Monbiot explains his thinking on carbon offsets in this UK Guardian article. As he says...Carbon offsetting makes sense if you are seeking a global cut of 5% between now and forever. It is the cheapest and quickest way of achieving an insignificant reduction. But as soon as you seek substantial cuts, it becomes an unfair, impossible nonsense, the equivalent of pulling yourself off the ground by your whiskers. Yes, let us help poorer nations to reduce deforestation and clean up pollution. But let us not pretend that it lets us off the hook.