Final report of the FP7 CSA project EuTRACE European Transdisciplinary Assessment of Climate EngineeringRemoving Greenhouse Gases from theAtmosphere and Reecting Sunlightaway from EarthThe European Transdisciplinary Assessment of Climate Engineering (EuTRACE)Editors: Stefan Schfer, Mark Lawrence, Harald Stelzer, Wanda Born, Sean Low Editors:Stefan Schfer, Mark Lawrence, Harald Stelzer, Wanda Born, Sean LowLead authors:Paola Adrizola, Gregor Betz, Olivier Boucher, Stuart Haszeldine, Jim Haywood, Peter Irvine,Jon-Egill Kristjansson, Mark Lawrence, Tim Lenton, Helene Muri, Andreas Oschlies, Alexander Proelss,Tim Rayner, Wilfried Rickels, Lena Ruthner, Stefan Schfer, Jrgen Scheran, Hauke Schmidt,Vivian Scott, Harald Stelzer, Naomi Vaughan, Matt WatsonContributing authors:Asbjrn Aaheim, Alexander Carius, Patrick Devine-Right, Anne Therese Gullberg, Katherine Houghton,Rodrigo Ibarrola, Jasmin S. A. Link, Achim Maas, Lukas Meyer, Michael Schulz, Simon Shackley, Dennis TnzlerProject advisory board:Bidisha Banerjee, Ken Caldeira, Mike Childs, Harald Ginzky, Kristina Gjerde, Timo Goeschl, Clive Hamilton, Friederike Herrmann, David Keith, Tim Kruger, Doug Parr, Katherine Redgwell, Alan Robock, David Santillo, Pablo SuarezProject coordinator:Mark Lawrence The EuTRACE project has received funding from the European Unions Seventh Framework Programme for research, technological development and demonstration under grant agreement no 306993. It brought together a consortium of 14 partner institutions that worked together to compile this assessment report. Consortium members represented various disciplines with expertise on the topic of climate engineering. The views expressed in this report are not necessarily representative of the views of the institutions at which the authors are employed. Citation: Schfer, S.; Lawrence, M.; Stelzer, H.; Born, W.; Low, S.; Aaheim, A.; Adrizola, P.; Betz, G.; Boucher, O.; Carius, A.; Devine-Right, P.; Gullberg, A. T.; Haszeldine, S.; Haywood, J.; Houghton, K.; Ibarrola, R.; Irvine, P.; Kristjansson, J.-E.; Lenton, T.; Link, J. S. A.; Maas, A.; Meyer, L.; Muri, H.; Oschlies, A.; Proel, A.; Rayner, T.;Rickels, W.; Ruthner, L.; Scheran, J.; Schmidt, H.; Schulz, M.; Scott, V.; Shackley, S.; Tnzler, D.; Watson, M.; Vaughan, N. (2015) The European Transdisciplinary Assessment of Climate Engineering (EuTRACE): Removing Greenhouse Gases from the Atmosphere and Reecting Sunlight away from Earth. Funded by the European Unions Seventh Framework Programme under Grant Agreement 306993. 2_ EuTRACE ReportEuTRACE is a joint project of EuTRACE Report_3Final report of the FP7 CSA project EuTRACE4_EuTRACE ReportContents1 1.11.21.31.422.12.1.12.1.22.1.32.1.42.1.52.1.62.1.72.1.82.1.92.1.9.12.1.9.2 2.22.2.12.2.22.2.32.2.42.2.52.2.62.2.72.2.8PrefaceExecutive SummaryIntroduction The context: climate changeEngineering the climate as a proposed response to climate changeUnderstanding climate engineering: the role of scenarios and numerical climate modellingHistorical context and overview of this report Characteristics of techniques to remove greenhouse gases or to modifyplanetary albedo Greenhouse gas removal Aorestation Biomass energy with carbon capture and storage (BECCS) Biochar Additional biomass-based processes: non-forest, burial, use in construction, and algal CO2 capture Direct air capture Enhanced weathering and increased ocean alkalinity Ocean fertilisation, including ocean iron fertilisation (OIF) Enhancing physical oceanic carbon uptake through articial upwelling Cross-cutting issues and uncertainties Lifecycle assessment of greenhouse gas removal processes CO2 storage availability and timescale Albedo modication and related techniques Stratospheric aerosol injection (SAI) Marine cloud brightening (MCB) / marine sky brightening (MSB) Desert reectivity modication Vegetation reectivity modication Cirrus cloud thinning Results from idealised modelling studies General eectiveness and constraints of modifying the planetary albedo Carbon cycle climate feedbacks between modifying the planetary albedo and removing greenhouse gases from the atmosphere1213161618222527272828313232333436373738404144464748495556EuTRACE Report_5Contents3 3.13.1.1 3.1.23.1.33.1.4 3.23.2.13.2.23.2.2.13.2.2.23.2.33.2.444.14.1.14.1.24.1.34.1.4 4.24.2.14.2.24.2.3Emerging societal issues Perception of