The correct option is Carbon dioxide.
Explanation
The heating of the Earth's atmosphere is primarily governed by the Greenhouse Effect. The atmosphere is largely transparent to shortwave solar radiation (incoming insolation) but is selectively opaque to longwave terrestrial radiation (outgoing infrared radiation). This selective absorption by specific gases maintains the Earth's heat balance.
Detailed Analysis
- Nitrogen (N2) and Oxygen (O2): Together, these constitute approximately 99% of the atmosphere. They are diatomic molecules and are transparent to both incoming solar radiation and outgoing terrestrial radiation. They do not play a significant role in the greenhouse effect.
- Argon (Ar): This is an inert noble gas constituting about 0.93% of the atmosphere. It is transparent to thermal radiation and does not contribute to atmospheric heating via the greenhouse effect.
- Carbon Dioxide (CO2): Carbon dioxide is a meteorologically significant gas. It is transparent to incoming shortwave solar radiation, allowing it to reach the Earth's surface. However, it absorbs a significant portion of the outgoing longwave terrestrial radiation emitted by the Earth. By trapping this heat, it warms the lower atmosphere, a process essential for sustaining life temperatures but also responsible for global warming when concentrations increase.
Key Takeaway
Carbon dioxide (along with water vapour, methane, and ozone) acts as a greenhouse gas by transmitting shortwave solar radiation while absorbing and re-radiating longwave terrestrial radiation, thereby heating the atmosphere.