Someone has misinformed you.   My search - UN FAO 2015 Carbon Flux Model "CO2 is the fertilizer of global plant growth" - did not match any articles.

Weeds do respond as if CO2 is a fertilizer but the exact opposite is true for some crops.  For example, peer-reviewed research shows that wheat yields fall as CO2 increases.  Many plants hit their maximum tolerable CO₂ limits at 500–800 ppmv, and most hit it by 800–1,000 ppmv, if they can survive the intense heat and drought conditions that will occur outside of a greenhouse situation. For most crops the saturation point will be reached at about 1,000–1,300 ppmv under the ideal circumstances that you would find in a greenhouse. 

We are headed towards 900 to 1,300 ppmv by the end of the century under a BAU scenario.