The BBC website discusses an intriguing idea – personal tradeable carbon allowances. The principle is the same as that used for large companies – you have an allowance, and if you exceed it you have to buy more, if you stay within it you can sell your surplus.
I think it’s a fascinating idea, myself – strong resemblance to rationing during the war, and might produce a similar community cohesion (though doubtless also a similar black market).