A recent article in the popular press (New York Times) reporting on the scientific press (Science) laid out the researched case that many of us had intuited. Use of biofuels will result in increases in CO2 emissions because of changes in land use -- increased deforestation -- despite vows to guard against it.
Biofuel crops grown in the US on previously plowed fields result in a decline in yields of whatever was planted there before, increasing the price of that former crop (soybeans, corn). Therefore:
• new land is deforested in other countries to plant this displaced crop which now has an economically more compelling value; and
• new land is deforested in other countries to plant the biofuel itself, which of necessity has a higher value than the crops it is displacing.
A nice quote puts the order of magnitude into perspective, from the New York Times article:
The clearance of grassland releases 93 times the amount of greenhouse gas that would be saved by the fuel made annually on that land, said Joseph Fargione, lead author of the second paper, and a scientist at the Nature Conservancy. “So for the next 93 years you’re making climate change worse, just at the time when we need to be bringing down carbon emissions.”
Another negative, which doesn't contribute to global warming but does contribute to global suffering, is the effect of higher prices of basic staples (corn and food oil) on the very poorest among us.
Using biofuels that are not a bi-product of some other crop (sugar cane stalks, for example) is just plain a bad idea. Conservation is the best alternative fuel in the transportation arena that we have in the near term. And of course, I have to say it, a carbon tax would go a long way towards encouraging (enforcing?) the judicious use of fossil fuels.