More CO2 means more plant growth.
A summary of 270 laboratory studies (Idso, 2013) of 83 food crops showed that increasing CO2 concentrations by 300 ppm will increase plant growth by an average of 46% across all crops studied. Conversely, a large number of studies show the adverse effects of a low-CO2 environment. For instance, Overdieck (1988) indicated that, compared to today, plant growth was reduced by 8% in the period before the Industrial Revolution, with its low concentration of 280 ppm CO2. Therefore, the proposed misguided attempts to reduce CO2 concentrations would be bad for plants, bad for animals, and bad for humankind.