It is well known that CO2 improves plants growth. It is also well known that plants grow better within a narrow band of temperatures.
Increasing both the CO2 contents of the atmosphere and Earth's temperature will have some complex results where on the places within that temperature band, plants grow slightly faster, on places that cross from too cold into the band plants grow much faster, and on places that cross from that band into too hot plants will grow much slower.
Also, solar incidence has a much larger effect than CO2 concentration, so the places with best growth rate can only cross into the too hot region, and polar places crossing into the good temperatures can't really compensate (besides, there isn't much area at the polar regions, unless you are talking about the hole Antarctic getting green).
And, anyway, water availability has a much larger effect than solar incidence, and this one is completely chaotic. So, good luck getting any conclusion.