How big is the bioenergy piece of the energy pie? Who cares—it's pie!

V IEWPOINT How Big is The Bioenergy Piece of the Energy Pie? Who Cares—It’s Pie! Cut my pie into four pieces, I don’t think I could eat eight. —Yogi Berra Estimating the practical limits to how much bioenergy could be produced in the future has been an academic sport in recent years, but is not a useful activity. Because biological photosynthesis usually results in less than 1% of available light energy being stored in biomass, there are easily calculated limits to the contribution that biological systems can make to human needs for energy. Recognition of this limitation provokes some authors to dismissive attitudes about bioenergy in favor of alternative technologies such as solar (Michel, 2012). However, biobased energy contributes approximately 10% of all human energy use at present (IEA, 2012), about 4.3 times as much as all hydro, and 11.4 times as much as all other renewables (e.g., solar, wind, and geothermal) combined. Thus, bioenergy is an important component of our overall energy system, and could be much larger without invoking dramatic changes in land use. For example, about 56% of the 1.2 billion tons of municipal solid waste in California is lignocellulose, which could be captured and converted to energy (Youngs and Somerville, Slade and colleagues (2014) recently reviewed the results of more than 120 estimates of the future contribution of bioenergy to the global energy use of about 550 EJ/year. The estimates vary widely, depending on the assumptions made, most of which reportedly had an optimistic bias. The estimates ranged from 22 to 1,272 EJ for energy crops, 10 to 66 EJ for agricultural residues, 12 to 120 EJ for wastes, and 60 to 230 EJ for forestry. Slade et al., concluded that nothing useful could be accomplished by further modeling studies or speculation about the magnitude of future bioenergy production and that the path forward is to learn by doing. Even at the pessimistic end of the estimates, bioenergy could contribute at least another 20% of total energy use. One of the drivers for the recent discourse about the potential contributions of different sources of energy appears to be the widespread recognition in the scientiﬁc community that carbon loading of the atmosphere may have undesirable Correspondence to: C. Somerville. Received 11 June 2014; Revision received 11 June 2014; Accepted 11 June 2014 Article first published online in Wiley Online Library (http://onlinelibrary.wiley.com/doi/10.1002/bit.25312/abstract). DOI 10.1002/bit.25312 s 2014 Wiley Periodicals, Inc. environmental consequences. Organizations that have grap- pled with how we might reduce the use of fossil fuels inevitably come to the recognition that, pending a break- through in fusion research, there is no straightforward path to the eventual displacement of fossil fuels (NAS, 2009). As participants in several of the large multidisciplinary analyses of our energy future, we have come to the opinion that relatively few people understand the whole picture. There is a natural tendency for colleagues working in one ﬁeld or another to think that their favorite technology is important and very promising, and that if there were just more research and development dollars in their ﬁeld things would be progressing much better. There are also a lot of people who do not welcome change in the energy sector because it is disruptive to their interests. Finally, many people are concerned about the ways in which we use land and water and are concerned about possible negative effects of energy technologies that impact these fundamental resources. Thus, consensus about the future of energy in general, and bioenergy in particular, is elusive for many reasons. It is clear that we will need a wide variety of solutions to meet our growing energy appetites sustainably. It is also clear that bioenergy has a place at the table. Several recent studies centered on curbing greenhouse gas emissions, using different approaches, have reached the conclusion that deploying all known low carbon technologies, including nuclear energy and carbon capture and storage (CCS), may be needed (Greenblatt et al., 2011; IPCC, 2014; Kainuma et al., 2013; McCollum et al., 2012; Williams et al., 2012). For example, even with other optimistic assumptions, such as electriﬁcation of all rail transport and a large percentage of the light duty ﬂeet, heroic efﬁciency improvements and renewable electricity from wind, solar and geothermal, biofuels are the only available option foreseeable at large scale in the next 40 years for reducing emissions from the heavy duty ﬂeet and aviation (Greenblatt et al., 2011; McCollum et al., 2012). Perhaps most controversially, biomass, when combined with CCS, is one of the few carbon negative options (IPCC, 2014; Keith, 2001). In other words, all the wedges are important (Pacala and Socolow, 2004), regardless of exactly how big they are. In some ways, it is challenging to consider quantitative estimates of probable future contributions of different energy technologies as a worthwhile scientiﬁc activity. All technolo- gies will be physically limited to some extent. While this can be estimated, social and economic limitations may be Biotechnology and Bioengineering, Vol. 111, No. 9, September, 2014