Greenhouse gases keep Earth warm enough for life
Definition
Greenhouse gas
A gas in the atmosphere that absorbs infrared radiation emitted by Earth and transfers energy to the surroundings, warming the atmosphere.
Definition
Greenhouse effect
The natural warming of Earth's surface and lower atmosphere because greenhouse gases absorb and re-emit some outgoing infrared radiation.
- The greenhouse effect keeps Earth's surface warm enough to support life.
- The gases that cause it are called greenhouse gases.
- The three you need to know are carbon dioxide, CO2\text{CO}_2CO2, methane, CH4\text{CH}_4CH4, and water vapour, H2O\text{H}_2\text{O}H2O.
- Without them, Earth's average surface temperature would be far below freezing.
Key Idea
Greenhouse gases act like a blanket: they let the Sun's energy in but slow the escape of heat, keeping the surface warm enough for life.
How the greenhouse effect traps heat, step by step
Definition
Methane
A gas with the formula CH4\text{CH}_4CH4 that acts as a greenhouse gas and is released by sources such as cattle, flooded rice fields and landfill sites.
Definition
Electromagnetic radiation
Energy transferred as waves, including visible light and infrared radiation.
- Radiation from the Sun reaches Earth, mostly as short-wavelength radiation, including visible light.
- Earth's surface absorbs this energy and warms up.
- The warmed surface then emits energy as longer-wavelength infrared radiation.
- Greenhouse gases in the atmosphere absorb this outgoing infrared radiation.
- They then re-radiate the energy in all directions, including back towards the surface.
- This keeps energy in the lower atmosphere, so Earth stays warmer than it otherwise would be.
Common Mistake
- Greenhouse gases absorb the outgoing infrared from Earth, not the incoming light from the Sun.
- Do not say they 'reflect' heat; they absorb and re-emit it in all directions.
Naming the main greenhouse gases
- Carbon dioxide and methane are the two the exam focuses on for human-caused warming.
- Water vapour is also a greenhouse gas and is part of the natural greenhouse effect.
- A gas is a greenhouse gas because its molecules can absorb infrared radiation.
Exam technique
- Describe the mechanism as a sequence: Sun's radiation absorbed, Earth emits infrared, greenhouse gases absorb it and re-emit it back.
- Name specific gases such as carbon dioxide, methane and water vapour; a vague 'greenhouse gases' can miss a mark.
Self review
- Name the three greenhouse gases you need to know.
- Which type of radiation from the Sun does Earth's surface mainly absorb?
- What type of radiation does the warmed Earth emit back out?
- What do greenhouse gases do to this outgoing radiation?
- Why would Earth be far colder without greenhouse gases?
