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  • Do biofuels actually cause more greenhouse gases?

    Posted on February 14th, 2008 Lee Devlin 2 comments

    I recently received a pointer to this blog article which references a NY Times piece about articles in Science that state that biofuels actually increase global warming by pulling land into the agricultural pool that was previously a carbon sink. The first of these Science papers is focused on the ethanol industry in the U.S.

    During the past 14 years, 15 separate studies have shown that ethanol has a net positive energy balance. Only one study has contradicted it, but the researchers of that study (Pimental and Patzek) wrote the same paper 4 times so you may hear that the ratio is 15:4. It’s the one that always gets quoted (usually unknowingly) when someone tells you it takes more energy to produce a gallon of ethanol than you can get out of it. Now it appears ethanol opponents will have another study to quote, this time about biofuels creating additional greenhouse gases.

    In looking in the supporting materials in Science Express, I found this curious assertion:

    If corn-based ethanol could not receive a credit for removing carbon from the atmosphere – deleting the feedstock uptake credit from the GREET model– it would increase greenhouse gas emissions by 48%. It follows that if the use of land to grow corn for ethanol has the net effect of reducing land-based carbon sequestration, the overall effect will be a bigger release of greenhouse gasses.

    In other words, they are stating that when comparing greenhouse gases from corn to gasoline, corn should not get a credit for having removed carbon from the atmosphere. Instead they think it should be compared to growing a forest or prairie in the place of farmland which would allow the carbon to be sequestered year after year. Forests and prairies give back carbon to the atmosphere every year when their leaves and grasses die. In the case of forests, every few decades the trees die, or burn, or are used for some other purpose and thus also give back their carbon in a brief instant of geological time. Unless you’re burying the carbon deep under the earth’s surface or oceans, any carbon taken in by plants is given off in a few months or decades. Soils also have a limited capacity to hold carbon and eventually reach a homeostasis after only a few decades. So I consider the logic used in this study to be flawed.

    But I will expect that every biofuel opponent will quote it with abandon, never realizing that the authors of the paper are not comparing biofuels with fossil fuels, but rather biofuels with some imaginary state of affairs where forests that capture but do not release carbon to the atmosphere have been replaced by farmland.

    All land capable of sustaining plants, whether it be used for farming, prairie, or forest eventually reaches a homeostasis when it comes to CO2 sequestration. Farming allows us to take advantage of the CO2 to carbohydrate conversion that occurs on land whereas prairies and rainforest that go unharvested do not. But in the end, they all return CO2 back to the atmosphere in a relatively short span of geological time. The only counter-examples are swamps that can, over the course of millions of years, turn vegetation into coal by trapping a tiny percentage of carbon each year.


    2 responses to “Do biofuels actually cause more greenhouse gases?”

