Excessive CO₂ is a greenhouse gas. It traps heat radiated from the Earth's surface in the atmosphere, preventing it from escaping into space. This process is known as the greenhouse effect. When the concentration of CO₂ increases, this effect is enhanced, leading to a gradual rise in the planet's average temperature, a phenomenon called global warming. It is not directly responsible for ozone depletion, smog formation, or polar vortex events.

Final Answer: Global warming