Answer. The CO2 forms a layer around the Earth like an envelope. It allows the heat rays of the Sun to pass through it, and reaches up to the Earth. These rays are reflected from the Earth surface and go back to upper atmosphere as shown in figure. Normal concentration of CO2 layer retains enough heat to keep the atmosphere warm. So, normal 1 concentration of CO2 is necessary and beneficial for keeping the temperature warm. Otherwise, the Earth would have been uninhabitable. The Earth's average temperature would be about -20°C, rather than presently average temperature 15°C.
The natural process of heat going out.