Trees are good but are largely a temporary store. Most of the carbon they capture ends up be re-released upon decay. Of course some does get more permanently sequestered in the ground, but a relatively small amount, and is a very slow process. I'm all for planting more trees, but I'm afraid the problem can't be entirely solved by merely planting more trees. It's also a rate problem - it may not be possible to plant enough trees to completely offset the rates we're adding carbon to the atmosphere.