Greenhouse gas implications of mobilizing agricultural biomass for energy: a reassessment of global potentials in 2050 under different food-system pathways

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

Gridded soil surface nitrogen surplus on grazing and agricultural land: Impact of land use maps

Excess N application on agricultural land greatly impacts the environment in multiple ways, driven by population growth and improving quality of human diets. Therefore, it is essential to quantify the sources of the emissions of N compounds and their determinants (e.g. biological N fixation (BNF), mineral fertilizer, manure N and N deposition) to develop adequate mitigation measures. Here we aim at comprehensively mapping and quantifying N fluxes on agricultural land to analyze these sources on different scales. As underlying grazing land maps used for such calculations are fairly different in terms of methodology and definition and thus spatial extent and pattern, we investigate how this diversity in grazing land maps affects quantification of N indicators. We compared three different global grazing land maps and analyzed the propagation of differences to discrepancies in N indicators calculated from them. We discovered that (i) area differences propagated to high discrepancies in N surplus mostly in Asia, and to a minor extent also in Europe and Northern Africa. (ii) BNF constitutes an important translator for differences on grazing land to N indicators, while also being a source of further uncertainty, which warrants further scrutiny. (iii) A more inclusive definition of grazing land results in overall less N surplus given the larger areas included but allows to provide a more comprehensive estimate of the influence of human activity on the N cycle. This study is the first to provide an in-depth analysis of the effect of grazing land and agricultural land area differences on various N budget terms and N indicator calculation, highlighting opportunities for further research, and the importance of a comprehensive accounting of N surplus when using an inclusive definition of grazing land

Zusammenfassung für Entscheidungstragende

Der Österreichische Sachstandsbericht Klimawandel 2014 (AAR14) stellt einen IPCC-ähnlichen Bericht dar, bestehend aus drei Bänden, für den bestehendes Wissen zum Klimawandel in Österreich, zu dessen Auswirkungen und den Erfordernissen und Möglichkeiten der Minderung und Anpassung zusammengefasst wird. Der Bericht verfolgt das Ziel, den wissenschaftlichen Kenntnisstand für Österreich kohärent und vollständig darzulegen und dies in Form von politikrelevanten Analysen an die Österreichische Bundesregierung und politische Entscheidungsgremien auf allen Ebenen zu übermitteln, und will darüber hinaus Entscheidungsgrundlagen auch für den privaten Sektor und einen Wissensfundus für akademische Institutionen bereitstellen. Ähnlich den IPCC-Sachstandsberichten liegt dem AAR14 das Prinzip zugrunde, entscheidungsrelevant zu sein, aber keinen empfehlenden Charakter zu haben. Rund 240 österreichische Wissenschaftlerinnen und Wissenschaftler haben in einer gemeinsamen, dreijährigen Anstrengung diesen ersten Sachstandsbericht zum Klimawandel in Österreich erarbeitet und damit den aktuellen Stand des Wissens zu Ausprägungen des Klimawandels in Österreich, seinen Folgen, Minderungs- und Anpassungsmaßnahmen zusammengestellt

Food systems in a zero-deforestation world: Dietary change is more important than intensification for climate targets in 2050

Global food systems contribute to climate change, the transgression of planetary boundaries and deforestation. An improved understanding of the environmental impacts of different food system futures is crucial for forging strategies to sustainably nourish a growing world population. We here quantify the greenhouse gas (GHG) emissions of global food system scenarios within a biophysically feasible “option space” in 2050 comprising all scenarios in which biomass supply – calculated as function of agricultural area and yields – is sufficient to cover biomass demand – derived from human diets and the feed demand of livestock. We assessed the biophysical feasibility of 520 scenarios in a hypothetical no-deforestation world. For all feasible scenarios, we calculate (in) direct GHG emissions related to agriculture. We also include (possibly negative) GHG emissions from land-use change, including changes in soil organic carbon (SOC) and carbon sinks from vegetation regrowth on land spared from food production. We identify 313 of 520 scenarios as feasible. Agricultural GHG emissions (excluding land use change) of feasible scenarios range from 1.7 to 12.5 Gt CO2e yr−1. When including changes in SOC and vegetation regrowth on spare land, the range is between −10.7 and 12.5 Gt CO2e yr−1. Our results show that diets are the main determinant of GHG emissions, with highest GHG emissions found for scenarios including high meat demand, especially if focused on ruminant meat and milk, and lowest emissions for scenarios with vegan diets. Contrary to frequent claims, our results indicate that diets and the composition and quantity of livestock feed, not crop yields, are the strongest determinants of GHG emissions from food-systems when existing forests are to be protected

