The Greenhouse Effect (Cambridge (CIE) IGCSE Physics)
Revision Note
Written by: Katie M
Reviewed by: Caroline Carroll
The greenhouse effect
Temperature of Earth
If the Earth had no atmosphere, the temperature on the surface would drop to about −180 °C at night, the same as the Moon’s surface at night
This would happen because the surface would be emitting all the radiation from the Sun into space
The temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted
The Earth receives the majority of its heat in the form of thermal radiation from the Sun
At the same time, the Earth emits its own thermal radiation, with a slightly longer wavelength than the thermal radiation it receives (this is caused by difference in surface temperature of the Earth and the Sun)
The greenhouse effect
Some gases in the atmosphere, such as water vapour, methane, and carbon dioxide (greenhouse gases) absorb and reflect back longer-wavelength infrared radiation from the Earth and prevent it from escaping into space
These gases absorb the radiation and then emit it back to the surface
These gases keep the Earth's temperature at a reasonable level, and the process is called the greenhouse effect
This process makes the Earth warmer than it would be if these gases were not in its atmosphere
Radiation and the greenhouse effect
The Earth receives thermal radiation from the Sun but emits its own thermal radiation at the same time. The greenhouse effect is where this radiation is trapped by gases
The temperature of the Earth, therefore, depends on several factors, such as the rate that infrared radiation is:
Reflected back into space
Absorbed by the Earth’s atmosphere or by the Earth’s surface
Emitted from the Earth’s surface and from the Earth’s atmosphere into space
Worked Example
Explain why adding an excess of carbon dioxide to the Earth's atmosphere would lead to an increase in the surface temperature of Earth.
Answer:
Step 1: Describe the greenhouse effect
Radiation from the Sun is absorbed by the Earth
Some is re-emitted by the Earth
Greenhouse gases, such as carbon dioxide, reflect a fraction of this emitted radiation and heat the Earth's surface
Step 2: Describe the effect of increasing the amount of carbon dioxide
If there is more carbon dioxide, more of the Earth's emitted radiation is reflected back to the Earth
The rate of radiation absorbed by the Earth's surface increases, so its temperature increases
Examiner Tips and Tricks
Global warming is an important current topic in the world. Generally it is important to understand how this process works, but this also means it appears in exams frequently. Learning more about this topic not only means that you are prepared for application questions on radiation, but it also means you are well-informed about the environmental issues we are facing as a society.
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