Introduction. The greenhouse effect is a natural process by which certain gases present in the earth’s atmosphere trap a part of the outgoing long-wave (infrared) radiation and keep the planet warm enough to support life. Without this natural greenhouse effect, the average temperature of the earth would be far below the freezing point of water. However, human activities have increased the concentration of some greenhouse gases, thereby intensifying this effect and causing global warming. Two important greenhouse gases are carbon dioxide (CO₂) and methane (CH₄).
1. Carbon dioxide (CO₂)
Carbon dioxide is the most significant anthropogenic (human-generated) greenhouse gas. It is released into the atmosphere by:
- Burning of fossil fuels such as coal, petroleum and natural gas in power plants, industries, vehicles and household energy use.
- Deforestation and burning of forests, which not only add CO₂ but also reduce the number of trees available to absorb it.
- Certain industrial processes, for example, cement manufacturing.
Although CO₂ is naturally present in small quantities and is a part of the normal carbon cycle, its concentration has increased sharply due to industrialisation and urbanisation. Because of its long residence time in the atmosphere and its large volume of emissions, carbon dioxide is considered the principal driver of the enhanced greenhouse effect and hence of global warming and climate change.
2. Methane (CH₄)
Methane is another important greenhouse gas, present in much smaller quantities than CO₂ but having a much higher heat-trapping capacity per molecule. Its major sources include:
- Decomposition of organic matter in waterlogged and anaerobic conditions, as in paddy fields and wetlands.
- Enteric fermentation in the stomach of cattle and other ruminants (e.g. cows, buffaloes, sheep, goats), which release methane during digestion.
- Leakages during extraction, processing and transport of natural gas, coal mining and oil exploration.
- Decomposition of organic waste in landfills and garbage dumps.
Molecule for molecule, methane is several times more effective than carbon dioxide in trapping heat in the atmosphere over a shorter time period. Therefore, even moderate increases in methane concentration contribute significantly to the greenhouse effect and to the warming of the lower atmosphere.
Other greenhouse gases (for context)
Besides carbon dioxide and methane, other greenhouse gases include water vapour (H₂O), nitrous oxide (N₂O), ozone (O₃) and several man-made gases such as chlorofluorocarbons (CFCs). However, for exam purposes, when only two greenhouse gases are asked, CO₂ and CH₄ are most commonly written because of their strong link with human activities and climate change.
Conclusion: Thus, two major gases that contribute to the greenhouse effect are carbon dioxide and methane. Both are naturally present in the atmosphere, but human activities have increased their concentrations, thereby strengthening the greenhouse effect and leading to global warming and climate-related environmental problems.