How long would human civilisation survive if all of the CO2 in the atmosphere, sea and freshwater suddenly simply disappeared?

about as long as it takes to suffocate-- CO2 is as necessary for respiration as oxygen.

Long enough to starve to death. All plant life would die without CO2.

My best guess would be six months. Then the food would run out.

Plants (all plants) Breath CO2. Without CO2 we can't grow food.

However all animals, including humans, breath Oxygen and give off C02. Plants breath CO2 and give off Oxygen.

It is a balanced system.

All the plants breathe in CO2 and exhale O2. So without CO2, the rain forests and such would stop producing oxygen. Soon our planet would be bereft of oxygen and all animal life, including humans, would die from hypoxia.

Humans, let alone civilized ones, would no longer be living on the planet.

It won’t happen. As long as they’re are living, breathing things on earth, CO2 will always be around. Assume that animals and humans don’t breathe out CO2, and only oxygen is around, then the earth will burn or explode. One little spark is all it takes to start a fire chain reaction.

don't worry, Skippy, won't happen .......................