The Natural Greenhouse Effect
Earth is wrapped in a thin blanket of gases that act like the glass panes of a greenhouse — letting sunlight in but trapping some of the heat that radiates back from the surface. This is the natural greenhouse effect, and it is absolutely essential for life.
Here is how it works, step by step:
- The Sun emits short-wave radiation (visible light and UV). Most of this passes straight through the atmosphere and is absorbed by Earth's surface, warming it.
- The warmed surface re-emits energy as long-wave infrared (heat) radiation directed back upward into the atmosphere.
- Greenhouse gases in the atmosphere absorb much of this outgoing infrared radiation and re-radiate some of it back toward Earth's surface.
- This keeps the surface warmer than it would otherwise be.
Without the greenhouse effect: Earth's average surface temperature would be approximately −18°C instead of the current +15°C — roughly 33°C colder. Most of Earth would be a frozen, lifeless world.
Greenhouse Gases
The main greenhouse gases (GHGs) in Earth's atmosphere are:
| Gas | Formula | Natural source |
|---|
| Water vapour | H₂O | Evaporation from oceans and lakes |
| Carbon dioxide | CO₂ | Respiration, decomposition, volcanism |
| Methane | CH₄ | Wetlands, termites, natural gas seeps |
| Nitrous oxide | N₂O | Soil bacteria, lightning |
| Ozone | O₃ | Stratosphere (UV reaction with O₂) |
Water vapour is the most abundant natural GHG. CO₂, though less than 0.05% of the atmosphere, is the most important human-influenced GHG.
The Enhanced Greenhouse Effect
Since the Industrial Revolution (~1750), human activities have been increasing the concentrations of greenhouse gases in the atmosphere far above natural levels. This enhanced greenhouse effect is trapping more heat than normal and causing Earth to warm — a process called global warming.
- CO₂ rises from: burning fossil fuels (coal, oil, natural gas) for electricity, transport, and industry; deforestation (trees store carbon — when cut and burned, CO₂ is released)
- CH₄ rises from: cattle digestion (enteric fermentation), rice paddies, decomposing waste in landfills, natural gas leaks from pipelines
- N₂O rises from: nitrogen-based fertilisers (bacteria in soil convert them to N₂O), certain industrial chemical processes
Current CO₂ level: approximately 420 ppm (parts per million) — the highest in at least 3 million years. Pre-industrial level was ~280 ppm. That is a 50% increase driven almost entirely by human activity.
Evidence of Climate Change
Scientists have gathered overwhelming evidence that Earth's climate is changing rapidly:
- Rising temperatures: Global average temperature has increased by approximately 1.1°C above pre-industrial levels since 1880, with most warming occurring after 1980.
- Melting ice: Arctic sea ice has declined by about 40% since 1979. Glaciers worldwide are retreating. The Greenland and Antarctic ice sheets are losing mass at accelerating rates.
- Rising sea levels: Sea levels have risen approximately 21 cm since 1900, with the rate accelerating in recent decades due to melting ice and thermal expansion of warming oceans.
- Extreme weather: Droughts, floods, heatwaves, and intense tropical storms are becoming more frequent and severe.
- Shifting seasons: Spring arrives earlier; animals migrate and breed at different times; plant flowering seasons have shifted.
Consequences of Climate Change
- Loss of biodiversity: Coral reefs bleach and die as oceans warm and acidify. Polar bears lose sea-ice hunting grounds. Many species face extinction as habitats shift faster than they can adapt.
- Food and water insecurity: Changing rainfall patterns cause droughts in some regions and floods in others, disrupting agriculture. Glaciers that supply freshwater to millions of people are shrinking.
- Flooding of coastal cities: Cities like Miami, Mumbai, and Durban face increasing flood risk as sea levels rise.
- Displacement of people: Low-lying island nations (e.g. Maldives, Tuvalu) may become uninhabitable, forcing entire populations to migrate.
- Spread of disease: Warmer temperatures allow mosquitoes carrying malaria and dengue fever to spread into previously cool regions.
The Ozone Layer — A Related but Separate Issue
The ozone layer sits in the stratosphere (20–35 km up) and absorbs harmful UV radiation from the Sun. It is a different problem from climate change:
- Chemicals called CFCs (chlorofluorocarbons) — once used in refrigerants and aerosol sprays — react with ozone and destroy it.
- A large "ozone hole" developed over Antarctica every spring.
- Ozone depletion causes more UV radiation to reach Earth's surface, increasing rates of skin cancer, cataracts, and harm to marine ecosystems.
- The Montreal Protocol (1987) banned CFCs; the ozone layer is slowly recovering — a rare environmental success story.
Do not confuse the two issues: Greenhouse gases trap heat and cause warming. CFCs destroy ozone and increase UV radiation. Some GHGs are ozone-depleting too, but the mechanisms and solutions are different.
Solutions to Climate Change
- Renewable energy: Solar, wind, and hydroelectric power produce electricity without CO₂ emissions.
- Energy efficiency: Better insulation, LED lighting, fuel-efficient vehicles — using less energy overall.
- Reforestation: Planting trees absorbs CO₂; protecting existing forests prevents its release.
- Changing diets: Reducing meat (especially beef) consumption lowers methane emissions from livestock.
- International agreements: The Paris Agreement (2015) commits nations to limit global warming to 1.5°C above pre-industrial levels. Requires massive reductions in emissions by 2050.
- Carbon capture technology: Machines and natural processes (wetlands, soil) that remove CO₂ directly from the atmosphere.
South African context: South Africa is one of the world's top 15 greenhouse gas emitters, mainly due to its heavy dependence on coal-fired power stations (Eskom). Climate change threatens SA's water supply (droughts in the Western Cape), agriculture (crop failures), and coastal infrastructure. SA committed to net-zero emissions by 2050 under the Paris Agreement.