It is such an unreasonable idea! Ignoring the loss of biomass (and the fact that there would be no way to implement this scheme without providing a very unwelcome financial incentive to cut down trees wherever they are found), you'd use as much CO2 in the machinery required to cut the trees down and dig a big hole! Unless you're suggesting we do it all by hand? In which case, the picture of a crazed, doomsday cult is complete. I suppose at least it involves less murder than the Aztecs and their sacrifices.