shouldn't we plant billions of trees now?....
'
No.
You do that, you are disturbing existing ecosystems without caring about them.
This right here, if you're planting BILLIONS of trees you're going to cause a shit ton of problems for ecosystems which have already grown accustomed to not having these areas around. Plus the fact that I don't know where people plan on planting these billions of trees.
No one is going to be happy when you ask them to leave their homes (or force them to leave their homes) so you can plant some trees.
A noble goal, but one which won't come to fruition.