The Greenhouse Effect

The Earth is warmed by the Sun and cools by giving out infrared (IR) radiation. The greenhouse effect explains how some gases in the atmosphere slow the escape of this IR, keeping the planet warm enough for life.

Key ideas

• Thermal radiation is infrared radiation. It transfers energy without needing a medium, so it can travel through space from the Sun to Earth.
• The Sun’s energy arrives mostly as visible and ultraviolet light. It passes through the atmosphere and warms the land and oceans.
• Warm surfaces emit infrared radiation back towards space.
• Greenhouse gases (mainly water vapour, carbon dioxide, and methane) absorb some of this outgoing IR and then re‑emit it in all directions, including back down to the surface. This is like a thin thermal blanket.

Energy balance and temperature

• If the rate of energy in from the Sun equals the rate of energy out as IR, the Earth’s average temperature stays steady: $\text{energy in} = \text{energy out}$.
• If more energy is kept than lost (for example, due to more greenhouse gases), the Earth warms until it emits more IR and a new balance is reached.
• Hotter objects emit IR faster and from more area if they are larger. As temperature rises, emission increases, helping restore balance.

How to picture it

• Shortwave in → warms surface → longwave out → gases absorb → re‑emit → some returns to surface.
• A real greenhouse warms mostly by stopping convection, but the atmospheric greenhouse effect is about absorption and emission of IR.

Everyday connections

• Cloudy nights are often warmer because clouds (water droplets) absorb and re‑emit IR back to the ground.
• Desert nights can be cold because dry air has fewer greenhouse gases, so more IR escapes to space.
• Bright ice and snow reflect more sunlight, while dark oceans and forests absorb more. This affects how much the surface warms.

Common misconceptions

• “Gases trap heat like a lid.” Not exactly. They absorb specific IR wavelengths and re‑emit energy in all directions.
• “The ozone hole causes the greenhouse effect.” The ozone layer mainly affects ultraviolet light, not IR warming.
• “Radiation needs air.” Thermal radiation does not need a medium; it travels through the vacuum of space.