Important announcement … 99.96% of the atmosphere is not CO2.
It wasn’t always so. The phanerozoic is the eon of abundant life on earth, that’s about 570 million years. All current life is descended from ancestors that existed throughout that entire span of time. For the first 150 million years or so our ancestors survived in an atmosphere that contained about 4000 parts per million of CO2, imagine that … only 99.6% of their atmosphere wasn’t CO2!
The graph is from Wikipedia, the present day is on the left, which may not be the best way to show it.
Carbon dioxide is essential to plants, the process of photosynthesis requires CO2 and releases oxygen. All the free oxygen in the atmosphere, essential for us to survive, is the result of photosynthesis. Oxygen is too reactive a substance to persist long in a free state. If photosynthesis stopped the atmosphere would soon be depleted of oxygen and animal life would cease.