Rather than take a fossil fuel and pollute and then require an offset say in the planting of an equivalent number of trees ( not that re-forrestating the earth wouldn't be anything, but a good thing. )

Instead, to use a biofuel, for example biodiesel, and using it ( + 1 amount of CO2 ), then mitigating that pollution by creating the biodiesel again whether from grain or algae ( -1 amount of CO2 ), thus creating a zero balance. You can then do this forever."

Graham Stoate