The Greenhouse Effect: Natural vs Enhanced

The Natural Greenhouse Effect

The natural greenhouse effect has operated throughout Earth's history and is essential for life. Here is the sequence:

1. The Sun emits short-wave radiation (visible light and UV) — this passes through the atmosphere largely unimpeded
2. Earth's surface absorbs this radiation and warms up
3. The warm surface re-emits energy as long-wave infrared radiation
4. Greenhouse gas molecules in the atmosphere absorb this infrared radiation
5. They re-emit it in all directions — including back towards Earth's surface
6. This keeps the surface warmer than it would otherwise be

Despite being called the "greenhouse effect," it works differently from a real greenhouse. A real greenhouse keeps warm by stopping air from moving out. The atmospheric greenhouse effect works instead by greenhouse gas molecules absorbing the infrared radiation emitted by Earth's surface and re-emitting it in all directions — including back towards Earth.

The Enhanced Greenhouse Effect

Human activities are increasing the concentrations of greenhouse gases in the atmosphere:

• More CO₂ from burning fossil fuels and deforestation
• More CH₄ from cattle farming, rice paddies, and landfill sites
• More N₂O from fertilisers and industrial processes

Higher concentrations of greenhouse gases mean more infrared radiation is absorbed and re-emitted back to Earth, causing additional warming above the natural level.