CO2 is a minor constituent in our atmosphere, with an abundance of less than 0.05% of the gases in this atmosphere. Why, then, is an increase in this gas considered to be important with respect to the continued viability of life on Earth?

(a) CO2 is poisonous, and an increase could endanger life on the Earth.

(b) An increase in CO2 will lead to less UV light reaching Earth from the Sun, thereby leading to a significant cooling.

(c) Small increases in CO2 will prevent more IR radiation from escaping from Earth, thereby leading to a dangerous warming of the Earth.

(d) CO2 causes acid rain, and an increase in CO2 would hurt agriculture and other human activities.

Option (C)

Explanation:

CO₂ is one of the main green house gases on earth. A small increase in its concentration can lead to significant changes on earth. When the incoming solar radiations are incident on the earth's surface, some of the energy is absorbed by the atmosphere and the earth's surface. And some of the energy from the land surface after absorption is released into the atmosphere in the form of infrared radiations. The escaping of these radiations from earth is directly dependent on the concentration of CO₂. An increase in the amount of CO₂ will not allow these IR radiations to be removed, and this will result in the global rising of the temperature, leading to catastrophic changes.

Thus, the correct answer is option (C).