Publications

The most recent IPCC assessment has shown an important role for negative emissions technologies (NETs) in limiting global warming to 2 °C cost-effectively. However, a bottom-up, systematic, reproducible, and transparent literature assessment of the different options to remove CO2 from the atmosphere is currently missing. In part 1 of this three-part review on NETs, we assemble a comprehensive set of the relevant literature so far published, focusing on seven technologies: bioenergy with carbon capture and storage (BECCS), afforestation and reforestation, direct air carbon capture and storage (DACCS), enhanced weathering, ocean fertilisation, biochar, and soil carbon sequestration. In this part, part 2 of the review, we present estimates of costs, potentials, and side-effects for these technologies, and qualify them with the authors' assessment. Part 3 reviews the innovation and scaling challenges that must be addressed to realise NETs deployment as a viable climate mitigation strategy. Based on a systematic review of the literature, our best estimates for sustainable global NET potentials in 2050 are 0.5–3.6 GtCO2 yr−1 for afforestation and reforestation, 0.5–5 GtCO2 yr−1 for BECCS, 0.5–2 GtCO2 yr−1 for biochar, 2–4 GtCO2 yr−1 for enhanced weathering, 0.5–5 GtCO2 yr−1 for DACCS, and up to 5 GtCO2 yr−1 for soil carbon sequestration. Costs vary widely across the technologies, as do their permanency and cumulative potentials beyond 2050. It is unlikely that a single NET will be able to sustainably meet the rates of carbon uptake described in integrated assessment pathways consistent with 1.5 °C of global warming.

Funding: The authors received no specific funding for this work. Competing interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests: Emma Archer is Editor-in-Chief of PLOS Climate.

Abstract In the UK and other temperate regions, short rotation coppice ( SRC ) and Miscanthus x giganteus ( Miscanthus ) are two of the leading ‘second‐generation’ bioenergy crops. Grown specifically as a low‐carbon (C) fossil fuel replacement, calculations of the climate mitigation provided by these bioenergy crops rely on accurate data. There are concerns that uncertainty about impacts on soil C stocks of transitions from current agricultural land use to these bioenergy crops could lead to either an under‐ or overestimate of their climate mitigation potential. Here, for locations across mainland Great Britain ( GB ), a paired‐site approach and a combination of 30‐cm‐ and 1‐m‐deep soil sampling were used to quantify impacts of bioenergy land‐use transitions on soil C stocks in 41 commercial land‐use transitions; 12 arable to SRC , 9 grasslands to SRC , 11 arable to Miscanthus and 9 grasslands to Miscanthus . Mean soil C stocks were lower under both bioenergy crops than under the grassland controls but only significant at 0–30 cm. Mean soil C stocks at 0–30 cm were 33.55 ± 7.52 Mg C ha −1 and 26.83 ± 8.08 Mg C ha −1 lower under SRC ( P = 0.004) and Miscanthus plantations ( P = 0.001), respectively. Differences between bioenergy crops and arable controls were not significant in either the 30‐cm or 1‐m soil cores and smaller than for transitions from grassland. No correlation was detected between change in soil C stock and bioenergy crop age (time since establishment) or soil texture. Change in soil C stock was, however, negatively correlated with the soil C stock in the original land use. We suggest, therefore, that selection of sites for bioenergy crop establishment with lower soil C stocks, most often under arable land use, is the most likely to result in increased soil C stocks.

Abstract Perennial bioenergy crops have significant potential to reduce greenhouse gas ( GHG ) emissions and contribute to climate change mitigation by substituting for fossil fuels; yet delivering significant GHG savings will require substantial land‐use change, globally. Over the last decade, research has delivered improved understanding of the environmental benefits and risks of this transition to perennial bioenergy crops, addressing concerns that the impacts of land conversion to perennial bioenergy crops could result in increased rather than decreased GHG emissions. For policymakers to assess the most cost‐effective and sustainable options for deployment and climate change mitigation, synthesis of these studies is needed to support evidence‐based decision making. In 2015, a workshop was convened with researchers, policymakers and industry/business representatives from the UK , EU and internationally. Outcomes from global research on bioenergy land‐use change were compared to identify areas of consensus, key uncertainties, and research priorities. Here, we discuss the strength of evidence for and against six consensus statements summarising the effects of land‐use change to perennial bioenergy crops on the cycling of carbon, nitrogen and water, in the context of the whole life‐cycle of bioenergy production. Our analysis suggests that the direct impacts of dedicated perennial bioenergy crops on soil carbon and nitrous oxide are increasingly well understood and are often consistent with significant life cycle GHG mitigation from bioenergy relative to conventional energy sources. We conclude that the GHG balance of perennial bioenergy crop cultivation will often be favourable, with maximum GHG savings achieved where crops are grown on soils with low carbon stocks and conservative nutrient application, accruing additional environmental benefits such as improved water quality. The analysis reported here demonstrates there is a mature and increasingly comprehensive evidence base on the environmental benefits and risks of bioenergy cultivation which can support the development of a sustainable bioenergy industry.

