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Abstract
Global bioenergy potentials have been the subject of extensive research and continued controversy. Due to vast uncertainties regarding future yields, diets and other influencing parameters, estimates of future agricultural biomass potentials vary widely. Most scenarios compatible with ambitious climate targets foresee a large expansion of bioenergy, mainly from energy crops that needs to be kept consistent with projections of agriculture and food production. Using the global biomass balance model BioBaM, we here present an assessment of agricultural bioenergy potentials compatible with the Food and Agriculture Organization’s (2018) ‘Alternative pathways to 2050’ projections. Mobilizing biomass at larger scales may be associated with systemic feedbacks causing greenhouse gas (GHG) emissions, e.g. crop residue removal resulting in loss of soil carbon stocks and increased emissions from fertilization. To assess these effects, we derive ‘GHG cost supply-curves’, i.e. integrated representations of biomass potentials and their systemic GHG costs. Livestock manure is most favourable in terms of GHG costs, as anaerobic digestion yields reductions of GHG emissions from manure management. Global potentials from intensive livestock systems are about 5 EJ/yr. Crop residues can provide up to 20 EJ/yr at moderate GHG costs. For energy crops, we find that the medium range of literature estimates (∼40 to 90 EJ/yr) is only compatible with FAO yield and human diet projections if energy plantations expand into grazing areas (∼4–5 million km2) and grazing land is intensified globally. Direct carbon stock changes associated with perennial energy crops are beneficial for climate mitigation, yet there are—sometimes considerable—‘opportunity GHG costs’ if one accounts the foregone opportunity of afforestation. Our results indicate that the large potentials of energy crops foreseen in many energy scenarios are not freely and unconditionally available. Disregarding systemic effects in agriculture can result in misjudgement of GHG saving potentials and flawed climate mitigation strategies.
Highlights
Substantiated knowledge of renewable energy potentials is pivotal for developing realistic energy and climate-change mitigation scenarios, and for planning energy futures on regional and global scale
Mobilizing biomass at larger scales may be associated with systemic feedbacks causing greenhouse gas (GHG) emissions, e.g. crop residue removal resulting in loss of soil carbon stocks and increased emissions from fertilization
In the BAU scenario, which is based on the narrative of the ‘SSP2 scenario’ (O’Neill et al 2017) and assumes failure in addressing challenges for food access and stability (FAO 2018b), the energy crop potentials without grazing intensification are close to 20 EJ/yr and about twice as high as in the ‘Towards sustainability’ (TSS) and the ‘Stratified societies’ (SSS) scenario
Summary
Substantiated knowledge of renewable energy potentials is pivotal for developing realistic energy and climate-change mitigation scenarios, and for planning energy futures on regional and global scale. The global sustainable potentials of biomass, currently the most important source of renewable energy (IEA 2019, REN21 2019), have been the subject of extensive research (e.g. Fischer and Schrattenholzer 2001, Berndes et al 2003, Hoogwijk et al 2003, Smeets et al 2007, Campbell et al 2008, Dornburg et al 2010, Haberl et al 2010, Haberl et al 2011, Deng et al 2015, Searle and Malins 2015, Fricko et al 2017, Strapasson et al 2017) as well as continued controversy (WBGU 2009, Smith et al 2014, Robledo-Abad et al 2017). Most long-term scenarios towards ambitious climate targets strongly rely on the large-scale implementation of energy crop potentials. High biomass supply is mainly derived from dedicated energy crops (figure 1(b)) rather than increased forest harvest
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