Although burning any fuel other than hydrogen (H₂) sends carbon dioxide in

the atmosphere, bio-fuels remove this CO₂ during the growth of the plants via

photosynthesis.  Although this process does consume some conventional

petroleum energy for fertilizer, harvesting, etc... there is a NET energy gain. 

For ethanol the net energy gain is somewhere around 1.5 to 1, and for

biodiesel this is somewhere around 3.5 to 1 (energy OUT  vs. energy IN).

Carbon dioxide is the most prolific of the global warming gasses that are

slowly but surely warming our Earth.