Understanding methane from livestock
Jane Court (Agriculture Victoria), Ainslie Macdonald and Richard Eckard (University of Melbourne)
As part of the CN30 Pathways project
The carbon cycle in the livestock system
Figure 1 shows the carbon cycle in a livestock grazing system, showing carbon (as C) existing in different phases in the system – as a gas (CO2) and as a carbohydrate in plants (CHO).
Plants capture CO2 from the atmosphere via photosynthesis to produce plant material as carbohydrate (CHO), which livestock eat for maintenance and production of food and fibre.
The main carbohydrate is cellulose as the largest energy source on earth. This ability to digest cellulose in plants is one of the key attributes of ruminants. They can use a largely indigestible energy source to maintain themselves and produce meat, milk and fibre.
Most of the carbon digested by the animal is respired back into the atmosphere again as CO2 and the products produced and consumed by humans are similarly respired back into the atmosphere (as CO2) over a 12-month period.
If that was all that happened the cycle would be completely in balance, with all the carbon absorbed and returned to the atmosphere in the same form. BUT, some of the carbon that the animal consumes is turned into methane (CH4) and belched out as a gas into the atmosphere (Figure 1).
Two key characteristics determine the impact of different greenhouse gases on the climate: the length of time they remain in the atmosphere and their ability to absorb energy.
Methane has a much shorter atmospheric lifetime than carbon dioxide (CO2) – around 12 years compared with centuries for CO2. After 12 years methane breaks down into CO2 and water. However, the other key issue is its contribution to warming while it is in the atmosphere. Methane absorbs much more energy while in the atmosphere - over a 20-year period, it is 80 times more potent at warming than CO2.
The emitting of methane makes the carbon cycle ‘imbalanced’ in its contribution to warming.
Methane sources and cycle
Figure 2. The life cycle of methane (CH4) from different sources as natural sources (volcanoes; wetlands etc); fossil fuel (oil; gas and coal extraction); from agriculture (livestock and rice fields) and landfill. CH4 breaks down after approx. 12 years to CO2 and H2O but whilst in the atmosphere it absorbs significant amounts of heat which is primarily absorbed by and stored in the deep ocean.
Methane is emitted into the atmosphere by a number of sources (Figure 2). These include natural sources (volcanoes and wetlands); fossil fuel production (e.g., oil, gas and coal extraction); from agriculture (e.g., livestock) and landfill. Fossil fuels are the largest emitters followed by ruminant livestock (e.g., sheep, goats and cattle).
Natural (Naturogenic) methane is not targeted for emissions reduction as these emissions aren’t caused by human actions. Methane from agriculture (biogenic) is created from CO2 already part of an otherwise balanced carbon cycle., Whereas fossil methane isn’t and therefore fossil fuel production contributes additional CO2 into the atmosphere once it is broken down, which is taken account of in total greenhouse gas emissions and warming potential.
Methane from all sources has the same significant absorption of heat from the atmosphere while it is there. The heat that methane absorbs during its lifetime is primarily transferred to the ocean, stored in the deep ocean, where it will continue to contribute to climate change for centuries after methane has broken down. Long-term, this heat significantly contributes to increases in global average temperatures.
So, in considering a constant sheep or beef herd over a 12-year period all the methane belched out in year one, has by year 12, been reverted back to CO2. Theoretically then, they are not putting any ‘new’ methane into the atmosphere. However, the methane has contributed to global warming while in the atmosphere and continues to beyond its lifetime and therefore can’t be ignored. In other words, with stable livestock numbers the amount methane in the atmosphere might be constant (i.e., no new direct warming) but heating potential has been cumulative into the ocean over this time – the damage is done and continues to do so.
The Global Methane Pledge is a non-binding agreement signed by 150 countries including Australia, which aims to reduce methane to keep net global increases in temperature below two degrees Celsius. However, unlike CO2 emissions, radiative forcing shows that methane does not need to be zero to keep the rise in global temperatures below two degrees Celsius, it just has to be reduced. Radiative forcing measures the impact of all greenhouse gas emissions by the amount of radiative energy they capture in the earth’s atmosphere, to determine the impact of greenhouse gas emissions on global warming.
This method was used to develop the Methane Pledge’s target of 30 per cent less methane by 2030 and the New Zealand target of 47 per cent less methane by 2050. This acknowledges that methane is different to carbon dioxide, and due to its short life and potency, methane can be reduced and have the same impact as sources of carbon dioxide reaching carbon neutrality (i.e., net zero). The Methane Pledge also identifies the energy sector, not agriculture, as the sector with the greatest potential for emissions reductions by 2030.
At an industry level the Australian red meat industry has set a target to be carbon neutral by 2030, not a requisite for every farmer. As a target to aim for, they are stimulating research and development (both public and private) to help the livestock industries reduce methane emissions. The most promising technologies are dietary supplements, most appropriate for intensive feeding but early life programming, vaccines, and genetics are being explored for grazing systems. Livestock producers won’t be able to significantly reduce or eliminate methane without new technology.
However, supply chains are driving the demand for lower emission meat, milk and fibre and this is generally for lower emission intensive products i.e. lower emissions per unit product rather than total emissions. Management practices that achieve this are doable and generally no-regrets options that increase productivity such as reproductive efficiency and focussing on productive animals and pastures.
“Every emission of methane makes the planet warmer, regardless of whether it arises from fossil fuel or biological sources.” – Professor Mark Howden, Vice Chair of the IPCC.
Key messages
- A key value of our grazing livestock is that they can digest widely available and largely indigestible carbohydrate in pastures to produce food and fibre
- The carbon cycle in livestock would generally be a balanced system but for the production of methane as part of the digestive process
- Whilst methane breaks down after about 12 years compared to decades for CO2, its warming potential is far greater, and longer lasting. The heat that methane absorbs in its lifetime is primarily transferred and stored in the deep ocean, where it continues to contribute to global warming for centuries, after the methane has broken down
- The largest emitters of methane are fossil fuel production followed by livestock. Unlike livestock, fossil fuels emit ‘new’ C into the atmosphere and therefore emit both methane and CO2 into the atmosphere
- The Global Methane Pledge recognises the difference in warming of CO2 and methane, by setting a target to reduce methane by 30 per cent and not zero, by 2030
- The red meat industry target for carbon neutral by 2030 is an industry target and not a target all farmers need to achieve
- Setting these targets has led to increased investment in technologies to help livestock farmers reduce emissions. If scientists can’t find a way to have ‘methane free’ sheep (or cows or goats), then we won’t be able to get methane to zero
- Supply chains are leading the drive for lower emission products but are focussed on lower emission intensity, which farmers can often do something about, and which is also positive for farm productivity and profitability.
References
IPCC Global Methane Pledge: Global Warming of 1.5°C: IPCC Special Report on Impacts of Global Warming of 1.5°C above Pre-industrial Levels in Context of Strengthening Response to Climate Change, Sustainable Development, and Efforts to Eradicate Poverty. (Cambridge University Press, 2022). doi:10.1017/9781009157940.
Myhre, G. et al. 8 Anthropogenic and Natural Radiative Forcing.
Nations, U. United Nations: The ocean – the world’s greatest ally against climate change.