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The biogenic carbon cycle explained

Methane emitted by ruminants like cattle, sheep and goats is recycled into carbon in plants and soil, in a process known as the biogenic carbon cycle. It’s an important natural cycle that’s been happening since the beginning of life.

Cows (and other ruminant animals like sheep) are often linked to climate change because they emit methane, a powerful greenhouse gas (GHG).

But the fact is, this methane is part of a natural – or biogenic – carbon cycle, in which the methane breaks down into carbon dioxide (CO2) and water after about 12 years. Grass then absorbs the CO2 through photosynthesis, cows eat the grass and the cycle continues.

Biogenic Carbon Cycle.png

Photosynthesis and carbon sequestration

Photosynthesis is an important part of the biogenic carbon cycle. It’s the process by which plants remove CO2 from the atmosphere and deposit it into leaves, roots and stems, while releasing oxygen back into the atmosphere. This carbon is then converted into cellulose – an organic compound that’s present in grasses, shrubs and trees. Cows digest the cellulose, then burp out methane, which returns the carbon to the atmosphere so the cycle can start all over again.

Photosynthesis also facilitates the sequestration of carbon (removal and storage) in soil, large shrubs and trees.

 

Are fossil fuels part of the biogenic carbon cycle?

While the natural carbon cycle between cows, plants and the atmosphere takes place over a relatively short period of time (about 12 years), CO2 from burning fossil fuels stays in the atmosphere for potentially 1000s of years.

That means that, if methane emissions from livestock remain stable or reduce, fossil fuels have a much more significant and long-term impact on our climate than the methane from cows. With stable livestock numbers, the amount of methane produced actually balances the methane that breaks down from the atmosphere.

The animation below shows how the environmental impact of methane emissions from cows is fundamentally different to the CO2 from fossil fuels.

 

Did you know?

  • Dung beetles can recycle nutrients in pastures and help store carbon in the soil. Carbon sequestration from these ecosystem engineers could be equivalent to carbon sequestration from 400,000 hectares of eucalypt plantation (source).
  • Grazing of pastures by livestock helps remove GHG from the air by stimulating more plant growth, which accelerates the absorption of CO2 from the air, turning it into carbon in plants and soil (source).
  • According to researchers from the University of California, the production of beef and sheep meat in Australia at current levels will not contribute to global temperature rise (source).

FAQs

  • If Australian livestock numbers increase over the next few years how will this impact emission levels?

    If the herd numbers increase, there will be an increase in methane. However, through the industry’s carbon neutral by 2030 (CN30) program of work, we are continuing to develop and implement technologies to reduce methane and increase carbon sequestration to achieve a net zero greenhouse gas position for the red meat industry.

    The solution includes increasing the use of natural supplements and additives to reduce emissions, as well as continuing to store carbon in the landscape. With increasing livestock numbers, the industry will continue to increase the rate of adoption of these practices to enable the industry to achieve CN30.

  • While the amount of methane being emitted by cattle may not change year to year how can that mean that there is no impact on temperatures?

    As outlined in the video above not all methane is created equal.

    Methane generated by ‘life’ is called biogenic methane. One notable source of biogenic methane is ruminants, such as sheep and cattle.

    Biogenic methane is any methane created by things alive today and things that have very recently died. That could be from cows, landfills, or microbes in stagnant ponds. This is a major contributor to the agricultural sector’s climate impact. The creation of biogenic methane is intricately linked to the drawdown of carbon dioxide by photosynthesis.

    Simply put, when ruminant animals eat grass, methane is released when they belch, and also from their manure. After 12 years or so, that methane breaks down into ‘natural’ or biogenic CO2 and water. The grass absorbs the CO2 through photosynthesis and turns it into carbohydrate. Cows eat the grass, and the whole cycle starts again. 

    Methane generated by burning fossil fuels is termed ‘fossil methane’ and its impact on our climate is far more destructive.

