The greenhouse effect and agriculture's greenhouse gases
How the greenhouse effect works, farming's gases (CO2, methane, nitrous oxide), global warming potential, CO2-equivalent sums and ways to cut them.
On this page
Without the greenhouse effect, Earth's average surface temperature would be about −18 °C instead of about 15 °C, too cold for crops, livestock or most life. The natural effect is essential. The problem is the enhanced greenhouse effect: since the industrial revolution, human activity has raised the concentrations of heat-trapping gases. The world has warmed by about 1.1–1.3 °C since pre-industrial times, and the warming is changing rainfall, heat extremes and growing seasons everywhere.
Agriculture sits on both sides of this problem. It is highly exposed to climate change, and it is a major source of greenhouse gases, especially methane and nitrous oxide. According to the IPCC, agriculture, forestry and other land use (AFOLU) produced about 22% of global net human emissions in 2019. The whole food system, including processing, transport and waste, accounts for roughly one-third.
How the greenhouse effect works
- The Sun emits mostly shortwave radiation: visible and near-infrared light.
- About 30% is reflected back to space by clouds, aerosols, ice and bright surfaces. This fraction is Earth's albedo.
- The rest is absorbed and warms the land and oceans.
- The warm surface emits longwave (infrared) radiation.
- Greenhouse gases, such as water vapour, CO2, methane, nitrous oxide and ozone, absorb much of this infrared radiation and re-emit it in all directions, including back toward the surface. This keeps the lower atmosphere and surface warmer.
Nitrogen and oxygen, which make up 99% of dry air, do not absorb infrared radiation. Greenhouse gases are effective because their molecules vibrate at infrared frequencies.
Water vapour is the largest natural contributor, but it acts as a feedback: warmer air holds more moisture, about 7% more per °C. The long-lived gases that humans add, mainly CO2, CH4 and N2O, drive the change.
The main gases and agriculture
| Gas | Pre-industrial | Recent level | Main agricultural sources |
|---|---|---|---|
| CO2 | about 280 ppm | over 420 ppm | Deforestation and land conversion, soil carbon loss from tillage and drained peat, farm fuel and fertiliser manufacture |
| CH4 (methane) | about 0.7 ppm | over 1.9 ppm | Enteric fermentation in ruminants, flooded rice paddies, manure storage, burning of crop residues |
| N2O (nitrous oxide) | about 270 ppb | over 335 ppb | Nitrogen fertilisers, manure and urine, legume residues and crop residues, via nitrification and denitrification in soil |
Agriculture produces about 40% of human methane emissions and about two-thirds of human nitrous oxide emissions.
Global warming potential
Gases differ in how strongly they trap heat and how long they last. Global warming potential (GWP) compares the warming from 1 kg of a gas, over a time horizon, with 1 kg of CO2. The IPCC's Sixth Assessment Report (AR6) gives 100-year values (GWP100) of:
- CO2: 1
- CH4: about 27 from biological sources, and about 30 from fossil sources
- N2O: 273
Methane is short-lived, lasting about 12 years, but potent. Its 20-year GWP is about 80. Cutting methane therefore slows near-term warming quickly. CO2 and N2O persist for a century or more.
Emissions from fertiliser on one hectare
A wheat field receives 120 kg N per ha. The IPCC default emission factor for direct soil N2O is about 1% of the applied nitrogen. Direct N2O–N = 1.2 kg, so N2O = 1.2 × 44 ⁄ 28 = 1.89 kg. In CO2 equivalent, that is 1.89 × 273 ≈ 515 kg CO2e per ha. That is before counting indirect emissions from leached and volatilised N, and the energy used to make the urea. For comparison, a dairy cow emitting about 100 kg of enteric methane a year produces about 100 × 27 = 2.7 t CO2e each year.
Agriculture's emissions in India
According to India's national inventory reported to the UNFCCC, agriculture produces roughly 14% of national greenhouse gas emissions, excluding land use. Enteric fermentation from India's large cattle and buffalo herd is the largest single share. Rice cultivation, agricultural soils, manure management and residue burning follow. Emissions per head are low, but the totals are large because of the scale of India's farming.
Reducing emissions
Mitigation options in agriculture include:
- Livestock: better feeds and feed efficiency, higher productivity per animal, which means less methane per litre of milk; feed additives such as 3-NOP and some oils; breeding; and better manure management, including biogas plants.
- Rice: alternate wetting and drying (AWD) and mid-season drainage, which cut methane by about 30–50% with little yield loss; direct-seeded rice; and suitable varieties.
- Fertiliser: the 4R approach (right source, rate, time and place), neem-coated urea, nitrification inhibitors, soil testing, and legumes in rotation.
- Carbon storage: agroforestry, reduced tillage, cover crops, compost, and restoration of degraded land.
- Avoiding residue burning by using Happy Seeders and in-situ or ex-situ residue management.
- Diets and waste: reducing food loss and waste, and shifting demand.
Many of these options also raise productivity or cut costs, which is the central idea of climate-smart agriculture.
Key points
- Greenhouse gases absorb outgoing infrared radiation and keep the surface about 33 °C warmer than it would otherwise be. Human emissions have enhanced the effect.
- About 30% of incoming sunlight is reflected (albedo); the rest is absorbed and re-emitted as infrared radiation.
- Agriculture's key gases are CO2 (land conversion, soil loss), CH4 (ruminants, rice, manure) and N2O (fertiliser and manure N).
- GWP100 (AR6): CO2 1, CH4 about 27–30, N2O 273. CO2e = Σ mass × GWP.
- AFOLU produces about 22% of global net human emissions; food systems as a whole produce about one-third.
- Mitigation includes AWD in rice, feed efficiency and biogas, 4R nutrient management, agroforestry and ending residue burning.
What the exams ask
- Explain the greenhouse effect and distinguish natural and enhanced effects.
- Name the main greenhouse gases, their sources in agriculture and their GWPs.
- Calculate CO2 equivalents.
- Explain why methane mitigation slows warming quickly.
- List mitigation options for rice, livestock and fertiliser use.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is the ozone hole the same as the greenhouse effect?
No. The ozone hole is the thinning of stratospheric ozone by chlorine and bromine compounds such as CFCs, which lets more ultraviolet light reach the surface. The Montreal Protocol has largely addressed it. The two issues are linked, because some ozone-depleting chemicals are also strong greenhouse gases, but they are different problems.
Does a cow really emit more than a car?
It depends on the car and the cow. A typical petrol car driven 12,000 km a year emits about 2–3 t of CO2, similar in order of magnitude to a dairy cow's enteric methane in CO2e. Methane's short lifetime means its warming behaves differently from CO2's, which is why scientists also use metrics such as GWP*.
Why is nitrous oxide so hard to control?
It forms through microbial nitrification and denitrification. These processes are driven by soil nitrogen, moisture, temperature and carbon, which vary greatly in space and time. A small share of the nitrogen applied becomes N2O, but its GWP of 273 makes even that small share important. Matching nitrogen supply to crop demand is the most reliable control.
Sources
- IPCC. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report. Cambridge University Press, 2021.
- IPCC. Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report, Chapter 7 (AFOLU). Cambridge University Press, 2022.
- Crippa, M. et al. "Food systems are responsible for a third of global anthropogenic GHG emissions." Nature Food 2, 198–209 (2021).
- IPCC. 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 4: Agriculture, Forestry and Other Land Use.
- Ministry of Environment, Forest and Climate Change. India: Third Biennial Update Report to the UNFCCC. Government of India, 2021.
More in environment and agro-meteorology
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.
Last reviewed