Helmut Haberl, Karl-Heinz Erb, Fridolin Krausmann, Steve Running, Timothy D Searchinger and W Kolby Smith 2013 Bioenergy: how much can we expect for 2050? Environ. Res. Lett. 8 031004
http://iopscience.iop.org/1748-9326/8/3/031004/article

Hello, my name is Bill Smith, a post-doc at the university of Montana and co-author of the perspective appearing in ERL 'Bioenergy: how much can we expect for 2050?'  I present this work on behalf of my co-authors including lead author Helmut Haberl.

Record-high prices for fossil fuels, concerns over peak conventional oil and natural gas production, and the necessity to reduce global greenhouse gas emissions motivate a global interest in biomass as a key future energy source.  But how much bioenergy can we -- or should we -- expect the terrestrial ecosystems of the earth to deliver in the coming decades?  Currently, diametrically opposed views on bioenergy's future prospects to deliver sustainable, low greenhouse energy abound in the scientific community, with estimates of global primary bioenergy potentials published in the last five years spanning almost three orders of magnitude, from thirty to thirteen hundred exajoules per year. Recently, the IPCC Special Report on Renewable Energy reported a similarly wide range, as did the influential Global Energy Assessment.  Reducing the range of variability associated with current estimates of bioenergy potential is a necessary precursor to effective incorporation of bioenergy into global energy policy. 

One crucial piece of information that can help to tackle this conundrum and realistically constrain global bioenergy potential is the current global annual biomass growth of green plants on the earth's lands, termed net primary production or NPP. Satellite measurements of NPP derived from the MODIS sensor, suggest that global terrestrial NPP has stayed near 53.6 petagrams of carbon per year, with stunningly low year-to-year variation. In other words, considerable global efforts to increase annual yields in agriculture and forestry through irrigation, fertilization or forest management have not increased total plant growth at the global scale.

In our perspective, we argue NPP represents an upper-envelop constraint on bioenergy potential, and we show that, under optimistic assumptions regarding the fraction of NPP that could be used for bioenergy, the biophysical limit for primary bioenergy is around 190 EJ/y. This capacity estimate would entail cultivating all vegetated lands outside dense forests, urban areas, croplands and the world's remaining wilderness areas at the highest conceivable exploitation rate. It is important to note that this is not an estimate of the upper limit for sustainable bioenergy potential.  For instance, if we consider just a few of the many sustainability issues that have been highlighted in the recent literature such as low productivity land, vulnerable drylands, areas that risk tropical forest encroachment, and areas that risk savannah degradation, we are left with only roughly 80 exajoules per year, and even that might result in trade-offs with food supply if not implemented well.

Given biospheric constraints on NPP, it is unlikely that bioenergy could provide more than 250 exajoules per year or roughly 20-30% of global primary energy demand in 2050, a figure substantially below many published bioenergy projections. Reaching such a level of supply would require roughly a doubling of global biomass harvest in less than four decades and would result in massive increases in humanity's pressures on land ecosystems. We therefore argue that any strategic planning or energy policy that includes an expectation for bioenergy to deliver more than 250 exajoules per year is unrealistic and fails to reflect current biophysical growth constraints.