It is the process of releasing carbon which has been sequestered for millions of years, and which is, upon being burnt, increasing the partial pressure of CO2 level in the atmosphere that is causing global warming.

There is an additional cost associated with biofuels, and that is the depletion of nutrients in the soil in which they are grown. A typical nutrient cycle - one undisturbed by man - is such that grass grows, is eaten by buffalo and other herbivores (for example) which then die, either of old age or predation, and return their bodies to the soil of the area in which they lived, either directly or by passing through the system of a preditor or preditors, which, in turn die…

When humans interrupt this cycle, it becomes necessary to replenish the soil with fertilizer. The energy costs of manufacturing the fertilizer and then tranporting and distributing it much be accounted for when trying to determine if a “biofuel” is “green”.