It's not enough to plant trees, you have to trap the carbon they gather somehow. Otherwise when the tree dies, the decomposition process throws all that carbon back in the atmosphere.

The early plants (billions of years ago) went through a few generations before other organisms evolved to feed on their carcasses, so they just ended up buried deep underground, trapping their carbon (and lowering the carbon concentration in the atmosphere) until we came along and decided to dig them up and burn them, throwing all that carbon in the air again.