why is co2 a greenhouse gas

Carbon dioxide (CO2) is a greenhouse gas because it absorbs and re-radiates heat energy (infrared radiation) that is emitted from the Earth's surface. This process traps heat in the atmosphere, much like the glass of a greenhouse traps heat inside, leading to the warming of the Earth's surface—an effect known as the greenhouse effect. When the Sun's energy reaches Earth, much of it is absorbed, and then some is emitted back as infrared radiation. CO2 molecules absorb this infrared radiation and re-emit it in all directions, including back toward the Earth's surface, effectively trapping heat and raising the planet's temperature. This natural process is essential for maintaining Earth's temperature at a level suitable for life. However, increased amounts of CO2 from human activities amplify this effect, causing global warming. Additionally, CO2’s ability to trap heat is significant despite it representing a small fraction of the atmosphere (less than 0.1%), because it interacts with heat energy in a way that retains warmth. Also, warmer air holds more water vapor, which is itself a greenhouse gas, further enhancing the effect. In summary, CO2 is a greenhouse gas because it absorbs thermal infrared radiation emitted from Earth and re-radiates it, effectively acting as a heat-trapping blanket around the planet, leading to increased surface temperatures.