We could convert the plant material to a carbon rich form, and bury it deep in the ground. Luckily, in some of the post industrial west we have holes prepared, which used to contain this form of carbon so we know they're suitable. That said, they sometimes leak methane. So we probably need to breed a bird of some kind, to be used to the idea of living in methane athmosphere and sing, if the methane levels vary. And since we cant use petrol engines to put the carbon blocks there, we maybe need to breed special little horses,pulling wooden carts, to work in this dark environment. The holes may be small due to local compression. It might be best if we also got kids involved, and used to confined spaces.<p>If we do this right we may wind up with a small amount of carbon we can store in pits. If we can find the pits that the romans and post romans dug.