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