potential eects of research and deployment Moral Hazard Environmental responsibility Public awareness and perception Participation and consultation: questions from example cases Societal issues around potential deployment Political dimensions of deployment Economic analysis Assessing costs and benets Socio-economic insights from climate engineering scenarios Distribution of benets and costs CompensationInternational regulation and governance Emerging elements of a potential climate engineering regime in the activities of international treaty bodies UNFCCC Climate engineering as a context-specic response to climate change? LC/LP Climate engineering as an activity or technical process? CBD Climate engineering judged in light of its eects on the environment? Outlook: bringing together the regulatory approaches of context, activities and eects The EU law perspective: considering a potential regulatory strategy for climate engineering including application of the approaches of context, activities and eects EU Primary Law An overarching context for climate engineering regulation and competences for its implementation within the EU EU Secondary Law Taking a regional perspective on climate engineering5858586061637073747476778082838486888990919293Final report of the FP7 CSA project EuTRACE6_EuTRACE ReportResearch options Background Arguments for and concerns with climate engineering research Arguments in favour of climate engineering research Concerns with climate engineering research Knowledge gaps and key research questionsPolicy development for climate engineering Policy context General policy considerations for climate engineering Urgency, sequencing and multiple uses of climate engineering research Urgency and timeliness of climate engineering research Sequencing: Advantages and disadvantages of a parallel research approach Multiple uses of knowledge: Connection to other research Outlook: a challenge and opportunity Policy considerations in developing principles for climate engineering governance Strategies based on principles Policy considerations for international governance of climate engineering The United Nations Framework Convention on Climate Change (UNFCCC) The Convention on Biological Diversity (CBD) The London Convention and Protocol (LC/LP) Possible future development of the emerging regime complex on climate engineeringTechnique-specic policy considerations Policy development for BECCS Policy development for OIF Policy development for SAI An EU perspective94949696979810310310510510510710710810811011211311311311411511511811912255.1 5.25.2.15.2.2 5.366.1 6.26.2.16.2.1.16.2.1.26.2.1.3 6.2.1.46.2.26.2.36.2.46.2.4.1 6.2.4.26.2.4.36.2.4.46.36.3.16.3.26.3.36.4EuTRACE Report_7ContentsExtended Summary Introduction Characteristics of techniques to remove greenhouse gases or to modify the planetary albedoGreenhouse gas removal Albedo modication and related techniques Emerging societal issuesInternational regulation and governance Research optionsPolicy development for climate engineering Development of research policy Development of international governance Development of technique-specic policy Potential development of climate engineering policy in the EU References 12512512612612812913313413513613613713814077.1 7.27.2.17.2.2 7.37.4 7.57.67.6.17.6.27.6.37.6.4 8List of Figures1.11.22.12.22.32.42.52.62.72.82.92.103.13.25.1Observed global mean surface temperature anomalies from 1850 to 2012 Global surfaceatmosphere solar and terrestrial radiation budget Contributions of various technologies and changes in end-use to two mitigation scenarios, with a signicant role for BECCS assumed in both scenarios Airsea CO2 ux and change in ux over time induced by ocean iron fertilisation in model simulations Sizes of fossil carbon supply (reserves and resources) and potential carbon stores (in Gt CO2)Surface shortwave radiative ux anomaly induced by a given annual rate ofinjection of stratospheric sulphate particles, for three dierent modelling studies Depiction of cloud brightening by aerosol particle injection Schematic of cirrus cloud thinning by seeding with ice nuclei, showing reduction in reection of shortwave solar radiation and absorption of longwave terrestrial radiation Idealised radiative forcing curves in each of the four original GeoMIP experiments (G1-G4) Dierence between the GeoMIP