opinion & comment risk in many different contexts (including science through the working group reports David Viner* is at Mott MacDonald, cultural, geographical and political), where and yet, the forthcoming Synthesis Report Demeter House, Station Road, Cambridge flexibility through use of cost–benefit would benefit significantly from incorporation CB1 2RS, UK. Candice Howarth is at the Global analyses is a standard practice. This process of practitioner experience of climate Sustainability Institute, Anglia Ruskin University, would benefit the IPCC WGII by widening solutions implementation. Co-production Cambridge CB1 1PT, UK. the pool of research and practical solutions of knowledge, across academic, political and *e-mail: [email protected] covered, making the reviews more relevant practitioner communities, would frame, to decision-makers and by incorporation of structure and deliver climate action. Such a References 1. http://go.nature.com/H9a8Nq more transparent language and terminology process will ensure that future IPCC reports 2. Coumou, D. & Rahmstorf, S. Nature Clim. Change (such as climate change resilience) in are more up-to-date, robust and complete 2, 491–496 (2012). future assessments. in their analysis and that the climate change 3. http://go.nature.com/DRhXIx 4. Conway, D. & Mustelin, J. Nature Clim. Change 4, 339–342 (2014). The IPCC process provides the most resilience solutions proposed incorporate the 5. Pidgeon, N. & Fischhoff, B. Nature Clim. Change compelling account of evidence about climate most practically viable research. ❐ 1, 35–41 (2011). COMMENTARY: Betting on negative emissions Sabine Fuss, Josep G. Canadell, Glen P. Peters, Massimo Tavoni, Robbie M. Andrew, Philippe Ciais, Robert B. Jackson, Chris D. Jones, Florian Kraxner, Nebosja Nakicenovic, Corinne Le Quéré, Michael R. Raupach, Ayyoob Sharifi, Pete Smith and Yoshiki Yamagata Bioenergy with carbon capture and storage could be used to remove carbon dioxide from the atmosphere. However, its credibility as a climate change mitigation option is unproven and its widespread deployment in climate stabilization scenarios might become a dangerous distraction. uture warming will depend strongly scenarios aimed at keeping warming with terrestrial carbon stocks inherently 8 on the cumulative CO2 emissions below 2 °C in the IPCC’s fifth assessment vulnerable to disturbance . released through to the end of this report (AR5)6. Indeed, in these scenarios, F 1,2 century . A finite quota of cumulative CO2 IAMs often foresee absorption of CO2 via The need for negative emissions emissions, no more than 1,200 Gt CO2, BECCS up to (and in some cases exceeding) The IPCC’s Working Group 3 (WG3) is needed from 2015 onwards to stabilize 1,000 Gt CO2 over the course of the considered in AR5 over 1,000 emission climate below a global average of 2 °C above century7, effectively doubling the available pathways to 2100 (Fig. 1a). Most scenarios pre-industrial conditions by 2100 with a carbon quota. (101 of 116) leading to concentration levels likelihood of 66%. This corresponds to BECCS is the negative emissions of 430–480 ppm CO2 equivalent (CO2eq), about 30 years at current emissions levels3. technology most widely selected by IAMs to consistent with limiting warming below However, during the past decade, emissions meet the requirements of temperature limits 2 °C, require global net negative emissions from fossil fuel combustion and cement of 2 °C and below. It is based on assumed in the second half of this century, as do production have increased substantially to carbon-neutral bioenergy (that is, the same many scenarios (235 of 653) that reach −1 36.1 ± 1.8 Gt CO2 yr in 2013 (refs 4,5), amount of CO2 is sequestered at steady state between 480 and 720 ppm CO2eq in 2100 −1 projected to reach 37.0 ± 1.8 Gt CO2 yr by biomass feedstock growth as is released (Fig. 1b, scenarios below zero). About half in 2014 (ref. 3), 65% above their 1990 level. during energy generation), combined with of the scenarios feature BECCS exceeding Staying within the 2 °C limit in a cost- capture of CO2 produced by combustion 5% of primary energy supply. Many of those effective way will require strong mitigation and its subsequent storage in geological or (252 of 581) have net positive emissions action across all sectors, with greater effort ocean repositories. In other words, BECCS is in 2100 (Fig. 1b). Thus, BECCS does not needed the longer mitigation is delayed. a net transfer of CO2 from the atmosphere, ensure net negative emissions (that is, its Actions that could stabilize climate as through the biosphere, into geological use need not completely offset all positive desired include the deliberate removal layers, providing in addition a non-fossil emissions). BECCS is an