An interesting and important article about a researcher gathering data that seem to show that although more carbon in the air means plants grow faster, the result is plants with more sugar but less protein, minerals, vitamins, and other nutrients:
http://www.politico.com/agenda/story/2017/09/13/food-nutrients-carbon-dioxide-000511
the protein content of goldenrod pollen has declined by a third since the industrial revolution—and the change closely tracks with the rise in CO2.