G1 simulation and the pre-industrial control simulation for surface air temperature and precipitation (mean of 12 GeoMIP models) Multi-model ensemble simulations (GeoMIP G2) of the impact on temperature due to albedo modication by reduction of the solar constant Enhanced carbon uptake due to a deployment of albedo modication (reducing globalaverage temperatures from an RCP8.5 scenario down to the pre-industrial level) Schematic overview of possible consequences of the deployment of SAI Schematic overview of possible consequences of the deployment of BECCSMain trends in scientic publications on climate engineering 1719293539424548515355577172958_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEList of Boxes1.11.22.12.22.33.13.23.33.43.53.63.73.84.14.26.16.2Denition of terms for responses to climate changeDetection and attribution of albedo modication consequencesPractical constraints surrounding SAI delivery mechanismsThe Geoengineering Model Intercomparison Project (GeoMIP)SAI and vegetation productivityWhat are public awareness, acceptance and engagementLOHAFEX Iron Fertilisation ExperimentBio-Energy with Carbon Capture and Storage in Greenville, OhioBio-Energy with Carbon Capture and Storage in Decatur, IllinoisStratospheric Particle Injection for Climate Engineering (SPICE)Cost typesSAI as the lesser evil?Climate engineering deployment as a question of justiceThree regulatory approaches for climate engineeringCCS under the Clean Development MechanismSummary of arguments in support of or against eld tests of albedo modicationProcedural norms212443505461636566677578798285107112EuTRACE Report_9Final report of the FP7 CSA project EuTRACEList of AcronymsADMAOGCMAR5ATPBECCSBMBFBPPCBDCCACCCSCCUCDMCFCsCLRTAPCMIPCO2COPDGDMSELDENGOENMODENSOESMETSArcher Daniels MidlandAtmosphereOcean General Circulation ModelIPCC Fifth Assessment ReportThe Ability to Pay PrincipleBioenergy with Carbon Capture and StorageGerman Federal Ministry of Education and ResearchThe Beneciary Pays PrincipleUnited Nations Convention on Biological DiversityClimate and Clean Air CoalitionCarbon Capture and Storage Carbon Capture and UtilisationClean Development MechanismChlorouorocarbonsConvention on Long-Range Transboundary Air PollutionCoupled Model Intercomparison ProjectCarbon DioxideConference of the PartiesEuropean Commission Directorate-GeneralDimethylsulphideEuropean Union Environmental Liability DirectiveEnvironmental Non-Governmental OrganisationUnited Nations Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modication TechniquesEl Nio Southern OscillationEarth System ModelEuropean Union Emissions Trading System10_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEEuTRACEFP7GHGIAGPIBDPILUCIMOIMPLICCIPCCITCZLCLPMCBMDGMGSCMMLMSBNGONSECNZECOFAFOIFPPPRCCRCPSAISBSTASDGSO2SRMGITEUTFEUUNCCDUNCLOS UNFCCCVCLTEuropean Transdisciplinary Assessment of Climate EngineeringEuropean Union Seventh Framework Research ProgrammeGreenhouse GasIntegrated Assessment of Geoengineering ProposalsIllinois Basin-Decatur ProjectIndirect Land Use ChangeInternational Maritime OrganizationImplications and Risks of Engineering Solar Radiation to Limit Climate ChangeIntergovernmental Panel on Climate Change Inter-Tropical Convergence ZoneLondon ConventionLondon ProtocolMarine Cloud BrighteningUnited Nations Millennium Development GoalsMidwest Geological Sequestration ConsortiumMobilisation and Mutual Learning Action PlanMarine Sky BrighteningNon-Governmental OrganisationNational Sequestration Education CenterEUChina Near Zero Emissions Coal projectOcean Fertilisation Assessment FrameworkOcean Iron FertilisationThe Polluter Pays PrincipleRichland Community CollegeIPCC Representative Concentration PathwaysStratospheric Aerosol InjectionSubsidiary Body for Scientic and Technological AdviceUnited Nations Sustainable Development GoalsSulphur Dioxide Solar Radiation Management Governance InitiativeTreaty on European UnionTreaty on the Functioning of the European UnionUnited Nations Convention to Combat DeserticationUnited Nations Convention on the Law of the SeaUnited Nations Framework Convention on Climate ChangeVienna Convention on the Law of TreatiesEuTRACE Report_11Final report of the FP7 CSA project EuTRACEPrefaceThe project EuTRACE (European Transdisciplinary Assessment of Climate Engineering) was funded from June 2012 