Focus on reactive nitrogen and the UN sustainable development goals

The scientific evidence assembled in this Focus Collection on 'Reactive nitrogen and the UN sustainable development goals' emphasizes the relevance of agriculture as a key sector for nitrogen application as well as its release to the environment and the observed impacts. Published work proves the multiple connections and their causality, and presents pathways to mitigate negative effects while maintaining the benefits, foremost the production of food to sustain humanity. Providing intersections from field to laboratory studies and to modelling approaches, across multiple scales and for all continents, the Collection displays an overview of the state of nitrogen science in the early 21st century. Extending science to allow for policy-relevant messages renders the evidence provided a valuable basis for a global assessment of reactive nitrogen

Data-driven estimates of global nitrous oxide emissions from croplands

Croplands are the single largest anthropogenic source of nitrous oxide (N2O) globally, yet their estimates remain difficult to verify when using Tier 1 and 3 methods of the Intergovernmental Panel on Climate Change (IPCC). Here, we re-evaluate global cropland-N2O emissions in 1961–2014, using N-rate-dependent emission factors (EFs) upscaled from 1206 field observations in 180 global distributed sites and high-resolution N inputs disaggregated from sub-national surveys covering 15593 administrative units. Our results confirm IPCC Tier 1 default EFs for upland crops in 1990–2014, but give a ∼15% lower EF in 1961–1989 and a ∼67% larger EF for paddy rice over the full period. Associated emissions (0.82 ± 0.34 Tg N yr–1) are probably one-quarter lower than IPCC Tier 1 global inventories but close to Tier 3 estimates. The use of survey-based gridded N-input data contributes 58% of this emission reduction, the rest being explained by the use of observation-based non-linear EFs. We conclude that upscaling N2O emissions from site-level observations to global croplands provides a new benchmark for constraining IPCC Tier 1 and 3 methods. The detailed spatial distribution of emission data is expected to inform advancement towards more realistic and effective mitigation pathways

Nitrogen budgets in Japan from 2000 to 2015: Decreasing trend of nitrogen loss to the environment and the challenge to further reduce nitrogen waste

The benefits of the artificial fixation of reactive nitrogen (Nr, nitrogen [N] compounds other than dinitrogen), in the form of N fertilizers and materials are huge, while at the same time posing substantial threats to human and ecosystem health by the release of Nr to the environment. To achieve sustainable N use, Nr loss to the environment must be reduced. An N-budget approach at the national level would allow us to fully grasp the whole picture of Nr loss to the environment through the quantification of important N flows in the country. In this study, the N budgets in Japan were estimated from 2000 to 2015 using available statistics, datasets, and literature. The net N inflow to Japanese human sectors in 2010 was 6180 Gg N yr-1 in total. With 420 Gg N yr-1 accumulating in human settlements, 5760 Gg N yr-1 was released from the human sector, of which 1960 Gg N yr-1 was lost to the environment as Nr (64% to air and 36% to waters), and the remainder assumed as dinitrogen. Nr loss decreased in both atmospheric emissions and loss to terrestrial water over time. The distinct reduction in the atmospheric emissions of nitrogen oxides from transportation, at -4.3% yr-1, was attributed to both emission controls and a decrease in energy consumption. Reductions in runoff and leaching from land as well as the discharge of treated water were found, at -1.0% yr-1 for both. The aging of Japan's population coincided with the reductions in the per capita supply and consumption of food and energy. Future challenges for Japan lie in further reducing N waste and adapting its N flows in international trade to adopt more sustainable options considering the reduced demand due to the aging population