Monday, October 24, 2016

Carbon balance effects of real-world biofuel use

Most of the debate about the environmental merits of biofuel use has been based on lifecycle analysis (LCA). Also known as life-cycle assessment, carbon footprint analysis, "cradle-to-grave" analysis, or (in the case of motor fuels) "well-to-wheels" analysis, LCA is a method for adding up all of the impacts of interest associated with a product. It includes the effects of producing a product and its component materials, associated shipping and packaging effects (as relevant), the use of the product and any related disposal effects. For assessing climatic impacts, LCA tallies the greenhouse gas (GHG) emissions associated with the production, use and disposal of the product. When the product is a motor fuel, the numerical result of such LCA modeling is called the fuel's "carbon intensity," as calculated for California's Low-Carbon Fuel Standard (LCFS), for example.

A cornfield does not absorb CO2 from the atmosphere
any more quickly 
when it is used to make ethanol than
when it is grown for food or feed. 
This basic fact of 
carbon mass balance is ignored by the 
lifecycle studies
that claim climate benefits for biofuels. 
The question, "how does the overall emissions impact of using a biofuel such as ethanol compare to that of a fossil fuel such as gasoline?" seems straightforward, and sounds like something that LCA can answer. However, that question is actually ill-posed scientifically speaking. In other words, when one looks carefully at what actually happens when a given biofuel substitutes for a fossil fuel, it turns out that LCA cannot give a straightforward, unambiguous answer. Properly qualified, the answer will always be, "it depends." And it doesn't just depend on the particular fuel and how it is produced; it also depends on the design of the LCA model and the assumptions it invokes.

Nevertheless, many lifecycle researchers obscure this "fine print" and are all to often happy to make unqualified (or poorly qualified) numerical claims based on their LCA modeling results. For reasons explained in previous posts about modeling mistakes, the problems with LCA logic, and related bookkeeping issues, such claims do not withstand careful scientific scrutiny. It is possible, however, to evaluate the directly measurable GHG emissions impacts of biofuel production and consumption without resorting to the inherent ambiguities of LCA.

Given the large expansion of U.S. biofuel use after the Renewable Fuel Standard (RFS) became law in 2005, data are available for assessing the increased ethanol and biodiesel use observed since then. Between 2005 and 2013, biofuel consumption more than tripled, rising from 4.2 billion to 14.6 billion gallons per year (see this chart). Field data on crop growth and fuel use over this period enable evaluation of the direct effects on CO2 emissions, which is the subject of our new paper (citation below).

A key finding is that the increased amount of CO2 taken up by crop growth over this period was not nearly enough to balance out the increased biomass-derived CO2 emissions due to rising biofuel use. This result contradicts the LCA models' assumption that biomass is carbon neutral, i.e., that the CO2 emitted when burning a biofuel is automatically and fully offset by CO2 uptake in the corn or soybeans used to make the biofuel. Farm data, taken from the U.S. Department of Agriculture (USDA) annual crop production summary reports, revealed that crop growth actually offset only 37% of the biogenic (biomass-based) CO2 emissions due to the expanded biofuel use between 2005-13.

Now, our results don't claim to be the whole story about the impacts of substituting biofuels for petroleum fuels. That would require modeling the entire globe and projecting impacts well into the future. Any such analysis faces overwhelming uncertainties; after all, there's no time machine that can give us data from the future and neither does anyone have sufficiently detailed data on the global changes in commodity markets and land use triggered by biofuel production.

But an evaluation of the physical movements of carbon -- both the CO2 emitted during fuel production and use and the CO2 absorbed during crop growth -- that were a direct result of the increase in biofuel use is possible using high-quality data. Such an evaluation puts a scientific constraint on the total net impact because the indirect effects only serve to increase biofuel-related GHG emissions overall.

Because the claimed climate benefits of biofuels hinge completely on the assumption that their CO2 emissions are balanced out by CO2 absorption when their feedstocks are grown, our results pull the rug out from under the popular (and government-sanctioned) belief that biofuels help reduce the emissions that cause global warming. As our paper notes, once previously published estimates processing emissions and other effects including land-use change are considered, it is clear that biofuel use has increased rather than decreased CO2 emissions compared to petroleum fuels to date.

DeCicco, J.M., et al. Carbon balance effects of U.S. biofuel production and use. Climatic Change 138(3): 667-80 (October 2016).

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