    1. Most scientists and informed citizens will agree that the current trend of escalating greenhouse gas (GHG) levels will cause major social, economic and environmental disruption over the next decades. Hence there is great interesting identifying approaches and practices that will mitigate this effect. Biofuels derived from food crops or other biomass offer an exciting and potentially important component of our overall portfolio of GHG mitigation, especially if they can be produced in a sustainable way. For example, the best recent calculations from several academic groups indicate that the burning of corn-derived ethanol rather than gasoline will result in a modest net reduction in GHG emissions, and that ethanol derived from grass or woody biomass has a somewhat greater potential. However, the element that is often disregarded in these studies (probably because it is much harder to determine) is the contingent effect of changes in land-use practices on overall atmospheric carbon balance.
      A huge amount of carbon is sequestered both in plants, but also in soils. In some environments such as rainforests and native grasslands, the levels of carbon are in quasi-equilibrium, with the rate of carbon fixation balancing the rate of oxidation of dead organic material. Some current farming practices also have established a new carbon balance. As a broad generalization, disruption of natural habits results in net carbon release, either due to burning or to stimulation of metabolism of soil organic material. Deforestation, such as that in the US over the past ~200 years has resulted in a major release of net carbon, but currently re-growth of these forest areas actually results in a substantial net uptake of carbon, estimated at 10% of US annual carbon emissions. Likewise, after extensive agriculture, the carbon-rich US native prairies have release substantial carbon in the past, but in many cases have reached a new quasi-equilibrium. Over-farming of corn in some places continues to cause a net degradation of soil quality and carbon balance. In contrast, in the US and Europe, land that was previously marginal or highly degraded can actually recover and constitute a net carbon-sink if it is left fallow, as a result of active conservation or market-driven forces.
      The two recent Science papers by Searchinger et al. and Fargione et al. make a strong case that the increased pressure to produce biofuels is resulting in a major shift in world-wide land-use practices. This puts pressure on habitats that are either in carbon balance or which are actually current carbon sinks.
      Both Science papers use models that compare estimates of carbon captured by biofuel crops with the predicted change release of soil- and vegetation-derived carbon due to producing those crops. It should be stressed that this effect on GHGs is quite distinct from the earlier calculations [mentioned by Lee D. above] that focus primarily on the energy used to produce biofuels or gasoline.
      Diverting current crops to fuel does not per se change GHG levels, but since the demand for food does not go away (and is actually increasing), there will be strong economic pressures to convert more land to agricultural production. Moreover this trend is currently being fueled by generous incentives from the US taxpayer, and it is has been argued that there would only be a modest bioethanol industry without these subsidies. In addition, increase in agricultural intensity on existing land will probably cause a degradation of soil carbon balance, and this could be magnified by increased use of synthetic fertilizers (which require a lot of energy to manufacture and cause increases in the release of N2O, a highly potent GHG).
      The current negative effects of the ongoing conversion of forests, grasslands and peaty soils over the past century have been well documented, and constitute a significant fraction of anthropogenic GHG release. The Science papers demonstrate that most of the current approaches for producing biofuels will only make matters worse. The 2007 IPCC reports state that global warming is already occurring, and would continue to do so for some time even if GHG emissions were reduced to zero (since most GHGs have a long atmospheric half-life). It has been argued that an 80% reduction in emissions by 2050 will be required in order to mitigate the worst effects over the next century. While new equilibria in GHG, temperature and other environmental variables will be reached over centuries or geological time, our current emphasis must be on the short-term impact of our actions.
      In the future, just as the developed world currently requires environmental impact statements for projects such as new hydroelectric construction, it will be important to do analogous studies on the impact of government policy changes and incentives on our global environment resulting from different kinds of biofuel production. Unfortunately, soil microbes and organic material do not have the same emotional appeal as polar bears and pandas, so it may be a while before there is a strong grass-roots lobby for sustainable agricultural practices.

      As with most papers in Science, unless one is an expert in a given field it is hard to critique the conclusions of an article: in fact, that’s why papers are peer-reviewed. To the best of my ability, I find the two papers and their extensive supporting data to provide a coherent case for the serious re-evaluation of the current US policies on biofuels, and they should help inform our discussions about where to place limited resources in our attempt to mitigate our GHG crisis. [In the context of an online discussion rather than a scientific paper, I have omitted making any literature citations to support my arguments above].
      Thanks to Lee for making space for this dialog, and I look forward to further debate of this important topic.

      Peter Olins, PhD

    2. Hi Peter,

      Thanks for your comment to my blog article. It’s so well written that I would not know where to begin if I had to refute it.

      There are two sides to every story, and you have done an excellent job at summarizing the supporting materials for case against the production of biofuels from crops.

      There is reason to believe that electrical and heating needs in the future could be handled by solar and wind energy sources, provided a means of timeshifting production and consumption demands becomes practical. But transportation fuels, which accounts for nearly 30% of our collective energy consumption, don’t seem to have anything on the horizon that is as safe, convenient, and energy dense as liquid hydrocarbon fuel (and I’m including all biofuels in that description). Some of this may be absorbed in electric vehicles, but for planes, ships, and large vehicles, only liquid hydrocarbon has the energy density to be carried onboard.

      It’s possible that fuels like hydrogen may never become practical for transportation because of the engineering problems associated with its generation, transportation, storage, and refueling. Battery energy density and corresponding cost reductions have been nearly moribund since the battery’s inception from a %/year standpoint. I’m trying to imagine a future that includes some method to produce enough transportation fuel to satisfy demands. It could be some enclosed algae-based solution where it’s grown on land that is rich in sunlight but where it is impossible to grow crops. That would produce a CO2 sink where none exists today and would not compete for land on which to grow crops.

      Biomass is responsible for 90% of our energy sources today. It’s unfortunate that nearly all of that is from biomass that is millions of years old, and harvesting that energy is neither sustainable nor wise if in the end it changes the climate of the planet to the point where our species is no longer part of it.

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