important of CO2 from the atmosphere by human fuel source of energy. Other options include mitigation technology, especially as the intervention — called here ‘negative afforestation, direct air capture and increases stabilization level is lowered, and if near- emissions’. Along with afforestation, the in soil carbon storage. Afforestation term mitigation is delayed. By eventually production of sustainable bioenergy with and increased soil carbon storage differ requiring deeper emissions reductions, carbon capture and storage (BECCS) is from BECCS in that these land-use and BECCS can help reconcile higher interim explicitly being put forth as an important management changes are associated with CO2eq concentrations with low long-term mitigation option by the majority of a saturation of CO2 removal over time, stabilization targets, particularly if integrated assessment model (IAM) and in that the sequestration is reversible overshooting of concentrations is allowed. 850 NATURE CLIMATE CHANGE | VOL 4 | OCTOBER 2014 | www.nature.com/natureclimatechange © 2014 Macmillan Publishers Limited. All rights reserved opinion & comment a b >1,000 ppm CO eq RCP8.5 2 >1,000 ppm CO eq 200 100 (172 scenarios, RCP8.5) 3.2–5.4 °C 100 2 720–1,000 ppm 720–1,000 ppm 150 Relative to Area of main figure ) (148 scenarios, RCP6) 580–720 ppm –1 1850 – 1900 580–720 ppm 480–580 ppm 100 yr 80 ) 80 2 (144 scenarios, RCP4.5) –1 430–480 ppm 50 CO 480–580 ppm yr (509 scenarios, no equivalent RCP) 2 Gt 0 60 430–480 ppm CO 60 (116 scenarios, RCP2.6) Gt RCP6 –50 2014 estimate 2.0–3.7 °C 0 20 40 60 80 40 40 emissions ( 2 CO emissions ( 2 20 20 Net RCP4.5 CO Historical 1.7–3.2 °C Net emissions 0 0 RCP2.6 Net-negative global emissions 0.9–2.3 °C –20 –20 1980 2000 2020 2040 2060 2080 2100 0102030405060 70 Year BECCS in 2100 (% of total primary energy) Figure 1 | Carbon dioxide emission pathways until 2100 and the extent of net negative emissions and bioenergy with carbon capture and storage (BECCS) in 2100. a, Historical emissions from fossil fuel combustion and industry (black) are primarily from the Carbon Dioxide Information Analysis Center4,6. They are compared with the IPCC fifth assessment report (AR5) Working Group 3 emissions scenarios (pale colours) and to the four representative concentration pathways (RCPs) used to project climate change in the IPCC Working Group 1 contribution to AR5 (dark colours). b, The emission scenarios have been grouped 5 into five climate categories measured in ppm CO2 equivalent (CO2eq) in 2100 from all components and linked to the most relevant RCP. The temperature increase (right of panel a) refers to the warming in the late twenty-first century (2081–2100 average) relative to the 1850–1900 average24. Only scenarios assigned to climate categories are shown (1,089 of 1,184). Most scenarios that keep climate warming below 2 °C above pre-industrial levels use BECCS and many require net negative emissions (that is, BECCS exceeding fossil fuel emissions) in 2100. Data sources: IPCC AR5 database, Global Carbon Project and Carbon Dioxide Information Analysis Center. Taking into account the full scenario range, emissions (that is, at the project level) and Concerning the capture and storage portion global net negative emissions would need to likely also for net negative emissions (that is, of the BECCS chain, the International Energy set in around 2070 for the most challenging the global net balance). Agency’s CCS roadmap clearly illustrates scenarios and progressively later for higher- that huge efforts would be needed to achieve temperature stabilization levels. The challenges ahead the scale of CCS (both fossil fuel emissions IAMs6 and Earth system models (ESMs2) The deployment of large-scale CCS and BECCS) foreseen in current provide different but complementary bioenergy faces biophysical, technical stabilization scenarios, as publicly supported approaches for quantifying negative and social challenges11, and CCS is yet demonstration programs are still struggling emissions requirements. ESMs simulate to be implemented widely. Four major to deliver actual large-scale projects13. the compatible net CO2 emissions based on uncertainties need to be resolved: (1) the It is difficult to estimate the actual costs mass balance between atmospheric changes physical constraints on BECCS, including of BECCS, as it is partially in competition in CO2 and land and ocean carbon sinks. A sustainability of large-scale deployment for resources (land, biomass and storage model intercomparison of ten ESMs found relative to other land and biomass needs, capacity, and cost of CCS) used in other that two-thirds of the models required net such as food security and biodiversity mitigation options and for objectives beyond negative emissions in