Bio diesel has carbon neutral property. Activities that don't disrupt this balance are described as carbon-neutral.
Bio diesel is plant based and using it adds no extra CO2 greenhouse gas to the atmosphere. In fact, there's no actual reduction in the amount of CO2 produced when biodiesel is burned instead of petro-diesel -- the same amount of CO2 will come out of the exhaust pipe with either fuel.
But the CO2 released by burning biodiesel is part of the current natural cycle, it does not raise the level of CO2 in the atmosphere and does not act as a greenhouse gas. Biodiesel is carbon-neutral and does not increase global warming.