Dr. Chris Guay, Vice President of Research and Development for Community Fuels, has been involved with climate change research for the past 15 years (see photos from Dr. Guay's field work in the Arctic and Antarctic).  He has prepared the following explanation about why using biodiesel results in lower emissions of greenhouse gases relative to petroleum diesel:

Combustion of both petroleum diesel and biodiesel produces carbon dioxide (CO2), the most prevalent of the greenhouse gasses (GHG) released to Earth's atmosphere as a result of human activity.  In the case of petroleum diesel, the CO2 produced during combustion was previously in the form of organic carbon derived from fossil fuel deposits.  The ultimate source of the organic carbon in the fossil fuel deposits was atmospheric CO2 that had been converted to organic carbon by photosynthesis -- i.e., the fossil fuel deposits were formed by the accumulation of material from plants or the animals that ate plants.  But the important thing to point out is that the carbon contained in the fossil fuel deposits was removed from the atmosphere millions of years ago and has been locked up within Earth's crust ever since.  When this fossil carbon is put back into the atmosphere as a result of burning fossil fuels, it represents a new input of carbon into the modern atmosphere.

In contrast, the CO2 produced from the combustion of biodiesel was previously in the form of organic carbon contained in modern plant material (i.e., the crops that were grown to produce the feedstock that was converted to biodiesel).  The CO2 produced during the combustion of biodiesel therefore does not represent a new input of carbon into the modern atmosphere because the CO2 being put back into the atmosphere had recently been removed from the atmosphere by photosynthesis during the growth of the feedstock crops.  So it's a closed cycle -- CO2 is removed from the atmosphere and converted into organic carbon in plant material, which is then released to the atmosphere during the combustion of plant-derived biodiesel, which is then removed from the atmosphere again by the growth of new crops, and so on -- and the ultimate result is no net input of new CO2 into the atmosphere.

So while fossil carbon and modern plant carbon both originated from atmospheric CO2 that was converted to organic carbon via photosynthesis, the key difference is the time scales on which these processes are occurring.  The carbon introduced to the atmosphere by combusting biodiesel will be converted back to plant material in a few months or years.  But it will take millions of years for the carbon introduced to the atmosphere by combusting petroleum diesel to be converted back to fossil fuel deposits.  This is why combusting biodiesel is considered a carbon-neutral process (i.e., no net input of new CO2 to the atmosphere), while combusting petroleum diesel is not.

But in order to make a complete comparison, it is also necessary to look at the net GHG emissions of petroleum diesel and biodiesel over their entire life cycle, which includes the process of producing the fuel as well as the combustion of the fuel.

Sources of GHG emissions during production of petroleum diesel are associated with:

• extraction of crude oil
• refining of crude oil
• transport of crude oil and finished diesel fuel

Sources of GHG emissions during production of biodiesel are associated with:

• growing and crushing of oilseed feedstock crops
• conversion of feedstock oils into biodiesel
• transport of raw materials used in production and finished biodiesel fuel

So the overall reduction of GHG emissions resulting from biodiesel combustion relative to petroleum diesel combustion is calculated as the difference between the total life cycle GHG emissions for a given amount of petroleum diesel and the total life cycle GHG emissions for an equivalent amount of biodiesel.  Several recent studies have quantified the life cycle of GHG emissions from petroleum diesel and biodiesel. One of the most frequently cited reports was published in 1998 from a joint study conducted by the U.S. Department of Agriculture and Department of Energy (NREL/SR-580-24089), which calculated a 78% reduction of GHG emissions from biodiesel combustion relative to petroleum diesel.  Another major report published in the Proceedings of the National Academy of Sciences in 2006 cited a 41% reduction in GHG emissions.

It should be pointed out that these studies were based on biodiesel produced from commodity soybean oil.  Greater reductions of GHG emissions are possible using alternative feedstocks (e.g., recycled fats and oils, non-conventional crops, algae, etc.) that are not as energy-intensive to produce as soybeans and other commercial oilseed crops.