What this lesson teaches
I can explain the natural greenhouse effect and how human activity has enhanced it.
Syllabus K226, Topic 3.2, Key Question 2. How do anthropogenic factors contribute to climate change?
Make a guess
Which kind of energy do greenhouse gases mainly absorb?
- Ultraviolet light, through the ozone hole.
- Heat given off by Earth's warmed surface.
- Incoming sunlight, before it reaches the ground.
- Every kind of energy, like a solid lid over Earth.
Show the answer
Heat given off by Earth's warmed surface.
Earth gives off longwave, infrared radiation. Greenhouse gases absorb some of it and send part back down, keeping the surface warmer.
Greenhouse gases absorb heat given off by Earth and send some back down, and adding more of them warms the planet.
Earth receives energy from the Sun mainly as shortwave radiation. Clouds, the air and bright surfaces reflect some of it. The rest is absorbed and warms Earth. The warmed surface then gives off longwave radiation, also called infrared or heat. These are the incoming and outgoing flows in the diagram.
Water vapour, carbon dioxide, methane and nitrous oxide are greenhouse gases. They absorb certain wavelengths of this infrared heat and give it off again in all directions, including back down towards the surface. This natural greenhouse effect keeps Earth much warmer than it would otherwise be, and helps make it habitable.
Adding more long-lived greenhouse gases strengthens the effect. At first, less heat escapes to space at the same surface temperature, so Earth gains energy and warms. As it warms, it gives off more heat, until a new balance is reached. Some heat escapes all the time.
The enhanced greenhouse effect is the extra warming caused by added greenhouse gases, mostly from human activity. Warming also increases water vapour in the air, which adds more warming: a feedback. The ozone hole is a different problem and is not the main cause of recent warming.
Put these in order
Put the steps of the greenhouse effect in order.
- Some heat returns to the surface and some escapes to space.
- Land and sea absorb the energy and warm up.
- Shortwave radiation from the Sun reaches Earth.
- Greenhouse gases absorb some of the heat and give it off in all directions.
- The warm surface gives off longwave radiation, which is heat.
Show the answer
- Shortwave radiation from the Sun reaches Earth. Some is reflected by clouds, air and bright surfaces, but most is absorbed by land and sea.
- Land and sea absorb the energy and warm up. A warm surface gives off energy of its own, at longer wavelengths than sunlight.
- The warm surface gives off longwave radiation, which is heat. Greenhouse gases in the air absorb certain wavelengths of this outgoing heat.
- Greenhouse gases absorb some of the heat and give it off in all directions. Part of that heat is sent back down, so the surface stays warmer than it would otherwise be.
- Some heat returns to the surface and some escapes to space.
- Shortwave in
- Sunlight arrives and warms Earth.
- Longwave out
- Earth gives off heat, some of which gases absorb.
- Enhanced effect
- Extra warming from added greenhouse gases.
Step by step
Take in sunlight
Earth absorbs shortwave energy and warms.
Give off heat
Greenhouse gases absorb some longwave heat and send it back.
Warm further
More gases mean less heat escapes to space.
Worked example: Improve a weak answer
Instead of "pollution traps the Sun", explain it: burning fossil fuels adds carbon dioxide. This absorbs more outgoing longwave heat, so less escapes to space and Earth warms.
Watch out for this
Greenhouse gases form a solid layer that reflects all heat back.
They are gases mixed in the air. They absorb and give off some heat, but some still escapes.
Check your understanding
Which statement links the natural and enhanced greenhouse effects correctly?
- Greenhouse gases stop heat escaping, so the enhanced effect is a new process.
- The natural effect is the ozone hole, and the enhanced effect is its growth.
- The natural effect warms Earth; more gases make it stronger.
Show the answer
The natural effect warms Earth; more gases make it stronger.
Yes. The enhanced effect adds to the natural one.
Check your understanding
How do greenhouse gases warm Earth?
- They make a hole in the atmosphere that lets more heat in.
- They absorb and re-emit heat given off by Earth's surface.
- They let more incoming sunlight pass through to the ground.
Show the answer
They absorb and re-emit heat given off by Earth's surface.
More gas means more heat is trapped.