The Atmosphere and Climate Change

Today's atmosphere

The Earth's atmosphere has been roughly stable for about 200 million years:

• ~78% nitrogen
• ~21% oxygen
• Small amounts of argon, carbon dioxide (~0.04%) and water vapour.

How the atmosphere evolved

• Early atmosphere: intense volcanic activity released gases — mainly carbon dioxide, plus water vapour, and small amounts of methane and ammonia. Little or no oxygen (similar to Mars/Venus today).
• Water vapour condensed as the Earth cooled, forming the oceans; lots of CO₂ dissolved into them.
• Algae and plants evolved and carried out photosynthesis, which removed CO₂ and added oxygen over billions of years — allowing animals to evolve.
• CO₂ also became locked up in sedimentary rocks (e.g. limestone) and in fossil fuels.

Greenhouse gases and global warming

Greenhouse gases — mainly carbon dioxide, methane and water vapour — trap heat in the atmosphere (the greenhouse effect), keeping Earth warm enough for life.

Human activities increase greenhouse gas levels:

• Burning fossil fuels and deforestation raise CO₂.
• Farming (livestock, rice) and landfill raise methane.

More greenhouse gases → more heat trapped → global warming and climate change.

Consequences of climate change

• Rising sea levels (melting ice, thermal expansion) → flooding.
• More extreme weather (storms, droughts, heatwaves).
• Changes to habitats and the distribution of species.

Reducing the impact — carbon footprint

A carbon footprint is the total greenhouse gases produced by a product, service or activity. Ways to reduce it:

• Use renewable energy and improve energy efficiency.
• Carbon capture and storage.
• Use fewer fossil fuels (electric transport), reduce, reuse and recycle.
• Carbon offsetting (e.g. planting trees).

Atmospheric pollutants from fuels

Burning fuels also releases harmful pollutants:

• Carbon monoxide (CO) — toxic, from incomplete combustion.
• Soot (particulates) — cause breathing problems and global dimming.
• Sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) — cause acid rain and respiratory problems.

Worked example

Explain how early photosynthesis changed the atmosphere.

• Algae and plants used up CO₂ and released oxygen during photosynthesis over billions of years, so CO₂ levels fell and oxygen levels rose, eventually allowing animals to live. ✓

Common mistakes

• Saying the early atmosphere was mostly oxygen — it was mostly carbon dioxide with little/no oxygen.
• Confusing pollutants: CO (toxic), SO₂/NOₓ (acid rain), soot (particulates).
• Forgetting the greenhouse effect is natural and necessary — the problem is the increase caused by humans.

Exam tips

• Learn the current atmosphere percentages (78% N₂, 21% O₂).
• Explain atmospheric change in terms of volcanoes → oceans → photosynthesis.
• Match each pollutant to its problem, and list several ways to reduce a carbon footprint.

Key facts to remember

• Atmosphere today: ~78% nitrogen, ~21% oxygen, ~0.04% CO₂; early atmosphere was mostly CO₂ from volcanoes.
• Photosynthesis removed CO₂ and added O₂; CO₂ also locked in oceans, rocks and fossil fuels.
• Greenhouse gases (CO₂, methane, water vapour) cause global warming; reduce your carbon footprint via renewables, efficiency and offsetting.