As pointed out elsewhere, we probably don't have enough land for that many trees, unless we figure out how to grow forests in the deserts or on floating islands

As pointed out elsewhere, we probably don't have enough land for that many trees, unless we figure out how to grow forests in the deserts or on floating islands in the ocean, etc. (technically possible, but not trivial). That misses the important fact, though: there are millions of acres where trees can be planted now. If we run out of land to plant trees on, we can solve that problem at that point.