The climate change deniers have a meme - carbon dioxide is plant food. Leaving aside what the word food means in the world of autotrophic nutrition, something occurred to me.
It is this: although plants as a kingdom and plant phyla and classes evolved at times when carbon dioxide levels were higher, the same is not necessarily true of individual plant species. Many plant species will have evolved less than 1 million years ago, plenty of them within the last 10,000 years. Those will have evolved in low carbon dioxide levels. The key is not the time of evolution but whether these plants are adapted to the current climate or not.
Most plants will be in some shape or form adaptable. They have to be. It is the only way to survive. But adaptability has limitations and since the climate is expected to change at rates never or only rarely seen in the history of life, there can be no guarantee that plants will be able to adapt. This is especially so for those plants that are wholly reliant on humans. Domesticated plants such as maize any not have the powers to adapt to a different climate.
Deniers miss another point about plant evolution - co-evolution. Many plants are co-evolved with insect pollinators, fungal mycorrhizae (filaments of fungi in the soil that link roots of plants, growing into the plant tissues and helping them to grow) or nitrogen-fixing bacteria. Even if the plants can adapt, their symbiotic fellow travellers may be unable to do so, or change at the required rate. Survival for many plants may be a matter of pure chance.
Simply put, the denier argument about carbon dioxide benefitting plants is true on a superficial lever, but climate change could produce unexpected results in plant adaptations, including some extinctions, as symbionts adapt at different rates. We shall see, of course.