Apart from the energy used in harvesting, transport and construction (if not carbon-neutral), yes. However, what you actually do is add a new carbon sink, with

Apart from the energy used in harvesting, transport and construction (if not carbon-neutral), yes. However, what you actually do is add a new carbon sink, with a flux in and a flux out, just like the eco-system, the ocean, the atmosphere etc. As long as you keep the outflow lower or equal than the inflow it's a net win. Edit: Another point - we might not be in a position to think about this at the moment, but we should not store all available carbon in rock form (some of the carbon sequestration ideas lead to rockform carbon basically) because our descendents may really need it one day.