Question

What would happen to the earth without carbon dioxide

Answer 1

The Earth would become somewhat cooler, the oceans would become somewhat less acidic, and no trees or green plants could grow, so there could be no grazing animals or insects.

Answer 2

The ability for CO2 to warm the surface of a planet through the absorption of infrared radiation is well known.  What is much less appreciated, however, is just how effective of a gas it is in maintaining the greenhouse framework that helps to characterize the modern climate.