Increasing efficiency in livestock production and reducing the share of animal products in human consumption are two strategies to curb the adverse environmental impacts of the livestock sector. Here, we explore the room for sustainable livestock production by modelling the impacts and constraints of a third strategy in which livestock feed components that compete with direct human food crop production are reduced. Thus, in the outmost scenario, animals are fed only from grassland and by-products from food production. We show that this strategy could provide sufficient food (equal amounts of human-digestible energy and a similar protein/calorie ratio as in the reference scenario for 2050) and reduce environmental impacts compared with the reference scenario (in the most extreme case of zero human-edible concentrate feed: greenhouse gas emissions −18%; arable land occupation −26%, N-surplus −46%; P-surplus −40%; non-renewable energy use −36%, pesticide use intensity −22%, freshwater use −21%, soil erosion potential −12%). These results occur despite the fact that environmental efficiency of livestock production is reduced compared with the reference scenario, which is the consequence of the grassland-based feed for ruminants and the less optimal feeding rations based on by-products for non-ruminants. This apparent contradiction results from considerable reductions of animal products in human diets (protein intake per capita from livestock products reduced by 71%). We show that such a strategy focusing on feed components which do not compete with direct human food consumption offers a viable complement to strategies focusing on increased efficiency in production or reduced shares of animal products in consumption.

The alignment of the environmental, economic and social sustainability of farms is necessary for enhancing the pro-vision of public goods in farming. This study combines the use of three tools for the assessment of farm sustainability. It provides first insights into the sustainability performance of farms at different stages of agro-ecological transitions in 15 case studies covering a range of different farming systems across Europe. Each case study reflects a different transition towards agro-ecological farming. The tools applied were COMPAS (an economic farm assessment tool); Cool Farm Tool (a greenhouse gas inventory, water footprint and biodiversity assessment tool); and the SMART Farm Tool (a ultidimensional sustainability assessment tool).First results of the use of combined sustainability assessments deepen the understanding of different farming systems. Sustainability performance varies greatly between farms, but overall, agro-ecological farms tend to enhance biodiversity and water quality. For soil quality, no clear patterns could be identified. The same applies to economic performance at different stages of the agro-ecological transition. Quality of life was generally rated medium to high on all investigated farms. The combined sustainability assessment enabled the identification of areas for further policy development. Aligning the tools required harmonising definitions, simplification and assumptions with regard to the input data of the tools.

Growth in World’s population, the expansion of human activities and the demand for land has and continues to damage the natural environment. Humans depend on the natural environment for essential life-preserving processes, for example food production. Damage to the environment has serious consequences for national (State) and human security and well-being. One concern is food security and, in particular, whether a growing World population will find enough food to eat. The consequences of using more land and resources to grow more food are an important area of research. The expansion of farming will leave less land for other important uses. A proposed solution to this land use problem is sustainable intensification: growing more food on less land with either the same, or a reduced environmental impact. On a purely technical level, sustainable intensification appears to offer a solution to food insecurity and environmental damage. However, the adoption of agricultural technologies such as sustainable intensification is influenced by ‘social forces’ which need to be considered. We are developing a hierarchical model that uses two ‘social forces’ as decision drivers: economics at the landowner level and security at the State level. In our prototype model, State security restricts the economic decisions of landowners. The purpose of the model is to understand the effect sustainable intensification has on land use patterns, when their distribution and quantity depend on State food-security in a global context.