Question

What would happen to the rate of photosynthesis in$C_3$ plants if the $C{O}_2$ concentration level almost doubles from its present level in the atmosphere

Answer 1

CO2 concentration leads to an increase in the rate of photosynthesis. But, beyond a certain limit, CO2 starts accumulating in the plant and this leads to slowing down of the process.

So, excessive CO2 inhibits photosynthesis especially when it starts to accumulate.