Expert assessment concludes negative emissions scenarios may not deliver

Many integrated assessment models (IAMs) rely on the availability and extensive use of biomass energy with carbon capture and storage (BECCS) to deliver emissions scenarios consistent with limiting climate change to below 2 °C average temperature rise. BECCS has the potential to remove carbon dioxide (CO2) from the atmosphere, delivering 'negative emissions'. The deployment of BECCS at the scale assumed in IAM scenarios is highly uncertain: biomass energy is commonly used but not at such a scale, and CCS technologies have been demonstrated but not commercially established. Here we present the results of an expert elicitation process that explores the explicit and implicit assumptions underpinning the feasibility of BECCS in IAM scenarios. Our results show that the assumptions are considered realistic regarding technical aspects of CCS but unrealistic regarding the extent of bioenergy deployment, and development of adequate societal support and governance structures for BECCS. The results highlight concerns about the assumed magnitude of carbon dioxide removal achieved across a full BECCS supply chain, with the greatest uncertainty in bioenergy production. Unrealistically optimistic assumptions regarding the future availability of BECCS in IAM scenarios could lead to the overshoot of critical warming limits and have significant impacts on near-term mitigation options

JULES-BE:Representation of bioenergy crops and harvesting in the Joint UK Land Environment Simulator vn5.1

We describe developments to the land surface model JULES, allowing for flexible user-prescribed harvest regimes of various perennial bioenergy crops or natural vegetation types. Our aim is to integrate the most useful aspects of dedicated bioenergy models into dynamic global vegetation models, in order that assessment of bioenergy options can benefit from state-of-the-art Earth system modelling. A new plant functional type (PFT) representing Miscanthus is also presented. The Miscanthus PFT fits well with growth parameters observed at a site in Lincolnshire, UK; however, global observed yields of Miscanthus are far more variable than is captured by the model, primarily owing to the model's lack of representation of crop age and establishment time. Global expansion of bioenergy crop areas under a 2 ?C emissions scenario and balanced greenhouse gas mitigation strategy from the IMAGE integrated assessment model (RCP2.6- SSP2) achieves a mean yield of 4.3 billion tonnes of dry matter per year over 2040-2099, around 30 % higher than the biomass availability projected by IMAGE. In addition to perennial grasses, JULES-BE can also be used to represent short-rotation coppicing, residue harvesting from cropland or forestry and rotation forestry

Long-term national climate strategies bet on forests and soils to reach net-zero

The deployment of carbon dioxide removal is essential to reach global and national net-zero emissions targets, but little attention has been paid to its practical deployment by countries. Here, we analyse how carbon dioxide removal methods are integrated into 41 of the 50 Long-term Low Emission Development Strategies submitted to the United Nations Framework Convention on Climate Change (UNFCCC), before 2022. We show that enhancing forest and soil carbon sinks are the most advocated strategies but are only explicitly quantified in 12. Residual emissions by 2050 are only quantified in 20 strategies and most of them use forests to achieve national net-zero targets. Strategies that quantify both residual emissions and carbon dioxide removal identify national constraints, such as wildfire risks to forests and limited geological CO2 storage capacity. These strategies also highlight the need for international cooperation. Taken together, we suggest that the UNFCCC should urgently strengthen its reporting requirements on long-term national climate strategies

Evaluating the use of biomass energy with carbon capture and storage in low emission scenarios

Biomass Energy with Carbon Capture and Storage (BECCS) is heavily relied upon in scenarios of future emissions that are consistent with limiting global mean temperature increase to 1.5 °C or 2 °C above pre-industrial. These temperature limits are defined in the Paris Agreement in order to reduce the risks and impacts of climate change. Here, we explore the use of BECCS technologies in a reference scenario and three low emission scenarios generated by an integrated assessment model (IMAGE). Using these scenarios we investigate the feasibility of key implicit and explicit assumptions about these BECCS technologies, including biomass resource, land use, CO2 storage capacity and carbon capture and storage (CCS) deployment rate. In these scenarios, we find that half of all global CO2 storage required by 2100 occurs in USA, Western Europe, China and India, which is compatible with current estimates of regional CO2 storage capacity. CCS deployment rates in the scenarios are very challenging compared to historical rates of fossil, renewable or nuclear technologies and are entirely dependent on stringent policy action to incentivise CCS. In the scenarios, half of the biomass resource is derived from agricultural and forestry residues and half from dedicated bioenergy crops grown on abandoned agricultural land and expansion into grasslands (i.e. land for forests and food production is protected). Poor governance of the sustainability of bioenergy crop production can significantly limit the amount of CO2 removed by BECCS, through soil carbon loss from direct and indirect land use change. Only one-third of the bioenergy crops are grown in regions associated with more developed governance frameworks. Overall, the scenarios in IMAGE are ambitious but consistent with current relevant literature with respect to assumed biomass resource, land use and CO2 storage capacity

Evaluating climate geoengineering proposals in the context of the Paris Agreement temperature goals

Current mitigation efforts and existing future commitments are inadequate to accomplish the Paris Agreement temperature goals. In light of this, research and debate are intensifying on the possibilities of additionally employing proposed climate geoengineering technologies, either through atmospheric carbon dioxide removal or farther-reaching interventions altering the Earth’s radiative energy budget. Although research indicates that several techniques may eventually have the physical potential to contribute to limiting climate change, all are in early stages of development, involve substantial uncertainties and risks, and raise ethical and governance dilemmas. Based on present knowledge, climate geoengineering techniques cannot be relied on to significantly contribute to meeting the Paris Agreement temperature goals