through September 2014 by the EU as a Coordination and Support Action (CSA) in the 7th Framework Programme (FP7). EuTRACE brought together a consortium of 14 partner institutions that worked together to compilethisassessmentreport.Consortiummembersrepresentedvariousdisciplineswithexpertiseonthe topic of climate engineering. This assessment report is the main result of the project.The EuTRACE assessment report is provided in three parts (all available via www.eutrace.org):the full report, which provides extensive details and references for any readers who are interested in an in-depth insight into the range of main issues associated with the topic of climate engineering;anextendedsummary,aimedatabroadrangeofreaders,providinganoverviewofthemainresultsofthe report,butleavingoutmostdetails;theextendedsummaryfollowstheoverallstructureoftheassessment report but does not include literature references in order to enhance readability;an executive summary, aimed especially at policy makers and other readers interested in an overview of the main actionable results of the assessment.12_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEExecutive Summary of the EuTRACE ReportBackground and General Considerations There is a broad scientic consensus that humans are changing the composition of the atmosphere and that this, in turn, is modifying the climate and other global systems. The likely harmful impacts on societies and ecosystems, along with possibilities for mitigation and adaptation, have been documented in the assessment reportsoftheIntergovernmentalPanelonClimate Change (IPCC). Inthiscontext,variousresearchers,policymakers, andotherstakeholdershavealsobeguntoconsider climateengineering(alsoknownasgeoengineer-ing or climate intervention) as a further response toclimatechange.Mostclimateengineeringtech-niques can be grouped into two broad categories:greenhousegasremoval:proposalsforreducing the rate of global warming by removing large amounts ofCO2orothergreenhousegasesfromtheatmos-phere and sequestering them over long periods;albedomodication:proposalsforcoolingthe Earths surface by increasing the amount of solar radi-ationthatisreectedbacktospace(albedoisthe fraction of incoming light reected away from a sur-face).TheEuTRACEassessmentreportprovidesanover-viewofabroadrangeoftechniquesthathavebeen proposedforclimateengineering.Researchoncli-mateengineeringhasthusfarbeenlimited,mostly basedonclimatemodelsandsmall-scaleeldtrials. To illustrate the range of complex environmental and societalissuesthatclimateengineeringraises,the EuTRACE assessment focuses on three example tech-niques:bio-energywithcarboncaptureandstorage (BECCS),oceanironfertilisation(OIF),andstrat-ospheric aerosol injection (SAI).In general, it is not yet clear whether it would be pos-sibletodevelopandscaleupanyproposedclimate engineeringtechniquetotheextentthatitcouldbe implementedtosignicantlyreduceclimatechange. Furthermore,itisunclearwhetherthecostsand impacts on societies and the environment associated withindividualtechniqueswouldbeconsidered acceptableinexchangeforareductionof global warming and its impacts, and how such accept-ability or unacceptability could be established demo- cratically.Againstthisbackground,abroadandrobustunder-standing of the topic of climate engineering would be valuable,werenationalandinternationalpolicies, regulation,andgovernancetobedeveloped.This could be supported by coordinated, interdisciplinary research combined with stakeholder dialogue, taking into account a range of issues, including the potential opportunities, the scientic and technical challenges, andthesocietalcontextwithinwhichwide-ranging concerns are being raised in discussions about climate engineering.Opportunities and Scientic and Technical ChallengesGreenhouse gas removal techniques could possibly be usedsomedaytosignicantlyreducetheamountof anthropogenic CO2 and other greenhouse gases in the EuTRACE Report_13Executive Summary of the EuTRACE Assessment Reportatmosphere.Thiscouldpresentanimportantlong-termopportunitytolimitorpartlyreverseclimate change, given that anthropogenic CO2, once emitted, remainswithintheclimatesystemformorethana hundred years on average. However, such techniques facenumerousscienticandtechnicalchallenges, including:determiningwhetherthetechniquescouldbe scaled up from current prototypes, and