But that isn't the case for sugar-cane ethanol or soybean biodiesel from Brazil's newly cleared lands, including tropical forests and savannas. Clearing land releases immense amounts of greenhouse gases into the air, because much of the material in the plants and soil is broken down into carbon dioxide.
Land clearing/burning has always been my biggest concern with biofuels - other than the fact that it's a half-ass answer to start with.