There's a problem with the policies now imposed on transportation fuels in an attempt to control CO2 emissions. The issue has to do with how carbon is counted when the policies are defined.
Good accounting needs to reflect the fact that motor fuels are liquids that contain carbon as their molecular backbone. By nature, liquids flow. That's one main reason why they are so valuable for fueling cars, trucks, ships and planes. Carbon flows with the fuels, and it's the rate at which the carbon flows to and from the atmosphere in the form of CO2 that matters for global warming.
Note the "to and from" in the previous sentence. A big focus of climate policy is reducing CO2 emissions, e.g., reducing the rate at which carbon flows into the air due to energy use. But the from part is also important, and in fact, the CO2 problem is best described as an imbalance in the global carbon cycle due to human activities that cause carbon to flow into the atmosphere faster than natural processes can remove it.
|The Global Carbon Cycle|
(Source: www.esrl.noaa.gov Carbon Cycle Toolkit)
The adjoining figure illustrates the global carbon cycle. As suggested by the relative thickness of the arrows depicting the flows, the overall cycle is much larger than the excess from fossil fuel use. In round numbers, the natural CO2 flow amounts to circulation of about 200 PgC/yr (petagrams, i.e., 1015 grams, or billion of metric tons, of elemental carbon per year) between the atmosphere and the earth's surface (both land and sea). The total flow into the atmosphere from fossil fuel consumption has been steadily climbing and is now about 10 PgC/yr. Roughly another 1 PgC/yr of emissions are due to deforestation. (The latest data can be found at the Global Carbon Project.)
The carbon cycle matters a lot when biofuels enter the picture. To protect the climate, biofuels must increase the from part of the carbon cycle; that is to say, they have to increase the net rate at which CO2 is removed from the atmosphere. After all, they don't decrease the to part of the cycle because simple chemistry tells us that a given biofuel emits essentially the same amount of CO2 as the fossil fuel it replaces. As I've put it elsewhere, if biofuels have a benefit, it's not when they're burned.
Unfortunately, these scientific basics were neglected, or at least badly misapplied, when the lifecycle analysis (LCA) ("carbon footprint") models now used to regulate transportation fuels were developed. These LCA models treat biofuels are inherently carbon neutral whether or not their production actually increases the net rate at which CO2 is flowing from the atmosphere into biomass. But if the rate of CO2 removal isn't increased, then replacing a fossil fuel such as gasoline with a biofuel such as ethanol doesn't have any climate benefit at all. In that case, all the emissions related to producing the ethanol, as well as any excess emissions due to indirect effects, only make matters worse. Thus, rather than reducing CO2 emissions, efforts to promote biofuels are almost certainly increasing emissions overall.
The resulting errors are hard-coded into the heart of policies such as the Renewable Fuel Standard (RFS) and Low-Carbon Fuel Standard (LCFS). In both cases, and also in the Argonne National Laboratory (ANL) GREET model (as sponsored by the U.S. Department of Energy and adapted for California's LCFS), the result is scientifically unsound bookkeeping when comparing fuels. This mistake in the carbon balance sheets was unintentional on the part of those who developed the models, but it's a serious mistake all the same.
The accounting problems are analyzed in my recent paper, The Liquid Carbon Challenge, which finds that all of the carbon footprint studies that have been used to promote biofuels are flawed and should be redone using correct carbon accounting methods. The principles are given in my prior paper on Biofuel's Carbon Balance.
By failing to properly account for the from part of the carbon cycle, LCA-based policies improperly credit the CO2 absorbed from the atmosphere when biofuel feedstocks such as corn, soybeans and sugarcane are grown. Most of the land now devoted to producing biofuels was already being used to produce crops for feed and food. Harvesting that land for making fuel does not significantly increase the net rate at which CO2 flows out of the atmosphere, and therefore offers no climate benefit.
The bottom line is that the very premise of the LCFS and the climate-related provisions of the RFS is incorrect. Because their bookkeeping is faulty to begin with, incorporating "the latest data" (as renewable fuel proponents put it) does not fix the problem. In short, these policies are scientifically unsound and, as far as climate is concerned, should just be scrapped.