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Global greenhouse gas (GHG) emissions can be traced to five economic sectors: energy, industry, buildings, transport and AFOLU (agriculture, forestry and other land uses). In this topical review, we synthesise the literature to explain recent trends in global and regional emissions in each of these sectors. To contextualise our review, we present estimates of GHG emissions trends by sector from 1990 to 2018, describing the major sources of emissions growth, stability and decline across ten global regions. Overall, the literature and data emphasise that progress towards reducing GHG emissions has been limited. The prominent global pattern is a continuation of underlying drivers with few signs of emerging limits to demand, nor of a deep shift towards the delivery of low and zero carbon services across sectors. We observe a moderate decarbonisation of energy systems in Europe and North America, driven by fuel switching and the increasing penetration of renewables. By contrast, in rapidly industrialising regions, fossil-based energy systems have continuously expanded, only very recently slowing down in their growth. Strong demand for materials, floor area, energy services and travel have driven emissions growth in the industry, buildings and transport sectors, particularly in Eastern Asia, Southern Asia and South-East Asia. An expansion of agriculture into carbon-dense tropical forest areas has driven recent increases in AFOLU emissions in Latin America, South-East Asia and Africa. Identifying, understanding, and tackling the most persistent and climate-damaging trends across sectors is a fundamental concern for research and policy as humanity treads deeper into the Anthropocene.
This study compares the short-term economic feasibility of six conversion pathways for renewable jet fuel (RJF) production. The assessment combines (i) a harmonized techno-economic analysis of conversion pathways expected to be certifi ed for use in commercial aviation by 2020, (ii) a pioneer plant analysis taking into account technological immaturity, and (iii) a quantifi ed assessment of the merits of co-producing RJF alongside existing European supply chains in the pulp, wheat ethanol, and beet sugar industries. None of the pathways assessed are able to reach price parity with petroleum-derived jet fuel in the short term. The pioneer plant analysis suggests that the hydroprocessed esters and fatty acids (HEFA) pathway is currently the best option; the technology achieves the lowest minimum fuel selling price (MFSP) of 29.3 € GJ -1 (1289 € t -1 ) and the technology is deployed on commercial scale already. In the short term, n t h plant analysis shows hydrothermal liquefaction (HTL) and pyrolysis emerging as promising alternatives, yielding MFSPs of 21.4 € GJ -1 (939 € t -1 ) and 30.2 € GJ -1 (1326 € t -1 ), respectively. The pioneer plant analysis shows considerable MFSP increases for producing drop-in fuels using HTL and pyrolysis as both technologies are relatively immature. Hence, further RD&D efforts into these pathways are recommended. Co-production strategies decrease the MFSP by 4-8% compared to greenfi eld production. Integration of process units and material and energy fl ows is expected to lead to further cost reductions. As such, co-production can be a particularly useful strategy to progress emerging technologies to commercial scale.
Using biomass to provide energy services is a strategically important option for increasing the global uptake of renewable energy. Yet the practicalities of accelerating deployment are mired in controversy over the potential resource conflicts that might occur, in particular conflicts over land, water, and biodiversity conservation. This calls into question whether policies to promote bioenergy are justified. Here we examine the assumptions on which global bioenergy resource estimates are predicated. We find there is a disjunct between the evidence that global bioenergy studies can provide and policy makers' desire for estimates that can straightforwardly guide policy targets. We highlight the need for bottom-up assessments informed by empirical studies, experimentation, and cross disciplinary learning in order to better inform the policy debate. 1 Conflicting aspirations for bioenergyUsing biomass to provide energy services is one of the most versatile options for increasing the global uptake of renewable energy and an important component in many climate change mitigation and energy supply scenarios [1][2][3][4] . The International Energy Agency (IEA), for example, estimates that biomass could contribute an additional 50EJ (~10%) to global primary energy supply by 2035, and states that "the potential supply could be an order of magnitude higher" 4 . Governments of the world's largest economies have also introduced policies to incentivise bioenergy deployment, motivated by concerns about energy security and climate change, and by the desire to stimulate rural development 5,6 . Yet the potential contribution from biomass to global energy supply is controversial. Sources of contention include concern about the inter-linkages between biomass, bioenergy and other systems. Most notably, land and resource conflicts are foreseen between bioenergy and food supply, water use, and biodiversity conservation. The fear is that the benefits offered by increased biomass use will be outweighed by the costs [7][8][9][10] . It is also argued that the wide range of estimates of biomass potential and the lack of standardised assessment methodologies confuses policy makers, impedes effective action and fosters uncertainty and ambivalence 11 . These broad points contribute to a general sense of unease about the future role of bioenergy, and whether it presents a genuine opportunity or is a utopian (or for some dystopian) vision that stands little chance of being realised.Here we analyse how scenarios for increasing bioenergy deployment are contingent on anticipated demand for food, energy, and environmental protection, and expectations of technological advances. We use a systematic review methodology 12,13 to identify and analyse the most influential estimates of the global bioenergy potential that have been published over the last 20 years. The technical and sustainability assumptions that lie behind these estimates are exposed and their influence on calculations of potential described. We find that the range of estimates is primar...
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