Global warming – are bio-fuels good or bad?
(fourth in series)
The emergence of bio-fuels such as ethanol has been touted as the answer to our energy problems and a boon for the agriculture sector. By defining the energy problem as the danger of relying on foreign sources of oil, domestically produced renewable fuels provide a logical solution. So, the question facing domestically produced renewable fuels is not “if it provides a solution” but “how much of a solution does it provide”.
Net greenhouse gas emissions
Another dimension of the energy problem has emerged. Scientific investigation has confirmed the dangers of global warming from greenhouse gas (GHG) emissions. Our reliance on energy from fossil fuels contributes to GHG emissions. Early analysis determined that bio-fuels, while not exempt from carbon emissions, emit less (GHG) emissions than gasoline. As shown in Table 1, corn ethanol results in a 20 percent reduction in emissions versus gasoline. Biomass ethanol shows a 70 percent reduction. Other studies provide similar results.
Although growing corn and biomass and refining them into ethanol produces as much or more emissions than pumping, transporting and refining crude oil into gasoline, the source and amount of carbon contained in the feedstock is the most important component. The carbon in crude oil has been sequestered from the atmosphere and now is being released into the atmosphere during consumption. So, it adds to the amount of atmospheric carbon. Conversely, the carbon contained in corn and biomass that is released during consumption was recently pulled out of the atmosphere during photosynthesis. So this carbon is part of the natural carbon cycle and does not increase the level of atmospheric carbon.
Carbon in the soil
A carbon sink is a place where carbon is stored or sequestered. We are aware that crude oil and coal are natural sinks where carbon was removed from the atmosphere millions of years ago. As we consume oil and coal, this carbon is released back into the atmosphere. We are also aware that forests, especially tropical rain forests, are natural carbon sinks where large amounts of carbon are stored in the wood. When forests are burned or otherwise destroyed through deforestation, the carbon is released into the atmosphere.
A less well-known but important carbon sink is soil. Large amounts of carbon are stored in the soil in the form of undecayed plant and organic matter. When virgin soils are disturbed by plowing or other tillage, large amounts of carbon stored as organic matter are released into the atmosphere. This causes a reduction in soil organic matter as shown hypothetically in Figure 1. However, over time the balance of emissions and sequestration is restored, but at a lower level of soil organic matter. Part of the organic matter loss from tillage is replaced by the organic matter increase from the decomposition of crop residue. Crop residue includes the stalks, stems, leaves, chaff, cobs, etc. left in the field after the grain is harvested.
Crop residue has been touted as a major biomass source for the production of cellulosic ethanol. However, removing residue for ethanol production will change the organic carbon balance in the soil. By removing the crop residue, it is not available for decay and sequestration as carbon in the soil. The soil will once again become a net emitter of carbon into the atmosphere.
Is there a limited amount of residue that can be removed for ethanol production without reducing soil organic matter levels further? This is a topic of current discussion and future research. Regardless of the answer, it appears that the potential of crop residues as a major ethanol feedstock is not as great as previously believed.
High levels of organic matter are also important for maintaining soil productivity and retaining soil moisture. In addition, crop residue contains important crop nutrients that are returned to the soil during decomposition. Crop residue left on the soil surface also helps reduce soil erosion.
This article has focused on the “direct” GHG emissions from both corn and cellulosic ethanol production. More controversial are the “indirect” emissions from “land use” changes that may be attributed to corn, biomass and ethanol production. In the next article we will explore these indirect effects and endeavor to shed light on the issues involved in this debate.
Fargione, J., J. Hill, D. Tilman, S. Polasky, and P. Hawthorne, 2008, “Land Cleaning and Biofuel Carbon Debt,” Sciencexpress. Accessed April 21, 2008.
Searchinger, T., R. Heimlich, R.A. Houghton, F. Dong, A. Elobeid, J. Fabiosa, S. Tokgoz, D. Hayes, and T.H. Yu, 2008, “Use of U.S. Croplands for Biofuels Increases Greenhouse Gases through Emissions from Land Use Change,” Sciencexpress. Accessed April 21, 2008.