Imagine a glass greenhouse on a sunny day. Sunlight passes through the walls and enters the interior. Some is absorbed by the objects inside, while the rest are reflected back as infrared radiation, or heat. This reflected heat gets trapped inside the greenhouse, increasing the internal temperature. The atmosphere of a planet works in much the same way as the glass walls of a green house. It lets sunlight in, but prevents heat reflected off the planet’s surface from escaping into outer space by absorbing it. This phenomenon is known as the greenhouse effect, which makes the Earth hospitable for human beings.
The atmosphere’s ability to trap heat is directly related to the concentrations of gas molecules it contains. Some of the major gases contributing to the greenhouse effect on Earth are water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons (CFCs). Water vapor causes the largest greenhouse effect on Earth, followed by carbon dioxide, methane, and ozone. This is due to the high concentration of water vapour and carbon dioxide molecules in the atmosphere, not their ability to absorb or reflect heat. A single nitrous oxide molecule contributes to the greenhouse effect 310 times that of one carbon dioxide molecule. However, there are relatively few nitrous oxide molecules in the atmosphere, making their overall contribution to Earth’s greenhouse effect minimal.
The right balance of greenhouse gas molecules are required to keep a planet habitable. Having too few can lead to frozen surface temperatures like those recorded on Mars. The greenhouse effect maintains Earth’s average surface temperature at 14 degrees Celsius—without it, Earth’s temperature would be -18 degrees Celsius. On the other hand, a higher level of molecules could lead to temperatures so hot even bodies of water couldn’t withstand them. This is the case on Venus, where the 96.5 per cent carbon dioxide atmosphere keeps the planet’s surface at a toasty 457 degrees Celsius.
