Groenestein, K. (2022). 10 questions and answers about methane, a short-lived greenhouse gas.
- Lynch, J., Cain, M., Pierrehumbert, R., & Allen, M. (2020). Demonstrating GWP*: A means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants. Environmental Research Letters, 15(4).
- Allen, M., Lynch, J., Cain, M., & Frame, D. (2022). Climate metrics for ruminant livestock.
- de Jong, P., Woudstra, F., & van Wijk, A. N. (2024). Sustainability Evaluation of Plant-Based Beverages and Semi-Skimmed Milk Incorporating Nutrients, Market Prices, and Environmental Costs. Sustainability, 16(5).
- Foley, J. A., Ramankutty, N., Brauman, K. A., Cassidy, E. S., Gerber, J. S., Johnston, M., Mueller, N. D., O'Connell, C., Ray, D. K., West, P. C., Balzer, C., Bennett, E. M., Carpenter, S. R., Hill, J., Monfreda, C., Polasky, S., Rockström, J., Sheehan, J., Siebert, S., ... Zaks, D. P. M. (2011). Solutions for a cultivated planet. Nature.
- Van Zanten, H. H. E., Simon, W., Van Selm, B., Wacker, J., Maindl, T., Frehner, A., Hijbeek, R., Ittersum, M. K. van, & Herrero, M. (2023). Circularity in Europe strengthens the sustainability of the global food system. Nature Food, 4.
