What was done
Using a process-based model of the land biosphere to simulate rain-fed and irrigated biomass yields driven by data from different climate models, and combining these simulations with a scenario-based assessment of future land availability for energy crops, the authors estimate "the global bioenergy potential from dedicated biomass plantations in the 21st century under a range of sustainability requirements to safeguard food production, biodiversity and terrestrial carbon storage," after which they explore the resulting spatial patterns of large-scale ligno-cellulosic energy crop cultivation with respect to their impacts on land and water resources.

What was learned
Beringer et al. report that their calculated bioenergy potentials "are in the lower range of previous assessments," but they say that all biomass sources may still provide some 15-25% of the world's future energy demand in 2050, with energy crops accounting for 20-60% of the total potential, depending on land availability and share of irrigated area. But therein lies the problem.

What it means
Noting that "human land use is already the most important driver behind environmental degradation (Foley et al., 2005), biodiversity loss (Butchart et al., 2010) and fresh water consumption (Rodell et al., 2009)" -- and that "if energy crops are not restricted to abandoned and surplus agricultural land, the spatial expansion of agricultural activities could affect a large number of natural ecosystems, many of which already are under significant pressure from habitat loss and fragmentation" -- the three German researchers conclude that "a possible twofold increase in irrigation water requirements, global cropland increasing by up to 30% for energy crops alone, and additional nitrogen demand that may exceed future fertilizer production," all illustrate the great challenges of integrating large-scale bioenergy into global sustainable land use. In addition, they report that "a spatial analysis with the 'Terrestrial Ecoregions of the World' data set (Olson et al., 2001) reveals that many of the affected regions feature a large diversity of wildlife," and that "converting these iconic landscapes into large-scale biomass plantations may not be regarded as socially acceptable."

In light of these several findings, it can readily be understood that the host of intractable problems associated with large-scale bioenergy production will in all likelihood prevent their full potential from ever being realized. Strong competing interests for finite land and water resources simply will not allow it to happen. And humanity thus needs to realize that we can't ride a hobbled bioenergy horse into the future and expect to prosper.

References
Butchart, S.H., Walpole, M., Collen, B., van Strien, A., Scharlemann, J.P.W., Almond, R.E.A., Baillie, J.E.M., Bomhard, B., Brown, C., Bruno, J., Carpenter, K.E., Carr, G.M., Chanson, J., Chenery, A.M., Csirke, J., Davidson, N.C., Dentener, F., Foster, M., Galli, A., Galloway, J.N., Piero Genovesi, P., Gregory, R.D., Hockings, M., Kapos, V., Lamarque, J.-F., Leverington, F., Loh, J., McGeoch, M.A., McRae, L., Minasyan, A., Morcillo, M.H., Oldfield, T.E.E., Pauly, D., Quader, S., Revenga, C., Sauer, J.R., Skolnik, B.,Spear, D., Stanwell-Smith, D., Stuart, S.N., Symes, A., Tierney, M., Tyrrell, T.D., Vié, J.-C. and Watson, R. 2010. Global biodiversity: indicators of recent declines. Science 328: 1164-1168.

Foley, J.A., DeFries, R.S., Asner, G.P., Barford, C., Bonan, G., Carpenter, S.R., Chapin, F.S., Coe, M.T., Daily, G.C., Gibbs, H.K., Helkowski, J.H., Holloway, T., Howard, E.A., Kucharik, C.J., Monfreda, C., Patz, J.A., Prentice, I.C., Ramankutty, N. and Snyder, P.K. 2005. Global consequences of land use. Science 309: 570-574.

Olson, D.M., Dinerstein, E., Wikramanayake, E.D., Burgess, N., Powell, G., Underwood, E.C., D'Amico, J., Itoua, I., Strand, H., Morrison, J., Louchs, C., Allnutt, T., Ricketts, T.H., Kura, Y., Wettengel, W. and Kassem, K. 2001. Terrestrial ecoregions of the world: a new map of life on earth. BioScience 51: 933-938.

Rodell, M., Velicogna, I. and Famiglietti, J.S. 2009. Satellite-based estimates of groundwater depletion in India. Nature 460: 999-1002.