The Current Atmosphere
The proportions of gases in the Earth's atmosphere today have been relatively stable for about 200 million years:
- Nitrogen (N₂): ~78%
- Oxygen (O₂): ~21%
- Argon (Ar): ~0.9%
- Carbon dioxide (CO₂): ~0.04%
- Water vapour and other trace gases: very small amounts
The Early Atmosphere
For the first billion years of Earth's existence, the atmosphere was very different. Intense volcanic activity released gases that formed the early atmosphere.
Composition of the Early Atmosphere
- Mainly carbon dioxide (CO₂) — similar to Mars and Venus today.
- Very little or no oxygen.
- Water vapour (from volcanic eruptions), which later condensed to form the oceans.
- Smaller amounts of methane and ammonia.
How Oxygen Increased
Stage 1: Photosynthesis by Early Life Forms
Around 2.7 billion years ago, simple organisms like cyanobacteria (algae) evolved. They photosynthesised, absorbing CO₂ and releasing O₂.
Over hundreds of millions of years, this gradually increased oxygen levels.
Stage 2: Reducing CO₂
Carbon dioxide was removed from the atmosphere by:
- Dissolving in the oceans — forming carbonate sedimentary rocks (limestone) and by marine organisms incorporating it into shells.
- Photosynthesis — locked up carbon in biomass.
- Formation of fossil fuels — dead organisms were buried, compressed, and transformed into coal, oil, and gas over millions of years.
Greenhouse Gases & the Greenhouse Effect
Some gases in the atmosphere absorb heat radiation (infrared) emitted by the Earth's surface and re-radiate it in all directions, including back towards the surface. This is the greenhouse effect, and it keeps the Earth warm enough to support life.
The Main Greenhouse Gases
- Carbon dioxide (CO₂): From burning fossil fuels, deforestation.
- Methane (CH₄): From cattle farming, rice paddies, landfill decomposition.
- Water vapour (H₂O): Natural evaporation.
Climate Change
Since the Industrial Revolution (mid-1800s), human activities have significantly increased the concentration of greenhouse gases. This enhanced greenhouse effect is causing global temperatures to rise — known as global warming.
Consequences of Climate Change
- Polar ice caps melting → sea level rise → flooding of low-lying areas
- More extreme weather events (droughts, floods, storms)
- Changes to ecosystems and habitats
- Impact on agriculture and food production
- Migration and redistribution of species
Why is There a Debate?
While the vast majority of scientists agree that human activity is the main cause, the evidence involves very complex climate models and long-term data. Media, politics, and economics also influence public perception.
Carbon Footprint
A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, or event.
Ways to Reduce Carbon Footprint
- Using renewable energy sources (solar, wind, tidal) instead of fossil fuels.
- Improving energy efficiency of buildings and vehicles.
- Carbon capture and storage (CCS) technology.
- Reducing waste, reusing materials, and recycling.
- Planting trees to absorb CO₂.
- Using public transport or cycling instead of driving.
Atmospheric Pollutants
Burning fossil fuels releases several harmful pollutants:
Carbon Monoxide (CO)
Produced by incomplete combustion. Colourless and odourless, making it very dangerous. It binds to haemoglobin, preventing blood from carrying oxygen.
Sulfur Dioxide (SO₂)
Produced from burning fuels containing sulfur impurities. Dissolves in water in the atmosphere to form acid rain, which damages buildings (especially limestone/marble), kills aquatic life, and damages plants.
Nitrogen Oxides (NOₓ)
Formed when nitrogen and oxygen in the air react at the high temperatures inside car engines. Also contributes to acid rain and photochemical smog.
Particulates (Soot)
Tiny particles of unburned carbon from incomplete combustion. Cause respiratory problems, darken buildings, and contribute to global dimming (reflecting sunlight back to space).