Carbon dioxide (CO2) is referred to as a "greenhouse gas" because it traps heat in the Earth's atmosphere, much like the glass walls of a greenhouse trap heat inside. When sunlight reaches the Earth, some of it is reflected back into space and the rest is absorbed by the land and oceans. However, some of the absorbed heat is also re-emitted back into the atmosphere as longwave infrared radiation. Greenhouse gases, such as CO2, absorb this longwave radiation and trap heat in the atmosphere, causing the planet's temperature to rise. This process is known as the "greenhouse effect", and CO2 is considered a key contributor to global warming and climate change.