Biofuels and afforestation

In all cases, forestation of an equivalent area of land would sequester two to nine times more carbon over a 30-year period than the emissions avoided by the use of the biofuel. Taking this opportunity cost into account, the emissions cost of liquid biofuels exceeds that of fossil fuels.

-- from Carbon Mitigation by Biofuels or by Saving and Restoring Forests? by Renton Righelato and Dominick V. Spracklen (Science, 17 August 2007).