Answer: the trapping of the sun's warmth in a planet's lower atmosphere, due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet's surface.
The greenhouse effect is where the greenhouse gases (methane, carbon dioxide, and nitrate) rise up and makes our atmosphere thicker. This then makes the suns rays bounce back off the atmosphere, which makes Earth colder. However, the rays trapped in the atmosphere, keep the Earth warmer but warmer than normal. I also included climate change in this answer.
