In the increasing discussions about how to address climate change there are conflicting claims about the need to address methane (CH4) emissions. Some people argue that methane emitted by cows is a primary cause of climate change. Others argue that methane from beef and dairy production is not of much concern because of its relatively short duration in the atmosphere. Unlike carbon dioxide (CO2), which lasts from 300 to 1,000 years in the atmosphere, methane typically breaks down in about 12 years. This second view suggests that because of its relatively short lifespan, methane won’t build up in the atmosphere, and methane emissions from livestock should not be a concern. How important is methane in global emissions and in our planning to mitigate emissions that contribute to climate change?
Methane’s overall contribution to agricultural greenhouse gas emissions
In the US the sectors that contribute the most emissions to climate change are transportation (28%), electric power (25%), and industry (23%) and these emissions are mainly related to fossil fuels (US EPA ). In the US and Wisconsin, agriculture accounts for about 10 and 18%, respectively, of total greenhouse gas emissions. Methane from cattle digestion (enteric emissions) and stored manure makes up roughly 45% of those agricultural emissions nationally (or 4.5% of total emissions in the US). Because Wisconsin has a prominent dairy sector, those enteric and manure emissions account for a little more than half of Wisconsin’s agricultural greenhouse gas emissions, or approximately 9% of the state’s total emissions. Thus, while it is not the largest contributor, methane from livestock is a significant greenhouse gas. And although emissions from transportation, industry, and electricity generation are greater than those from agriculture in the US, electricity and industry emissions are going down, while agricultural emissions continue to go up (Figure 1). In Wisconsin, methane from liquid manure storage accounts for most of this increase in agricultural greenhouse gas emissions (Figure 2).
Figure 1. Wisconsin Greenhouse Gas Emissions by Economic Sector, 1990-2020 in CO2-eq. Source: US EPA Greenhouse Gas Inventory Data Explorer https://cfpub.epa.gov/ghgdata/inventoryexplorer/
Figure 2. Wisconsin Greenhouse Gas Emissions from Agricultural Activities by Category in CO2-eq, 1990-2020. Source: US EPA Greenhouse Gas Inventory Data Explorer https://cfpub.epa.gov/ghgdata/inventoryexplorer/
Comparison of potency and duration of Greenhouse Gasses
What about the fact that methane only lasts about 12 years in the atmosphere, while other greenhouse gases persist for more than 100 years? Figure 3 shows how the expected lifespans of the three main greenhouse gases affect how much they build up in the atmosphere over time.
Figure 3. Buildup of methane, nitrous oxide, and carbon dioxide in the atmosphere over time, given constant emissions of 1 tonne (metric ton) per year, starting in 2023. Because methane breaks down in about 12 years, its levels do not build up as quickly over time given equal emission rates.
When greenhouse gases are assesed, climate scientists compare two factors to our most prevalent greenhouse gas, carbon dioxide (CO2): how much one unit of each gas contributes to warming, and how long that gas is expected to last in the atmosphere. Taking both of those factors into account, scientists calculate CO2 equivalence measures (CO2-eq) for other greenhouse gases such as methane. For example the IPCC estimates the 100 year CO2 equivalent for methane is 28, meaning that one unit of methane will cause 28 times as much warming as an equal amount of carbon dioxide. Thus, the CO2 equivalence measures used in most climate change discussions (and in Figures 1 and 2) already take the longevity of each greenhouse gas into account.
However, there is considerable debate about how to weigh the atmospheric lifespan of different gases. Table 1 shows estimates of the warming potential of key greenhouse gases over both a 20-year period and a 100-year period using two different measures: Global Warming Potential (GWP) and Global Temperature change Potential (GTP). The measure used in most climate change literature is the 100-year Global Warming Potential.
Table 1. Carbon dioxide equivalent (CO2-eq) values for methane (CH4) and nitrous oxide (N2O) “IPCC, 2014
Source: IPCC, 2014. Climate Change 2014: Synthesis Report, p. 87
Global Warming Potential (GWP) averages the warming a unit of gas causes over a span of years compared to the warming caused by the same amount of CO2. Using this measure and a 100-year time span, methane’s warming potential is about 28 times greater than that of CO2. Figure 4 provides a visual depiction of the relative impacts of one unit of CO2, methane, and nitrous oxide (N2O) using the GWP model over both 20 and 100 years.
Figure 4. Visualization of comparative warming impact of a single emission of a tonne of methane (CH4) or nitrous oxide (N2O) compared to a ton of carbon dioxide, averaged over 20 years and 100 years.
Rather than averaging the warming that occurs throughout the specified time period, Global Temperature change Potential (GTP) estimates how potent the warming potential of a gas emitted today will still be after a certain time period has passed. Because on average methane molecules break down (oxidize) to CO2 and water in the atmosphere in just 12 years, the GTP-100 for methane is 4, meaning that after 100 years have gone by, a ton of methane emitted today will “only” be warming the Earth 4 times as much as a ton of CO2 emitted today.
For both GWP and GTP, using a 20-year time frame rather than a 100-year frame makes the warming impact of methane compared to CO2 even greater, as shown in Table 1 and Figure 4.