what the costs of this might be;determiningtheconstraintsimposedbyvarious technique-dependentfactors,suchasavailablebio-mass;developing the very large-scale infrastructures and energy inputs, along with the accompanying nancial and legal structures, that most of the proposed tech-niqueswouldrequire;basedonexistingknowledge and experience, this could take many decades before it could have a signicant impact on global CO2 con-centrations. Foralbedomodication,initialmodelsimulations haveshownthatseveralproposedtechniquescould potentiallybeusedtocooltheclimatesignicantly and rapidly (within a year or less, and possibly at rela-tively low operational costs). This would be the only known method that could potentially be implemented to reduce the near-term impacts of unmitigated global warming.However,inadditiontothesocietalcon-cernsoutlinedinthenextsection,itisunclear whetheranyoftheproposedalbedomodication techniques would ever be technically feasible. There are numerous scientic and technical challenges that wouldrstneedtobeaddressedtodeterminethis, including:very large and costly infrastructures that land-based techniques would require; delivery mechanisms for techniques based on injec-tion of aerosol particles into the atmosphere, includ-ingdeliveryvessels(e.g.,high-yingaircraftorteth-ered balloons) and associated nozzle technologies;amuchdeeperunderstandingoftheunderlying physical processes, such as the microphysics of parti-cles and clouds, as well as how modication of these would aect the climate on a global and regional basis.Afurtherchallengethatgenerallyappliestoboth greenhousegasremovalandalbedomodicationis that their application could result in numerous tech-nique-specicharmfulimpactsonecosystemsand the environment, many of which are presently uncer-tain or unknown. Societal Context The development and implementation of any of these proposedclimateengineeringtechniqueswould occurwithinacomplexsocietalcontextwhere numerous concerns arise, including: public awareness and perception;themoralhazardargument(theconcernthat research on climate engineering would discourage the overall eorts to reduce or avoid emissions of green-house gases);thesenseofenvironmentalresponsibilityinthe Anthropocene;possible eects of various climate engineering tech-niques on human security, conict risks, and societal stability;expected economic impacts;justice considerations, including the distribution of benets and costs, procedural justice for democratic decisionmaking,andcompensationforharms imposedonsomeregionsbymeasuresthatbenet others.It can be expected that these concerns, as well as the scienticandtechnicalchallengesdiscussedabove, would take considerable time to resolve, if this is at all possible. Thus, it appears imprudent to expect either greenhousegasremovaloralbedomodicationto play a signicant role in climate policy developments inthenextdecade,orevenwithinthenextseveral decades, although it is possible that one or more of the climateengineeringtechniquesthatarecurrently beingdiscussedwillbecomeanoptionforclimate policy in the latter half of this century.14_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEDevelopment of Policies, Regulation, and Governance Developingeectiveregulationandgovernancefor the range of proposed climate engineering techniques would require researchers, policy makers, andother stakeholderstoworktogethertoaddresstheuncer-taintiesandrisksinvolved.Atpresent,noexisting internationaltreatybodyisinapositiontobroadly regulategreenhousegasremoval,albedomodica-tion, or climate engineering in its entirety. The devel-opmentofsuchadedicated,overarchingtreaty(or treaties)forthispurposewouldpresentlybeapro-hibitively large undertaking, if at all realisable. Thus far, two treaty bodies, the London Convention/London Protocol (LC/LP) and the Convention on Bio-logical Diversity (CBD), have taken up discussions and passedtherstresolutionsanddecisionsonclimate engineering. Furthermore, it has often been suggested thattheUnitedNationsFrameworkConventionon Climate Change (UNFCCC) could contribute to reg-ulatingvariousindividualtechniquesoraspectsof climate engineering. In light of this, one option that the EU could follow if itweretodecidetotrytopromoteamorecoordi-nated approach to the regulation of climate engineer-ing would be to bring together the LC/LP, CBD, and UNFCCC