Discipline: environment; Keywords: methane, nitrous oxide, GHG, shale gas, denitrification, life cycle analysis, N fertilization.
Recent research shows that the contribution of methane (CH4) to the atmosphere by shale gas and other earth sources is much higher than accepted in IPCC calculations, which implies that much less is from ruminants (Howarth, 2019). Secondly, in carbon footprint calculations the CO2 capture by photosynthesis through the biogenic cycle in plant foods (say maize) (which apart from CO2 may also have resulted indirectly from atmospheric CH4) is not taken into account (Frankelius, 2020). This is actually a carbon sink. Thirdly, the lifespan of CH4 in the atmosphere is 10-12 years where after a substantial portion is removed by oxidation with tropical hydroxyl radicals (Badr et al., 1992). This implies that if X-amount of CH4 from agricultural (enteric etc) practices (all coming from the biogenic cycle) is added it will be in the atmosphere for about 12 years and if what is added thereafter remains at the same level, influx of CH4 into the atmosphere and removal from the atmosphere will be in balance. If the CH4 from agricultural practices is reduced, the warming effect will be reduced, if the CH4 is only coming from agriculture. However, it is not: the major portions will be coming from shale gas, coal mining, peat and most concerning, the CH4 trapped under ice caps, the release of which will escalate as the ice melts due to global warming. Why is this different to the CH4 from agriculture; simply because these sources were never released into the atmosphere before. CONCLUSION: CH4 from agriculture (ruminants) is not the major culprit as the world tends to believe; its influence in the atmosphere is much smaller than the figures accepted by the IPCC, FAO, IATP and others.
What then is the information on nitrous oxide (N2O). It is released into the atmosphere during the processes of denitrification (reduction of NO3- to N2 by soil microbes) under anaerobic conditions and nitrification (oxidation from NH4+ to NO3-) under aerobic conditions. Globally agricultural soils serve as a primary source of anthropogenic N2O emission. The greenhouse gas (GHG) national inventory report of South Africa (DEA, 2015) stated that the energy (10.7%) and Agriculture, Forestry and Other Land Use (AFOLU) (84.5%) sectors were the largest contributors to the total N2O emissions. The report estimates that N fertilizers in all agricultural soils are responsible for 28% of N2O emissions, whereas Smith (2020, pers. comm.) found in the grain industry that such fertilisers contribute 60-70%. Dairy farmers generally use high concentrations of fertilizer to promote pasture growth and maximize herbage yield to sustain milk production. However, the levels used are often way above benefit and, in addition, used inefficiently. A study by the University of Stellenbosch (Swanepoel, 2020; pers. comm.) showed no response in herbage yield above 220kg N per ha per year for kikuyu-perennial ryegrass pastures, whereas farmers may use 400 – 800kg. The N2O release into the atmosphere in the study rose exponentially from 220kg N to 440kg N, to 660kg N and to 880kg N per ha per year. On similar pastures Trace and Save in a life cycle analysis (Galloway, 2020; pers. comm.) estimated that manure management contributes 10.8%, fertilizer application 9.9%, fertilizer manufacture 4.9% and concentrate/roughage supplements 13.0% of N2O in CO2 equivalent terms, which is more than anticipated.
The lifespan of N2O in the atmosphere is 110-120 years and its global warming potential (GWP) is 298 times that of CO2 at GWP100, i.e. about exactly as calculated by the IPCC over the 100 year period, whereas the GWP of CH4 as a short-live polluter of the atmosphere (10-12 years), is much less than the 26-28 times CO2 calculated by the IPCC etc (Allen et al., 2018). Thus, although N2O's concentration in the atmosphere is only 1/1000 of CO2, it is 5-6% of all GHG in the atmosphere and increasing at a rate of 0.25% per year (Crutzen et al., 2008). Thus, its warming effect is substantial and increasing. Other problems due to or associated with N2O: following the ban on HFC's, N2O is now the dominant ozone-depleting substance in the atmosphere (Ravishankara et al., 2009); chemical N fertilizers are linked to cancer, eutrofication, groundwater pollution, poor soil health, high pest pressure in crop production and sub-optimum food quality (Otto, 2020; pers. comm.). IMPLICATIONS: Agricultural practices need to limit N fertilization with chemical fertilizers rather quickly because of its negative effects to soils, pollution and foods, in addition to the major effect of N2O as an atmospheric polluter!
Allen, M. R., Shine K.P., Fuglestvedt, J.S., Millar, R.J., Cain, M., David J. Frame, D.J. & Macey, A. H., 2018. A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation. Climate and Atmospheric Science (2018) 1:16 doi:10.1038/s41612-018-0026-8.
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Crutzen, P.J., Mosier, A.R., Smith, K.A. & Winiwarter, W., 2008. N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. Atmos. Chem. Phys. 8, 389–395.
DEA, 2015. “GHG National Inventory Report South Africa,” GHG Invent. South Africa, March 2015, Department of Environmental Affairs, Pretoria.
Frankelius, P., 2020. A proposal to rethink agriculture in the climate calculations. Agronomy Journal 112, 3216–3221.
Howarth, R.W., 2019. Ideas and perspectives: is shale gas a major driver of recent increase in global atmospheric methane? Biogeosciences 16, 3033–3046.
Ravishankara, A.R. et al., 2009. Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century. Science 326, 123 (2009); DOI: 10.1126/science.1176985