Energy in, energy out: The Earth gets its energy from the Sun as short-wave radiation and sends it back to space as long-wave radiation. The balance sets our temperature.
The points to remember
- The Sun gives out a broad spectrum: ultraviolet, visible light and some infrared.
- It arrives as short-wave radiation, which passes easily through the air.
- Of every 100 units: about 30 reflected to space, 20 absorbed by the air, 50 absorbed by the surface.
- The warmed surface gives off long-wave (infrared) radiation.
- For a steady temperature, energy in = energy out. If more comes in than leaves, the Earth warms.
Remember it as: Sun = short, Earth = long.
Real example: NASA's CERES instruments on satellites measure the energy arriving and leaving. Averaged over the whole Earth, about 340 watts of sunlight arrive on each square metre, and about 100 of them, nearly 30%, are reflected straight back to space.
Name both kinds: Questions often ask for the difference: short-wave sunlight comes in and passes through the air; long-wave infrared goes out and is absorbed by greenhouse gases.
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The greenhouse effect works in five steps. Learn them in order: a full answer needs the complete chain.
How the greenhouse effect works
- Sunlight (short-wave) passes through the greenhouse gases and warms the surface.
- The warm surface re-radiates energy upwards as infrared (long-wave) radiation.
- Greenhouse gases in the troposphere absorb this outgoing infrared ...
- ... and re-radiate it in all directions, some back down to the surface.
- So much of the heat is kept, raising the average surface temperature to about 15 °C.
- The main natural greenhouse gases: carbon dioxide, methane, water vapour, nitrous oxide.
Real example: the Moon is the same distance from the Sun as the Earth but has no atmosphere. Its surface reaches about 120 °C by day and falls to about -170 °C at night: with no greenhouse gases, the heat escapes straight back to space.
Two answers that score nothing: The surface re-radiates infrared; it does not 'reflect heat'. And the ozone hole is not the cause of the greenhouse effect: ozone near the ground and CFCs are human-made extras, not the main gases.
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Warm enough for life: The greenhouse effect is not a problem in itself. Without it the Earth would be frozen solid; with it, the average is a mild 15 °C.
About 15 °C, not about -18 °C
- With the greenhouse effect the Earth's average is about 15 °C; without it, about -18 °C.
- That is about 33 °C warmer: warm enough for liquid water and life.
- It is a natural process: it worked long before people.
- The Earth's temperature depends on how much greenhouse gas is in the air.
- More greenhouse gas: warmer. Less: colder.
Remember it as: Minus 18 without it, plus 15 with it: 33 degrees of help.
With the greenhouse effect
- Average about 15 °C
- Liquid water: oceans, rivers, rain
- Mild nights
Without it
- Average about -18 °C
- Oceans frozen
- Most life impossible
Real example: in the Sahara, the dry, cloudless air holds little water vapour, so after a 40 °C day the ground can cool to below 15 °C at night. In humid Singapore, nights rarely fall below 24 °C, because water vapour and clouds absorb and re-emit the outgoing infrared.
People have added greenhouse gases faster than nature removes them. The extra gas strengthens the natural effect, which is called the enhanced greenhouse effect.
Natural, enhanced, and climate change
- Enhanced greenhouse effect: the extra warming from greenhouse gases added by people.
- It comes from burning fossil fuels, clearing forests, farming cattle and rice, and fertilisers.
- It causes global warming: a rise in the mean global temperature.
- Climate change includes global warming but is broader: changes in rain, storms, ice and sea level.
- Natural = keeps us alive. Enhanced = the problem.
Natural greenhouse effect
- Gases that were always there
- Keeps the Earth about 15 °C
- Essential for life
Enhanced greenhouse effect
- Extra gases added by people
- Raises the mean global temperature
- Leads to climate change
Real example: carbon dioxide has risen from about 280 ppm before 1750 to about 420 ppm today, and the world is about 1.2 °C warmer. In 2022, record monsoon rains flooded about a third of Pakistan: climate change is more than a rise in temperature.
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How this comes up: Paper 2, Section B (a): the greenhouse effect as a chain of four or more separate steps.
Mars has a very thin atmosphere with few greenhouse gases, and its average temperature is about -60 °C.
Outline the role of the greenhouse effect in regulating the temperature on Earth.
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