Large-scale bioenergy from additional harvest of forest biomass is neither sustainable nor greenhouse gas neutral".
"Owing to the peculiarities of forest net primary production humans would appropriate ca. 60% of the global increment of woody biomass if forest biomass were to produce 20% of current global primary energy supply. We argue that such an increase in biomass harvest would result in younger forests, lower biomass pools, depleted soil nutrient stocks and a loss of other ecosystem functions. The proposed strategy is likely to miss its main objective, i.e. to reduce greenhouse gas (GHG) emissions, because it would result in a reduction of biomass pools that may take decades to centuries to be paid back by fossil fuel substitution, if paid back at all. Eventually, depleted soil fertility will make the production unsustainable and require fertilization, which in turn increases GHG emissions due to N2O emissions. Hence, large-scale production of bioenergy from forest biomass is neither sustainable nor GHG neutral."
Now, you might think that a proponent of biomass energy, such as I, would go to great pains to dissect and challenge this article. Not so. In fact, I believe the authors of the paper are contributing a valid issue to the bioenergy discussion, simply from the specific use of the qualifier "Large-scale bioenergy" in their title. You see, the gist of their article is, that large-scale biomass energy doesn't make sense, which is precisely what I proposed in past posts like Size Matters. The difference in their point is that they believe large-scale is not globally sustainable from an ecological standpoint, and I believe that it is not locally sustainable from an economic standpoint. So, for different reasons, we agree that size matters.
The debatable point is what "large-scale" means. The authors of this paper, I believe, are basing their definition of large-scale on the proposed production level of bioenergy targeted by many governments, including the United States. They point out that many European countries have targets of 20% of primary energy production to come from bioenergy. Further, the recent update to the USDA's Billion-Ton Supply study essentially lays out a road map by which our primary energy production from biomass could more than double by the year 2030.
So, let us clarify that the USDA Billion-Ton Study is not suggesting that we could or should produce 20% of our primary energy from natural forests, or even natural forests plus short-rotation woody crops. The 2030 projection would increase our forest energy production from about 2%, currently, to about 3.5% of our primary energy production, far short of the 20% argued against by the authors of the paper cited at the top of the post. When woody energy crop production is included (let's assume half of the green component above, the other half going to agricultural crops), that means in essence that the USDA report is suggesting in reality that about 5% of our primary energy production could be sustainably produced from forest production and short-rotation woody crops combined.
So, the recent paper argues a straw man position that I think most natural resource scientists would agree with...that our forest resources are not capable of sustainably producing 20% of the world's primary energy supply. They are correct in suggesting that attempts by governments to impose a 20% target of this nature, and to therefore encourage "large-scale" wood energy projects, especially large-scale power, are misguided and will not achieve the ecological impacts they are hoping for.
But I think we have ample room under the "biomass cap" to work with at this point. Once we get closer to 5%, which seems to be, with our best projections (for the United States, at least) a sweet spot on woody biomass energy production, we will be pushing the limits of our biomass energy capabilities, and the energy markets will certainly be pushing back, due to the increasing price of biomass at that level.
Helpfully, the discussion started by these scientists sets an outer limit for our discussion. We know 20% woody biomass energy is too "large-scale" from a ecological standpoint; we know that here in the U.S., 2% is what we currently do; and we know that 5% is "doable", at least from the production standpoint. Would the authors argue that 5% is as ecologically unsustainable as 20%?
Probably not. But their points set some nice boundaries for future research. What is the right level? Only our future scientists and wood producers will be able to answer that with a reasonable level of certainty. Until then, let's not push the biomass bandwagon too hard... we might hit a wall where none was expected. But let's not unnecessarily constrain it, either. Let's just keep moving it down the road at the rate that the energy markets will comfortably allow.