Carbon Dioxide

• Carbon dioxide (CO₂) is the primary greenhouse gas contributing to the enhanced greenhouse effect. While CO₂ has a variety of natural sources and sinks, human activities are responsible for the considerable increase in CO₂ since the industrial revolution.
• CO₂ is part of the Earth’s carbon cycle, circulating among air, water, and ecosystems. Burning of fossil fuels (coal, oil, natural gas) and wood is the primary source of human-caused CO₂ emissions. Deforestation and land use changes reduce the effectiveness of natural sinks.
• According to the IPCC, greenhouse gas emissions have increased since the pre-industrial era, with half of all emissions since 1970.
• About 40% of emissions remain in the atmosphere, while the rest is stored in plants, soil, and oceans. Oceans absorb around 30%, but this causes ocean acidification. The future efficiency of these sinks remains uncertain.

Methane

• Methane (CH₄) is the second most important GHG after CO₂. It is emitted from both natural and human sources. Natural sources include wetlands, tundra, oceans, sediments, and termites (about 36%). Human sources (about 64%) include landfills, livestock, rice paddies, biomass burning, and fossil fuel usage.

Nitrous Oxide

• Nitrous oxide (N₂O) is the third most important GHG. Though it exists in small amounts, it has a long atmospheric lifetime (~120 years) and has nearly 300 times the global warming potential of CO₂.
• N₂O levels are now highest in 800,000 years. Since the industrial revolution, they’ve increased by 16%.
• Natural sources: soils, tundra, oceans.: Human sources: agriculture, fertilizers, manure, combustion of fossil fuels, and industrial processes.

Water Vapour

• Water vapour (H₂O) is the most abundant natural greenhouse gas. Its concentration is not directly controlled by human actions but responds to warming of the atmosphere.
• As temperatures rise, more evaporation occurs from rivers, oceans, and soil, increasing water vapour, which in turn causes more warming — a positive feedback loop crucial for climate change projections.

Ozone

Ozone (O₃) plays dual roles:

• Tropospheric ozone (ground-level): acts as a warming greenhouse gas and affects GHG lifetimes.
• Stratospheric ozone: provides a cooling effect by blocking UV radiation from the Sun.

• Tropospheric ozone is formed from pollution and sunlight, contributing to ~1/3 of GHG warming since the industrial revolution. It’s hard to track due to its short lifespan and regional variation.
• Hydroxyl (OH) radicals formed from ozone breakdown help reduce methane levels.