at the operational level. This could be done, for example, through parallel action, common assess-ment frameworks, and Memoranda of Understanding. A further option for EU member states (which are all parties to both the UNFCCC and the CBD) could be topursueanagreementonacommonpositionon various techniques or general aspects of climate engi-neering.Inparticular,suchanagreementcouldbe made consistent with the high degree of importance that EU primary law places on environmental protec-tion. Forthemoregeneraldevelopmentofclimateengi-neering governance (in addition to formal regulation), the EuTRACE assessment highlights ve overarching principles for guiding the academic research commu-nity and policy makers: minimisation of harm; the precautionary principle; the principle of transparency; the principle of international cooperation;research as a public good.Based on these principles, the EuTRACE assessment proposesseveralstrategiesthatcouldbroadlybe appliedacrossallclimateengineeringapproachesin support of developing eective governance:early public engagement, including targeted public communication platforms;independent assessment;operationalising transparency through adoption of research disclosure mechanisms;coordinatinginternationallegaleffortsthrough activitieslikethosediscussedabove,e.g.,common assessment frameworks, as well as through develop-mentandjointadoptionofacodeofconductfor research;applying frameworks of responsible innovation and anticipatory governance to natural sciences and engi-neering research.ShouldtheEUdecidetodevelopclearandexplicit policiesforresearchonclimateengineering,orits potentialfuturedeployment,thenaconscientious application of the principles and strategies discussed in the EuTRACE assessment may help ensure coher-enceandconsistencywiththebasicprinciplesupon which broader European research and environmental policy are built.EuTRACE Report_15Executive Summary of the EuTRACE Assessment Report16_EuTRACE Report1. IntroductionThere is a broad scientic consensus that humans are changing the composition of the atmosphere, and that this is leading to global climate change (IPCC, 2013a). The implications of climate change have been recog-nisedinternationally,reflectedforexampleinthe UnitedNationsFrameworkConventiononClimate Change (UNFCCC). However, the national and inter-national mitigation eorts encouraged by this recog-nitionhavenotyetbeensucienttostoptheglobal increaseingreenhousegasemissions(IPCC,2013a, pp. 486). In light of this, numerous studies have been conducted and plans developed, from the local to the internationallevel,foradaptingtoclimatechange, with the general recognition that while adaptation can reduce the vulnerability to some impacts, it can be dif-cultandoftencostly,andinsomecasesmightnot even be possible (Klein et al., 2014). 1.1 The context: climate change The threats posed by global climate change are widely acknowledgedandhaverecentlybeenextensively describedintheIPCCsFifthAssessmentReport (IPCC,2013a),thekeyresultsofwhicharebriey summarised here. One of the most important global environmentalchangescausedbyhumansisthe increaseinthecarbondioxide(CO2)contentofthe atmosphere from about 0.028% to about 0.04% over the last two centuries. This increase in CO2 concen-tration has arisen mainly from the combustion of fos-silfuelsandisresponsibleforapproximatelyhalfof the current anthropogenic global warming. The com-binedwarminginfluenceofotheranthropogenic greenhousegases,togetherwithsunlight-absorbing soot particles, is of a similar magnitude to that of CO2 (IPCC, 2013a). At the same time, other anthropogenic aerosolparticlescontainingsulphateandnitrate reectsunlight,andalsocausecloudstobemore reective, partially masking the warming trend. How-ever, the strength of this aerosol eect on the climate is uncertain, shows signicant regional variations, and does not simply reduce temperatures, but also aects other aspects of the climate such as precipitation pat-terns,sothatitcannotmerelybeseenascancelling out a fraction of the global warming. Taken together, thesechangeshaveresultedinanetincreaseinthe average surface temperature of the Earth, as depicted inFigure1.1.TheIPCCindicatesthatthebestesti-mate of the human-induced contribution to warming is similar to the observed warming (IPCC, 2013b, p. 15), which is about 0.8C over the last