Greenhouse gases and the carbon cycle

• Livestock contributes approximately 14% of anthropogenic greenhouse gas emissions; agriculture emits mainly biogenic methane and nitrous oxide, while industry and transport release fossil CO2. [1]
• Combustion of coal and oil releases CO2 from geological carbon stocks, which remains in the atmosphere for centuries and raises its CO2 concentration. [2]
• Ruminant methane is oxidized to CO2 within approximately a decade, and plants take up this CO2 during photosynthesis to support renewed grass growth. [2]
• At stable ruminant numbers, atmospheric methane inputs and removal can balance, whereas 100% of industry and transport emissions add new CO2 from fossil carbon stocks. [2]
• In the bathtub analogy, atmospheric CO2 is the water, emissions the inflow, and limited natural removal the drain; industrialization increased inflow beyond removal and raised CO2 concentrations and global temperatures.
• Atmospheric CO2 stabilization requires reduced fossil-carbon inflow, renewable energy adoption, and lower energy consumption.

European sheep and goats

• Constant ruminant numbers correspond to balanced methane inflow and removal; methane emissions from European sheep and goats have declined by approximately 3% annually since 1990. [3]
• The Oxford warming-linked emissions metric yields no additional warming from the declining methane emissions of European sheep and goats. [4]
• Continued constant methane emissions would stabilize their warming impact, while a 1% annual reduction in sheep methane through 2050 would cause faster cooling. [4]
• Similar proportional reductions in fossil CO2 emissions would still produce substantial warming because CO2 accumulates. [2][4]

Nitrous oxide and revised estimates

• Nitrous oxide has an atmospheric lifetime exceeding 100 years, and one tonne is equivalent to 265 tonnes of CO2.
• The IPCC Climate Change and Land assessment attributes over half of agricultural nitrous oxide emissions to livestock on managed pastures and rangelands. [5]
• The default factors in the 2006 IPCC guidelines yielded overestimates of nitrous oxide emissions from grazing and manure application. [6]
• Emission factors in the 2019 IPCC refinement are no more than half the earlier values for these sources. [7]

Grass-fed livestock, feedlots, and mitigation

• Many people support removing ruminant livestock to address global warming, although grass-fed systems occupy biodiverse grasslands with low fossil-fuel requirements.
• Diets containing more grass and fibre generate more methane.
• Domestic grazing livestock occupy the historical niche of wild grassland herbivores; their removal would allow wild herbivores to occupy that niche and produce comparable methane emissions, as in the past. [8]
• Preserving livestock production without grass-fed systems would require additional feed-crop cultivation.
• Under that replacement scenario, feedlot livestock would generate less methane but more than twice the fossil-fuel use and nitrous oxide emissions because of fertilizer inputs and dense livestock populations, together with land conversion and biodiversity loss.
• Methane reductions accompanied by increased fossil-fuel consumption provide little near-term benefit and a worse longer-term outcome; mitigation centers on fossil-fuel reduction and low-fuel food production.

References

1. [00:06] Tackling Climate Change through Livestock: A Global Assessment of Emissions and Mitigation Opportunities (FAO, 2013) — https://www.fao.org/4/i3437e/i3437e00.htm
2. [00:28] Losing Time, Not Buying Time (Pierrehumbert, 2010) — https://www.realclimate.org/index.php/archives/2010/12/losing-time-not-buying-time/
3. [02:08] Del Prado et al., iSAGE Deliverable 4.4: New Trajectories Towards Innovative Sheep and Goat Production Systems — https://ec.europa.eu/research/participants/documents/downloadPublic?appId=PPGMS&documentIds=080166e5cc083f12
4. [02:17] Improved Calculation of Warming-Equivalent Emissions for Short-Lived Climate Pollutants (Cain et al., 2019) — https://doi.org/10.1038/s41612-019-0086-4
5. [03:00] Climate Change and Land: An IPCC Special Report (IPCC, 2019) — https://doi.org/10.1017/9781009157988
6. [03:16] 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 4: Agriculture, Forestry and Other Land Use — https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol4
7. [03:21] 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories — https://www.ipcc-tfi.iges.or.jp/public/2019rf/index.html
8. [04:02] Intensifying Pastoralism May Not Reduce Greenhouse Gas Emissions: Wildlife-Dominated Landscape Scenarios as a Baseline in Life Cycle Analysis (Manzano and White, 2019) — https://doi.org/10.3354/cr01555