i 6 Marks

Write down the different causes of global warming.

Answer: Causes of Global Warming

1. Introduction

Global Warming refers to the long-term increase in Earth's average surface temperature, primarily due to enhanced greenhouse effect caused by rising concentrations of greenhouse gases (GHGs) in the atmosphere from human activities.

Since the pre-industrial era (1850-1900), global average temperature has risen by approximately 1.1°C. The IPCC states with >95% confidence that human activities are the dominant cause of observed warming since the mid-20th century.

Key Statistic: Atmospheric CO2 has increased from 280 ppm (pre-industrial) to ~420 ppm (2024) - highest in 800,000 years. Current rate of increase is unprecedented in geological history.

2. Primary Anthropogenic (Human-Caused) Causes

A. Fossil Fuel Combustion (~75% of CO2 emissions)

The burning of coal, oil, and natural gas for energy is the largest single contributor to global warming.

  • Power Generation: Coal and gas-fired power plants release ~40% of energy-related CO2. Coal emits ~1000g CO2/kWh vs ~450g for natural gas.
  • Transportation: Vehicles, aircraft, ships burning petrol, diesel, and jet fuel contribute ~20% of emissions. Global transport fleet emits ~8 Gt CO2/year.
  • Industrial Energy Use: Manufacturing processes, heating, and industrial operations.
  • Residential/Commercial: Heating, cooking, air conditioning.

Total Impact: Fossil fuels release approximately 36 billion tonnes of CO2 annually.

B. Deforestation and Land Use Change (~10-12% of emissions)

  • Forest Clearing: Converting forests to agriculture, pasture, or urban areas releases stored carbon. Trees store ~200 tonnes CO2/hectare.
  • Forest Fires: Both natural and human-caused fires release stored carbon and reduce future absorption capacity.
  • Peatland Drainage: Draining carbon-rich peatlands for agriculture exposes organic matter to decomposition.
  • Reduced Carbon Sinks: Fewer trees means less CO2 absorption through photosynthesis.

Scale: ~10 million hectares of forest lost annually; tropical deforestation releases ~4.8 Gt CO2/year.

C. Agriculture (~14% of direct emissions)

  • Livestock (Enteric Fermentation): Ruminant animals (cattle, sheep, goats) produce methane during digestion. Global livestock emit ~100 million tonnes CH4/year.
  • Rice Cultivation: Flooded rice paddies create anaerobic conditions producing methane. ~500 million tonnes CO2e annually.
  • Nitrogen Fertilizers: Synthetic fertilizers release nitrous oxide (N2O) through soil processes. ~3 million tonnes N2O/year.
  • Manure Management: Animal waste decomposition releases CH4 and N2O.
  • Agricultural Machinery: Tractors, pumps running on fossil fuels.

D. Industrial Processes (~6% of emissions)

  • Cement Production: Calcination of limestone (CaCO3 → CaO + CO2) releases ~0.9 tonnes CO2 per tonne cement. Cement industry: ~8% of global CO2.
  • Steel Manufacturing: Blast furnaces using coke release CO2 from carbon used to reduce iron ore.
  • Chemical Industry: Production of ammonia, petrochemicals, and other chemicals.
  • Aluminum Production: Electrolysis process and anode consumption.

E. Waste Management (~3-5% of emissions)

  • Landfills: Decomposing organic waste in anaerobic conditions produces methane (landfill gas is 50% CH4).
  • Wastewater Treatment: Sewage treatment releases CH4 and N2O.
  • Open Burning: Uncontrolled waste incineration releases CO2 and other pollutants.

F. Fluorinated Gases (~2% of emissions)

  • HFCs (Hydrofluorocarbons): Used in refrigeration and air conditioning; leaks during use and disposal.
  • SF6 (Sulfur Hexafluoride): Used in electrical equipment; extremely potent GHG (GWP = 23,500).
  • PFCs: Released during aluminum smelting and semiconductor manufacturing.

3. Natural Causes (Minor Contribution to Recent Warming)

Natural Factor Mechanism Current Contribution
Solar Variability Changes in solar irradiance affecting Earth's energy input Minimal (~0.1°C since 1900); cannot explain recent warming
Volcanic Activity Eruptions release CO2; aerosols cause temporary cooling Volcanic CO2 is <1% of human emissions
Ocean Currents El Nino/La Nina cycles redistribute heat Causes short-term variability, not long-term trend
Natural GHG Cycles Wetland emissions, permafrost thawing Amplifying human-caused warming (feedback)

4. Feedback Mechanisms (Amplifying Warming)

5. Sectoral Breakdown of Global Emissions

Sector Share (%) Primary Gases
Energy (Electricity, Heat) 25% CO2
Agriculture, Forestry, Land Use 24% CO2, CH4, N2O
Industry 21% CO2, F-gases
Transportation 14% CO2
Buildings 6% CO2
Other Energy 10% CO2, CH4

Scientific Consensus: "It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred." - IPCC AR6 (2021)

Conclusion

Global warming is primarily caused by human activities, particularly fossil fuel combustion and deforestation, which release greenhouse gases that trap heat in the atmosphere. While natural factors play a role, they cannot explain the rapid warming observed since the mid-20th century. Understanding these causes is essential for developing effective mitigation strategies. Addressing global warming requires transitioning to clean energy, transforming agriculture, protecting forests, and reforming industrial processes - a comprehensive transformation of how human society produces and consumes energy and resources.

Sources: Module 1 Notes | IPCC AR6 (2021) | NASA Global Climate Change | Global Carbon Project