Are estimates of bioenergy emissions broken?

Tim Searchinger has a history of questioning assumptions and justifications put forward by proponents of biofuels.  His points are usually valid, or at least adds to the conversation.  In his latest article with Keith Smith in Global Change Biology, he questions two assumptions used in most lifecycle analyses (LCA) in regards to crop based biofuels.  These LCAs attempt to estimate or measure inputs and outputs from a system or process – in this case, the process of making bioenergy.  First, some basics: when plants grow they capture CO2.  When they are used for bioenergy, this CO2 is released again: this is net neutral CO2 (not including emissions from other aspects of growing and processing the plant).

” The problem is not that biofuels reduce GHG emissions, and land-use change increases them; the problem more accurately in such a case is that biofuels result in no positive land use or other market-based change that leads to greenhouse gas reductions in the first place. “

The assumptions he questions:

1. CO2 captured during plant growth is not “additional” unless nothing would otherwise have grown on that land.  The implication here is that bioenergy is not reducing CO2 emissions.

Yes this is true.  But even if bioenergy is no better than fossil energy, at least it isn’t worse.  I think Searchinger is really bothered by the fact that bioenergy is not net negative emissions.  But while every gallon and pound of fossil fuel we burn is adding to CO2 emissions, plants can recapture the CO2 … so it is net neutral!  I would also argue that bioenergy is more sustainable and renewable than fossil energy.  Once you start thinking about other environmental issues with extracting fossil fuels, you realize there is more to think about than LCAs and math.  Of course this is something societies need to decide at regional levels, based their understandings of costs/benefits and what resources they have available.  Obviously there are other great renewable energy technologies that are clearly net negative: solar, wind, nuclear, etc.. we should be using all of these.

Furthermore, this is specific to a certain type of bioenergy: from crops that would be grown otherwise.  When we use agricultural residues (what is leftover after harvesting grain for food) I would argue that this is additional CO2 that is captured.

2. Ignoring both land use emissions (like ILUC), and final emissions from burning the biomass, fails to account for the emissions in the LCA

This really hinges on how you handle #1.  I am by no means an expert but it is much easier to estimate and include direct emissions than to go down the road of ILUC..

3. Most LCAs underestimate N2O emissions

This is probably true – more research and measurements need to be done to better estimate this.  Something that my colleague and fellow contributor John Field is working on!


Smith, K., & Searchinger, T. (2012). Crop-based biofuels and associated environmental concerns GCB Bioenergy DOI: 10.1111/j.1757-1707.2012.01182.x

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2 Responses to Are estimates of bioenergy emissions broken?

  1. John Farmer says:

    You say, in reply to point 1:- “But even if bioenergy is no better than fossil energy, at least it isn’t worse”.
    But it IS worse! …because the total amount of CO2 released in preparing and burning biofuels or biomass is almost always higher than the CO2 released by fossil fuels for the same amount of energy. It only APPEARS less if you ASSUME that the biological carbon content doesn’t count. That would only be a good assumption if the same amount of CO2 were additionally sequestered from the atmosphere in growing the biomass. However if the biomass comes from an existing field, that field was already sequestering CO2 before you decided to burn it. So there is no additional sequestration of CO2 and your assumption is wrong.

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