Does the world have low-carbon bioenergy potential from the dedicated use of land?☆
Introduction
Estimates of potential low or zero carbon bioenergy can be 500 EJ or higher even as authors assume bioenergy will not displace food or wood products (Chum et al., 2011, Creutzig et al., 2015). Dedicated energy crops provide the largest estimated sources, with some estimates also based on harvesting wood from forests. We call these sources bioenergy from “dedicated use of land” because they require the dedication of some or all the productive capacity of land even if some harvested wood, crop or biofuel by-products continue to serve other uses.
These large estimates of bioenergy from such dedicated uses of land raise attention because of their competition for land and biomass:
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One, all the world's harvested biomass in 2000 had a gross energy content of about 230 EJ, including all harvested crops, crop residues, wood, and forages consumed by livestock (Chum et al., 2011, Haberl et al., 2012, Haberl et al., 2007). These bioenergy estimates are therefore claiming that bioenergy is both low or no carbon and sustainable at levels that would double or triple total human plant harvest.
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Two, land use changes necessary to supply current biomass harvests (and its 230 EJ of energy) have contributed around one third of the world's cumulative CO2 emissions since 1750 (Le Quéré et al., 2016). Nearly all strategies for stabilizing the climate at acceptable temperatures assume a rapid phase out of deforestation, and many assume increases in forest cover by 2050 (IPCC, 2014), in effect providing no room for additional depletion of land-based carbon stocks.
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Three, virtually all analyses project increases in demand for crops, milk, and meat on the order of 60–100%, plus increases in urban areas, and in the demand for wood products (Searchinger et al., 2013, Valin et al., 2014). Because of these projected increases in food demand, even with little or no increase in bioenergy, the vast majority of models estimate expansion of agricultural land by 2050, including several by more than half a billion hectares (Bajželj et al., 2014, Schmitz et al., 2014, Tilman and Clark, 2014).
Given this land use competition, as Bajželj et al. (2014) wrote pithily, “unless food demand patterns change significantly, there seems to be little spare land for bioenergy developments without a reduction of food availability” or, we add, without adverse effects on climate from losses of terrestrial carbon. What then explains the estimates of large, low carbon, bioenergy potential from dedicated uses of land?
This paper seeks to explain these different viewpoints by exploring where and how the optimistic bioenergy estimates find the land or biomass for bioenergy and their claimed sources of greenhouse gas (GHG) benefits. We start by exploring the basic principles of biomass accounting and why biomass is not inherently carbon neutral. The benefit of bioenergy from dedicated use of land is the reduced emissions of fossil fuels, but the cost is not using that land to produce other plant outputs, including food and carbon storage. Proper accounting must examine net effects. Using basic opportunity cost calculations, we show why bioenergy even under highly favorable assumptions is unlikely to produce net climate benefits.
With this background, we explore the estimates of large bioenergy potential or GHG benefits from biophysical mapping, and economic models and show how they count the benefits of using land or biomass for bioenergy but not the costs. In effect, they double-count land or biomass as available for bioenergy even as the analyses assume they also continue to serve existing uses. Some integrated assessment models (IAMs) do not double-count in the same way, but we find that their bioenergy benefits are contingent on a variety of optimistic and uncertain assumptions. They provide not plausible bioenergy estimates, but idealized thought experiments if governments took many implausible actions to maximize global land use outputs.
Section snippets
In general
Burning biomass releases carbon, and must release at least some more carbon than burning fossil fuels because of the lower energy per gram of carbon in biomass (IPCC, 2006). Regardless, typical lifecycle and other GHG analyses of bioenergy (Argonne National Laboratory, 2016), start with the assumption that biomass itself is an inherently “carbon neutral” fuel (Searchinger et al., 2009, Searchinger et al., 2015a). These analyses will typically count the emissions from trace greenhouse gases (N2O
Direct cost-benefit calculations for bioenergy
Direct comparisons of GHG benefits in using a hectare of land for bioenergy and its alternatives show the likely net costs of using land for bioenergy when properly factoring in land opportunity costs, including the enormous advantages of solar energy.
Global estimates of bioenergy potential based on biophysical categories
Many scientific bodies have now pointed out that biomass is not automatically carbon neutral, including the Science Advisory Board of the U.S. Environmental Protection Agency (Swackhamer and Khanna, 2011), the Science Committee of the European Environmental Agency (European Environment Agency Scientific Committee, 2011), and the authors of the most recent Working Group III bioenergy appendix of the Intergovernmental Panel on Climate Change (IPCC) (Creutzig et al., 2015). Yet this last group
Favorable modeling of indirect land use change (ILUC)
In addition to showing the carbon costs per hectare of land, Table 1 also shows the land use opportunity cost per MJ of fuel of directing different kinds of land to bioenergy use. In the case of maize ethanol, for example, it shows that even preventing any theoretically surplus temperate cropland from growing forests imposes a cost equal to 82 g/MJ, far more than enough to cancel out the benefits of displacing fossil emissions. When farmers merely divert crops from existing cropland, governments
Bioenergy potentials estimated by some long-run integrated assessment energy models
IAMs and energy models can provide another group of projections of increased bioenergy at least by 2100. In one recent model comparison of 15 separate models, bioenergy ranged from negligible up to 330 EJ in a 450 ppm mitigation scenario depending on the model (Rose et al., 2014). What do these high bioenergy estimates signify about the land use trade-off implicit in bioenergy from dedicated use of land?
A first answer is that most of the 15 models involved in this inter-comparison treat land as
Conclusions and recommendations
The general lessons from this analysis are that the carbon costs of dedicating land to bioenergy will exceed the benefits. Alternative analyses in a variety of ways have been counting the benefits of using land or biomass without counting the costs. They therefore also do not plausibly contradict that many analyses that the world lacks land to dedicate to bioenergy.
Models that rely on demand- and price-induced yield gains do not make this error but they lack significant evidence and ignore much
Acknowledgements
We wish to thank the David & Lucille Packard Foundation and the World Resources Institute for financial support.
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This article is part of a Virtual Special Issue entitled 'Scaling Up Biofuels? A Critical Look at Expectations, Performance and Governance'.