According to environmental think tank World Resources Institute, Biofuels Are Not a Green Alternative to Fossil Fuels. (http://www.wri.org/blog/2015/01/biofuels-are-not-green-alternative-fossil-fuels)
In a blog post was originally published in The Guardian on January 29, 2015, and assert
Powering cars with corn and burning wood to make electricity might seem like a way to lessen dependence on fossil fuels and help solve the climate crisis. But although some forms of bioenergy can play a helpful role, dedicating land specifically for generating bioenergy is unwise. It uses land needed for food production and carbon storage, it requires large areas to generate just a small amount of fuel, and it won’t typically cut greenhouse gas emissions.
I’ll skip over the first two points (you can go to the website and read them yourself, if you like), and allow the authors to elaborate on the third point.
Burning biomass, whether directly as wood or in the form of ethanol or biodiesel, emits carbon dioxide just like burning fossil fuels. In fact, burning biomass directly emits a bit more carbon dioxide than fossil fuels for the same amount of generated energy. But most calculations claiming that bioenergy reduces greenhouse gas emissions relative to burning fossil fuels do not include the carbon dioxide released when biomass is burned. They exclude it based on the assumption that this release of carbon dioxide is matched and implicitly offset by the carbon dioxide absorbed by the plants growing the biomass.
Yet if those plants were going to grow anyway, simply diverting them to bioenergy does not remove any additional carbon from the atmosphere and therefore does not offset the emissions from burning that biomass. Furthermore, when natural forests are felled to generate bioenergy or to replace the farm fields that were diverted to growing biofuels, greenhouse gas emissions go up.
The entire article raises important points, as do the reader comments.