Not an expert on climate either, but I own an aquarium so have a decent experience with artificially high CO2 levels. It's a common trick to inject pressurised CO2 to aquarium to speed up growth in planted freshwater aquariums. It works great for plants, but only if you provide them sufficient levels of other two necessary components: light and nutrients. If either of those is lacking plants will actually suffer, become unhealthy or algae will take over. Nature is all about balance, so you better increase all variables or not touch anything. Increasing just one almost always generates instability which can easily cascade into a disaster for farmers and the whole ecosystem. And it's such a dynamic system with so many variables in play that it's impossible to account for all possible side-effects this can create.