Greenhouse Effect

1. Definition

• The greenhouse effect is a natural process in which certain atmospheric gases (called greenhouse gases) trap heat radiated from the Earth’s surface, preventing it from escaping into space.
• This maintains Earth’s average surface temperature at ~15°C (instead of -18°C).
• Enhanced greenhouse effect occurs due to anthropogenic (human-induced) emissions, leading to global warming and climate change.

🔹 2. Major Greenhouse Gases (GHGs)

3. Mechanism of Greenhouse Effect

• Incoming solar radiation (shortwave) passes through the atmosphere.
• Earth’s surface absorbs radiation and re-emits it as infrared (longwave) radiation.
• Greenhouse gases absorb infrared radiation and re-radiate it back toward Earth’s surface.
• Result → warming of lower atmosphere (troposphere).

Without this, Earth would be frozen. But excess GHGs lead to overheating.

4. Natural vs. Enhanced Greenhouse Effect

• Natural effect: Essential for life. Maintains habitable climate.
• Enhanced effect: Due to human activities (post-industrial revolution). CO₂ concentration has risen from 280 ppm (1750s) to 420+ ppm (2024).

5. Consequences of Enhanced Greenhouse Effect

• Global Warming → average global temperature increase (~1.2°C since pre-industrial).
• Climate Change → unpredictable rainfall, floods, droughts.
• Sea Level Rise → melting glaciers, thermal expansion.
• Extreme Weather → heat waves, cyclones, heavy rainfall events.

6. Implications for Agriculture (Agronomy Focus)

1. Crop Growth & Yield
   • Higher temperature → faster crop growth but shortened life cycle → reduced yields in cereals (wheat, rice).
   • CO₂ fertilization effect → C₃ crops (rice, wheat, soybean) may benefit in biomass production, but quality (protein content) decreases.
   •  C₄ crops (maize, sorghum, millet) show little benefit from increased CO₂.
2. Soil & Water; Increased evapotranspiration → soil moisture stress. More frequent droughts/floods.
3. Weeds, Pests & Diseases; Weeds (e.g., Parthenium, Cynodon) respond more positively to high CO₂. Warmer climate → wider spread of pests & pathogens.
4. Livestock; Heat stress reduces milk and meat productivity. More methane emissions from enteric fermentation.

7. Mitigation & Adaptation Strategies (Agronomic Practices)

(a) Mitigation – Reducing GHG Emissions

• Carbon sequestration: Agroforestry, conservation tillage, residue management.
• Efficient fertilizer use: Site-specific nutrient management, nitrification inhibitors, neem-coated urea.
• Alternate wetting and drying (AWD) in rice to reduce methane.
• Renewable energy use in agriculture (solar pumps, biogas).

(b) Adaptation – Coping with Climate Change

• Develop & adopt climate-resilient crop varieties (heat-, drought-, and flood-tolerant).
• Change sowing dates to match new climatic conditions.
• Diversification of crops and farming systems.
• Improved irrigation: micro-irrigation, rainwater harvesting.

8. International Agreements

• Kyoto Protocol (1997) – binding emission reduction targets.
• Paris Agreement (2015) – limit global temperature rise to below 2°C (preferably 1.5°C).
• IPCC (Intergovernmental Panel on Climate Change) provides scientific basis.

General Facts

• The term “Greenhouse Effect” was first used by Joseph Fourier in 1824.
• Earth would be about –18°C without greenhouse effect; with it → +15°C (difference ~33°C).
• Major greenhouse gases (GHGs): CO₂, CH₄, N₂O, O₃, CFCs, water vapour.
• The IPCC (Intergovernmental Panel on Climate Change) was established in 1988 to assess climate change.
• Agriculture contributes about 25% of global GHG emissions.
• Rice fields → major source of CH₄.
• Fertilizer use (urea, ammonium nitrate) → major source of N₂O.
• Livestock (cattle, buffaloes) → enteric fermentation emits CH₄.
• Kyoto Protocol (1997): first binding treaty on GHG reductions.
• Paris Agreement (2015): limit warming to below 2°C (preferably 1.5°C).
• India’s NAPCC (2008): includes National Mission on Sustainable Agriculture (NMSA).
• First World Climate Conference: 1979 (Geneva).
• CO₂ accounts for ~76% of total GHG emissions.
• Methane accounts for ~16%, nitrous oxide ~6%.
• Average global temperature rise since pre-industrial era: ~1.2°C.