Greenhouse gases absorb and re-emit outgoing heat energy. Adding more of these gases warms the surface and lower atmosphere.
Earth receives energy from the Sun mainly as shortwave radiation. Some is reflected by clouds, the atmosphere and surfaces. The rest is absorbed and warms Earth. The warmed surface then emits longwave radiation, also called infrared radiation. These are the incoming and outgoing flows in the diagram.
Water vapour, carbon dioxide, methane and nitrous oxide are greenhouse gases. They absorb particular wavelengths of infrared radiation and emit infrared energy in all directions, including back towards the surface. This natural greenhouse effect makes Earth much warmer and helps keep it habitable.
Adding more long-lived greenhouse gases strengthens the effect. At first, less energy escapes to space at the same surface temperature. Earth gains energy and warms. As it warms, outgoing energy increases towards a new balance. Some energy continues to escape throughout this process.
The enhanced greenhouse effect is the extra warming caused by increased greenhouse gases, especially from human activities. Warming can also increase atmospheric water vapour, which amplifies the warming: a feedback. Ozone depletion is a different process and is not the main cause of recent global warming.
Step by step
Receive energy
Earth absorbs some incoming shortwave radiation and warms.
Emit and absorb
The surface emits longwave radiation. Greenhouse gases absorb and re-emit some of it.
Warm further
More greenhouse gases initially reduce energy escaping to space, causing additional warming.
Worked example: Improve a weak explanation
Instead of "pollution traps the Sun", identify the mechanism: fossil-fuel burning adds carbon dioxide; this strengthens absorption and emission of outgoing longwave radiation, creating an energy imbalance that warms Earth.
Watch out for this
Greenhouse gases form a solid layer that reflects all heat back down.
They are gases mixed through the atmosphere that absorb and emit infrared at particular wavelengths. They do not block all energy from escaping.
Check your understanding
Which statement correctly links the natural and enhanced greenhouse effects?
- The natural effect is an ozone hole; enhancement closes it.
- Greenhouse gases stop every form of radiation from escaping.
- The natural process warms Earth; increased greenhouse gases strengthen it.