CO2 is a greenhouse gas. The more of it in the atmosphere the more heat we trap, just like a greenhouse trapping hot air. It isn't about an "even more fertile" planet, there have been times in Earth's history when there were entire forests covering the poles that are frozen now(And more CO2 doesn't directly mean more plants anyway, as they need other things to grow. There's plenty of dirt and water on the planet too, but plants live in the balance of those resources, increasing the density of one only will no more cause plants to thrive than drowning them in too much water.). It's about keeping the planet in the temperature range it naturally would be in, which is what we and all life on the planet evolved to cope with. By artificially pumping billions of tons of CO2 into the air we're trapping more and more heat, warming the planet extremely quickly from a geological/ecological perspective. Yes, that will mean Canada is more temperate, and hence more habitable. Good for them. But it will mean catastrophic changes elsewhere, here, the ME, Africa, Asia, that will make current habitable areas much less habitable. 'Destroying the planet' is figurative, not literal. The planet is fine, it's about what type of environment we want that planet to be since we, you know, have to live on it.