    Fossil methane is made from carbon that has been stored underground for millions of years far from the surface and the global atmosphere. Virtually all gas burned for energy today is fossil methane, and the production of all fossil fuels involves releasing significant amounts of methane. Releasing this methane involves re-introducing old carbon to the active biosphere that had long ago been removed from the system.

    Because of the biogenic cycling of carbon, if livestock numbers stay the same, eventually (in about 12 years), the methane produced by livestock will not contribute additional global warming.

    In contrast, CO2 and methane produced from burning fossil fuels, are  new to the atmosphere. They do not stem from the natural carbon cycle. New additions of these gases build on what’s already there, day after day, year after year.

    Biogenic methane explainer

    Diagram courtesy of the Climate Council Australia

  • How can livestock be a part of the climate solution? Doesn’t methane have more warming power than CO2?

    Yes. Using the ‘carbon dioxide equivalent’ (CO2e) metric, methane is commonly referred to as being anywhere from 28 to 100 times more powerful than carbon dioxide, but it is important to factor the lifespan of the gases into the equation. When it comes to the difference between methane and carbon dioxide, the most important aspect that is missed by the simple metrics is full consideration of the gases’ different lifespans in the atmosphere. It takes the CO2 released from fossil fuels around 1000 years to be redeposited back into the earth but it only takes methane belched by cattle around 12 years.

    Methane is a live-fast, die-young gas. Once released into the atmosphere, methane traps heat much more efficiently than carbon dioxide, but only over the course of around a decade. After this point, it is broken down into carbon dioxide and water, among other things, in a complex set of chemical reactions. Once methane becomes carbon dioxide is remains stable in the atmosphere until it is drawn down.

    Carbon dioxide, on the other hand, is a ‘slow and steady’, tortoise-type greenhouse gas. It has a lower potential to heat the atmosphere, but it is a relatively stable gas. This means that unless it is drawn out of the atmosphere through a natural or human process, it continues to heat the atmosphere more-or-less indefinitely. It doesn’t get broken down in the same way that methane is.

    Adding carbon dioxide to the active biosphere is a bit like adding water to a sealed water tank. Just like adding water to the tank raises the water level in the tank, so too does adding carbon dioxide to the biosphere. Unless it is removed by some additional means, adding more of both water and carbon dioxide over time results in more being present. This is pretty simple.

    Adding methane works differently, it’s a bit more like a tank with an old-slow-to-start, automatic pump attached. This pump tries to remove water at the same rate that it’s added, but takes a long time to start. In the short term, adding more to the tank lifts the level in the tank, but in time – once the pump kicks in – as much is removed as is added. If the rate that water is added remains stable, eventually the amount in the tank will stabilise at a higher level. The breakdown of methane in the atmosphere works in a similar way. Changing the rate that methane is added causes a short-term shift in the amount of methane present, but if the same rate is sustained, then after about a decade the total amount in the atmosphere stabilises as additions are balanced by removals.

    Methane water tank explainer diagram

  • Do cattle emit more GHG than they currently sequester?

    In Australia, methane emissions from fossil fuels are rising due to expansion of the natural gas industry, while agriculture emissions are falling.

    Under a stable livestock population within the red meat agricultural sector, there is no additional impact on temperature as emissions would not increase.

    Under the Australian red meat industry’s goal to be carbon neutral by 2030 (CN30), carbon neutral means net zero GHG emissions on an annual basis. This means that the amount of GHGs released to the atmosphere by industry is equivalent or less than the amount of additional carbon stored in soils or vegetation in grazing lands in a given reporting year. The sources and sinks of emissions under the CN30 initiative are illustrated below.

    CN30 - diagram explaining how the red meat industry will achieve carbon neutrality by 2030

    According to CSIRO, it is possible to achieve CN30 without reducing livestock numbers below the rolling 10‑year average to 2015 (25 million cattle, 70 million sheep and 0.5 million goats).

    The red meat industry is working to reduce emissions through initiatives such as production efficiencies, dietary improvements, renewable energy sources and increase CO2 stored on farm in the soil and in trees and shrubs.