prize1-8, Exercise 2: The greenhouse effect

Model answer:

Sunlight passes through the atmosphere and warms the surface, which gives off infrared radiation. Gases such as water vapour, carbon dioxide and methane absorb some of that infrared and re-emit it in all directions, including back toward the surface, so the surface is warmer than it would be otherwise. This is natural, and without it the planet would be well below freezing. It can be enhanced because adding more of these gases, as happens when fossil fuels are burned, makes the atmosphere absorb more infrared and so traps more heat.