Speculating that plant biomass might help mitigate global warming doesn't really deserve the label "climate skeptic." It's already confirmed that rising CO2 affects crop growth rates.
For instance, the Saharan desert seems to be greening.
I hadn't heard about that, and my first few Googles came back without any good, definitive articles; do you have more information?
https://www.nature.com/articles/s41467-018-04616-8
The study is about sub saharan Africa, mostly the African savanna, and the identified drivers are human causes (decline in burned area, changes in herbivores density). They explicitly exclude CO2 levels increase as a driver.
> CO2 fertilization effects explain most of the greening trends in the tropics
It's unrelated to the specific positives and negatives.