The question of what time frame and what model to use when calculating the impact of different greenhouse gasses is difficult. There is no single right answer (and scientists use other models beyond GWP and GTP). Importantly, looking over a variety of models and methods can increase understanding and be beneficial for different applications of the data (e.g., short and long term planning). It is important to understand and think about the impacts over several different time frames: in the next ten or twenty years, over a human lifetime (roughly 100 years), and over multiple generations (500 years or longer). The world is experiencing severe climate impacts right now, and reducing methane emissions can help mitigate those impacts in the short term. On the other hand, if we fail to bring down our CO2 emissions because we rely too much on the short-term climate benefits of reducing methane, then we will not, ultimately, stem our emissions to the recommended levels. At the same time, comparing the impacts of different greenhouse gases is confusing even when a single time frame is used, which is why the GWP approach of averaging warming over 100 years is a useful compromise for most discussions.
Two types of emissions: Biogenic versus Non-biogenic
Another argument that is sometimes made is that methane emissions from livestock should not be a cause for concern because they are part of a natural biological cycle (referred to as biogenic emissions). According to this argument, this makes those natural emissions very different from methane that is released from fossil fuels.
It is true that methane emissions from fossil fuel production and use are fundamentally more serious than those from animal agriculture. While methane from both sources has the same warming effect, after the methane from fossil fuels breaks down into water and carbon dioxide, the CO2 that remains in the atmosphere contains carbon that was previously stored securely in the Earth’s crust. In contrast, methane from animals is part of what is called the biogenic carbon cycle. Biogenic methane is created when animals eat plants that have used photosynthesis to pull CO2 out of the atmosphere and turn it into carbohydrates, and then microorganisms in the animal’s digestive system and in liquid manure storage turn those carbohydrates into methane. In this biogenic carbon cycle the same carbon molecules keep cycling between atmosphere, living organisms, and soil in different forms. Thus, unlike methane emissions resulting from fossil fuel use, livestock methane does not cause an increase in the total amount of carbon contained in the atmosphere; instead, the carbon is in a continuous cycle. And at least in the US, fossil fuel production and distribution actually releases slightly more methane than livestock (Figure 5).
Figure 5. US Methane Emissions by Sector in 2021Source: US EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2021. https://cfpub.epa.gov/ghgdata/inventoryexplorer/index.html#iallsectors/allsectors/methane/inventsect/current
Total Biogenic methane emissions matter
Carbon in the form of methane (CH4) is a much more powerful greenhouse gas than carbon in the form of carbon dioxide (CO2), so because livestock agriculture is converting more carbon to methane, that is a problem. Agriculture is converting more carbon to methane in at least two ways. First, with modern livestock production there are more ruminants than there were historically in wild ecosystems. The estimate for North America is that total ruminant animal units before European settlement were about 85% as high as current domesticated plus wild ruminant numbers. That growth alone indicates that livestock production has caused at least a 20% increase over that estimated natural methane baseline from ruminant digestion. Second, wild ruminants deposit their manure on the ground where it decomposes aerobically and does not emit methane. In contrast, much of the manure from livestock is stored in slurry form, where it decomposes anaerobically, emitting significant amounts of methane. In the US and Wisconsin the increase of slurry manure storage systems is the fastest growing source of agricultural greenhouse gas emissions (see Figure 2).
Ways agriculture can reduce methane emissions
In order to prevent catastrophic climate change all sectors, including agriculture, need to reduce emissions. Methane, while a relatively short-lived greenhouse gas, is a contributor to climate change. Increases in methane emissions due to growing livestock numbers have significant climate impacts. Conversely, reductions in methane emissions from livestock could have immediate climate benefits and could be a large component of reducing greenhouse gas emissions from agriculture.
So how can farmers reduce methane emissions? There are several pathways to reduce livestock methane emissions, including:
- Management of stored manure. Slurry manure storage produces high methane emissions. Farmers can cover manure storage facilities to reduce methane emissions. The methane can be collected and burned, converting CH4 to CO2, with its much lower warming intensity. Farmers can also use manure digesters to generate methane to use on-farm for heating or to sell (methane is natural gas). Timing and placement of manure applications also affect greenhouse gas emissions.
- Management of feed. In general, feed that is highly digestible results in lower enteric methane emissions. In addition, farmers and researchers are experimenting with feed additives that reduce enteric emissions, at least in the short term. Graziers can optimize forage quality on pasture which will reduce enteric emissions from grazing livestock.
- Over the long term, breeding for lower enteric emissions may also help. There appears to be significant variation in the level of methane emissions between animals that is not correlated with production.
For both methane and CO2, moving away from our dependence on fossil fuels to sustainable renewable energy sources is a critical step to reduce climate impacts. Methane emissions from enteric fermentation and manure storage account for more than half of animal agriculture’s greenhouse gas emissions. This needs to be reduced if the US and the world are to meet emissions targets. Fortunately, farmers and researchers are working on ways to reduce methane emissions and still provide delicious cheese and meat.