C-LLAMA 1.0: a traceable model for food, agriculture, and land use

We present C-LLAMA 1.0 (Country-level Land Availability Model for Agriculture), a statistical–empirical model of the global food and agriculture system. C-LLAMA uses simplistic and highly traceable methods to provide an open and transparent approach to modelling the sensitivity of future agricultural land use to drivers such as diet, crop yields, and food-system efficiency. C-LLAMA uses publicly available FAOSTAT food supply, food production, and crop yield data to make linear projections of diet, food-system, and agricultural efficiencies, as well as land use at a national level, aiming to capture aspects of food systems in both developing and developed nations. In this paper we describe the structure and processes within the model, outline an anchor scenario, and perform sensitivity analyses of key components. The model land use output behaves as anticipated during sensitivity tests and under a scenario with a prescribed reduction in animal product consumption, in which land use for agriculture is reduced by 1.8 Gha in 2050 when compared with the anchor scenario

Bringing greenhouse gas removal down to earth: Stakeholder supply chain appraisals reveal complex challenges

Greenhouse gas removal (GGR) approaches are considered essential in several projections to meet the climate mitigation ambition of the Paris Agreement. Biomass Energy with Carbon Capture and Storage (BECCS) and afforestation are included extensively in mitigation scenarios but there are concerns about the feasibility of these approaches. This was explored with stakeholders from industry, non-governmental organisations (NGOs) and policy who were involved in interviews and a one-day participatory workshop. Multicriteria mapping (MCM) methodology was used to appraise the ‘real-world’ feasibility of four specific greenhouse gas removal supply chains at a granular level in the UK context. The MCM analysis shows that afforestation performs better in comparison to three BECCS supply chains, on criteria such as business model, social acceptability, and environmental sustainability. This innovative application of the MCM methodology enables the abstract representations of GGR in integrated assessment models to be explored at a more granular level through a supply chain analysis and thus gain a deeper understanding of the issues facing these approaches. The data gathered allows a wide range of technical, environmental, social and political criteria to be systematically applied in appraising the practical performance of different future implementation options for afforestation and BECCS. If these GGR supply chains are to become a reality on the scale required for 1.5 °C global warming, factors such as global cooperation, land availability, and the longevity of policies and incentives were found to be major challenges

Messing with nature? Exploring public perceptions of geoengineering in the UK

Anthropogenic influence on the climate – and possible societal responses to it – offers a unique window through which to examine the way people think about and relate to the natural world. This paper reports data from four, one-day deliberative workshops conducted with members of the UK public during early 2012. The workshops focused on geoengineering – the deliberate, large-scale manipulation of the planetary environment – as one of three possible responses to climate change (alongside mitigation and adaptation). Here, we explore one of the most pervasive and wide-ranging themes to emerge from the workshops: whether geoengineering represented an unprecedented human intervention into ‘nature’, and what the moral consequences of this might be. Using the concept of ‘messing with nature’ as an analytical lens, we explore public perceptions of geoengineering. We also reflect on why ‘messing with nature’ was such a focal point for debate and disagreement, and whether the prospect of geoengineering may reveal new dimensions to the way that people think about the natural world, and their relationship to it

Uncertain effectiveness of Miscanthus bioenergy expansion for climate change mitigation explored using land surface, agronomic and integrated assessment models

Large-scale bioenergy plays a key role in climate change mitigation scenarios, but its efficacy is uncertain. This study aims to quantify that uncertainty by contrasting the results of three different types of models under the same mitigation scenario (RCP2.6-SSP2), consistent with a 2°C temperature target. This analysis focuses on a single bioenergy feedstock, Miscanthus × giganteus, and contrasts projections for its yields and environmental effects from an integrated assessment model (IMAGE), a land surface and dynamic global vegetation model tailored to Miscanthus bioenergy (JULES) and a bioenergy crop model (MiscanFor). Under the present climate, JULES, IMAGE and MiscanFor capture the observed magnitude and variability in Miscanthus yields across Europe; yet in the tropics JULES and IMAGE predict high yields, whereas MiscanFor predicts widespread drought-related diebacks. 2040–2049 projections show there is a rapid scale up of over 200 Mha bioenergy cropping area in the tropics. Resulting biomass yield ranges from 12 (MiscanFor) to 39 (JULES) Gt dry matter over that decade. Change in soil carbon ranges from +0.7 Pg C (MiscanFor) to −2.8 Pg C (JULES), depending on preceding land cover and soil carbon.2090–99 projections show large-scale biomass energy with carbon capture and storage (BECCS) is projected in Europe. The models agree that <2°C global warming will increase yields in the higher latitudes, but drought stress in the Mediterranean region could produce low yields (MiscanFor), and significant losses of soil carbon (JULES and IMAGE). These results highlight the uncertainty in rapidly scaling-up biomass energy supply, especially in dry tropical climates and in regions where future climate change could result in drier conditions. This has important policy implications—because prominently used scenarios to limit warming to ‘well below 2°C’ (including the one explored here) depend upon its effectiveness