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Carbon dioxide is absorbed from the atmosphere by grass and other plants in the process of photosynthesis.
Pasture that is not eaten by animals breaks down naturally, and the carbon it contains returns to the atmosphere again as carbon dioxide through plant respiration and decay of dead plants. These processes cause no net change in carbon dioxide levels in the atmosphere.
If we now add grazing livestock such as sheep and cattle, the carbon cycle becomes more complex but it remains a cycle.
Livestock eat some of the grass and absorb the carbon it contains, but all of this carbon re-renters the atmosphere as carbon dioxide. Some of it re-enters it very quickly through the carbon dioxide that animals exhale. Some of the carbon is deposited in animal manure, which microbes break down into carbon dioxide. Some of the carbon is stored in animal products that we humans consume, such as meat and dairy; that carbon is released once we digest it or as food waste and dead animals decompose. Finally, some carbon is contained in animal leather, where it may be stored for many years before ultimately the leather decays and is again broken down by microbes.
Even though the individual processes are complex, there is no net change over time in carbon dioxide concentrations.
This cycle explains why growing grass and having livestock eat it is not a climate solution – it all remains a closed cycle. The only exception would be if some of the carbon is stored permanently in soil, by increasing soil carbon. This is possible in principle and would be very promising – but the evidence in New Zealand suggests that there has been very little net change in soil carbon across our agricultural lands.
If this cycling of carbon was the only thing happening in livestock systems, it would not make a substantial contribution to climate change.
But the digestive systems of ruminant animals also produce an additional and very powerful by-product: methane.
Methane is produced by microbes in the fore-stomach of ruminants, from the plants that animals eat (see What is enteric fermentation?). Methane is relatively a short-lived gas that decays back into carbon dioxide with a half-life of around 12 years. But while methane is in the atmosphere, it makes a powerful contribution to the overall warming effect of greenhouse gases because it is much more effective at absorbing heat radiation than carbon dioxide.
Current estimates are that emissions of methane to date have contributed almost 40 percent of the total warming effect from all human activities.
Given the importance of methane and livestock generally as a source of greenhouse gases, the New Zealand Agricultural Greenhouse Gas Research Centre is investing in a broad range of research, funded by the New Zealand government and in collaboration with industry, to find ways of reducing methane emissions from livestock in New Zealand. We are also working with partners internationally to help accelerate the search for solutions and to ensure technologies we develop in New Zealand can be adopted in other countries that find themselves in similar positions.