Carbon dioxide is called green- house gas because—

Carbon dioxide (CO2) is classified as a greenhouse gas primarily because of its ability to absorb and re-emit infrared radiation [2]. While the atmosphere is largely transparent to incoming short-wave solar radiation, the Earth's surface re-radiates this energy as long-wave thermal or infrared radiation [5]. Greenhouse gases like CO2 are opaque to this outgoing terrestrial radiation; they absorb the infrared energy, preventing it from escaping directly into space [3]. This absorbed energy is then re-emitted in all directions, including back toward the Earth's surface, which traps heat and raises the temperature of the lower atmosphere [5]. Although CO2 is used in photosynthesis and its concentration is rising due to human activity, these factors do not define its physical property as a greenhouse gas [5]. Its defining characteristic is the specific absorption of infrared wavelengths, such as the 15 μm band.

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 7: Climate Change > 2. greenhouse gases > p. 9
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 17: Climate Change > Role of Greenhouse Gases > p. 255
3. [5] https://www.noaa.gov/basics-of-carbon-cycle-and-greenhouse-effect
4. [3] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 7: Composition and Structure of Atmosphere > Gases > p. 64