Re: CO - @AC
Worstall does not state that arable land should count twice - merely that it is wrong to only count it once. You never eat all of the crop, so it is not all going straight back out as CO2. A large percentage of a field of wheat is straw and if I use that as a building material, then it can be tied up in the fabric of a house for the next 100 years.
There is a complex equation here involving the type of crop, uses of waste, impact of farming methods, etc. The problem is that everyone wants a simple equation (a x b x c) to predict the future, when what is needed is a far more complex model. By oversimplifying the model (never mind just getting it wrong), you get ridiculous or dangerous conclusions.