There are also plenty of ways to take CO2 out of the air (several of which literally grow on trees, or are trees), the question for both organic and technological CO2 sequestration is economics.

For a sense of scale, human emissions are about 40 Gt CO2, global primary production is 104.9 Pg, so making the world about 10% more fertile would have the same effect as decarbonising the economy, or equivalently remove 1 year of existing excess carbon if we also decarbonised the economy: http://www.wolframalpha.com/input/?i=%2840%20Gt%20%2F%20mass...

(10% is a lot, but not so much it would be crazy to consider).