Answer. The CO2 in the atmosphere acts like a glass wall of the greenhouse. It has the capability of absorbing some of the infrared radiations emitted by the Earth. Hence, increased concentration of CO2 layer absorbs the infrared radiations emitted by the Earth's surface and prevents heat energy escaping from the atmosphere. As a result, less heat energy is lost from the atmosphere. Therefore, the average temperature of the atmosphere gradually increases. So CO2 is responsible for increasing the atmospheric temperature. This is called the greenhouse effect. Therefore, CO2 is called as greenhouse gas.