Let's say a tree takes 25m^2 in space. The average tree can bind roughly 50kg of CO2 per year. If a human emits ~10.000kg of CO2 per year, you need to maintain 200 trees for each human. Let's take Germany as reference. You need to plant 80 million (people) times 200 trees. In total this is an area of 4*10^11 m^2=400.000km^2

Funnily, this is roughly the size of Germany! So that Germany is long term CO2 neutral, you need to plant (and keep healthy) a forest as big as Germany.

No scale this up for the world, impossible.

That's the problem.