two centuries, and that it is extremely likely that more than half of the observed increase in global average surface tem-perature from 1951 to 2010 was caused by the anthro-pogenicincreaseingreenhousegasconcentrations andotheranthropogenicforcingstogether(IPCC, 2013b, p. 15). Final report of the FP7 CSA project EuTRACEEuTRACE Report_17Figure 1.1: (a) Observed global mean combined land and ocean surface tem-perature anomalies, from 1850 to 2012 from three data sets. Top panel: annual mean values; Bottom panel: decadal mean values including estimated uncertainty for one dataset (for both panels, the colours rep-resent different datasets: black HADCRUT4 (ver-sion 4.1.1.0); blue NASA GISS; orange NCDC MLOST (version 3.5.2); the shaded area in the bottom panel shows the uncertainty in the HADCRUT4 dataset). Anomalies are relative to the mean of the period 1961 1990. (b) Map of observedsurface temperature change from 1901 to 2012 derived from tempera-ture trends determined by linear regression from one dataset (orange line in panel a). Source: IPCC AR5 Working Group 1 Summary for Policymakers; see report for further details.Observed change in surface temperature 1901 2012 (b)Observed globally averaged combined land and ocean surface temperature anomaly 18502012(a)YearTemperature anomaly (C) relative to 196119900.60.40.20.0-0.2-0.4-0.60.60.40.20.0-0.2-0.4-0.6Annual averageDecadal average1850 1900 1950 2000(C)1. IntroductionAs part of the IPCC report, projections of the evolu-tion of global mean temperatures for a range of future scenarioshavebeenmadeusingclimatemodels. Compared to the contemporary climate (19862005 average), the projected warming for the end of the 21st century (20812100 average) ranges from 0.3 to 1.7C under a scenario of stringent mitigation, and from 2.6 to 4.8C under a fossil-fuel-intensive scenario (IPCC, 2013b). The large range of temperatures for each sce-nario is due to uncertainties in the magnitude of the climate feedbacks that both amplify and dampen the warmingresponse,aswellastouncertaintiesinthe treatmentofmanyclimate-relevantatmospheric processes,suchastheformationofcloudsandpre-cipitation and how these are inuenced by anthropo-genic aerosol particles. Discussion of these scenarios often focuses on the change in the global mean tem-perature; however, within each scenario there are also considerable regional dierences, with greater warm-ingexpectedoverlandthanoveroceans,andthe greatestwarmingoccurringintheArcticregion (IPCC, 2013b). The accumulation of CO2 and other greenhouse gases intheatmospherewillhaveaprofoundeffecton human societies and ecosystems, as will the broader changesintheclimateandEarthsystemthatwill accompanytheriseinglobaltemperatures(IPCC, 2014c). Higher temperatures are likely to increase the frequencyandintensityofheatwaves(Meehland Tebaldi, 2004), and lengthen the melting and growing seasons(Bitzetal.,2012;Tagessonetal.,2012)with far-reaching ecological consequences in cold regions (Post et al., 2009). The distribution of precipitation is expectedtochange,withdryregionsfrequently becomingdrierandwetregionsbecomingwetter (HeldandSoden,2006),althoughuncertainties remain in both this and the dierences between con-tinental and marine responses. In general, the inten-sity of precipitation is expected to increase, with rain occurring in more intense downpours between longer periods of low precipitation (Liu et al., 2009), which could lead to more oods and more intense droughts (HeldandSoden,2006).Highertemperatureswill also continue to cause rising sea levels as the warming oceanexpandsandglaciersandicesheetsmelt (Schaeeretal.,2012).ElevatedCO2concentrations will also have a direct fertilising eect on vegetation, generally increasing net primary productivity (photo-synthesisminusautotrophicrespiration)andwater-useeciency(Franksetal.,2013).However,climate changeswillalsostressplants,potentiallyreducing netprimaryproductivityinsomeregions(Lobellet al., 2011), with substantial consequences for terrestrial ecosystems and hydrology (Heyder et al., 2011). Rising CO2 concentrations are also causing ocean acidica-tion,aectingmanymarineorganisms,particularly shell-forming organisms such as coral reefs and mol-luscs(Kroekeretal.,2010).Thesechangestothe physicalenvironmentandthebiospherewillaect human societies, for example through changes to nat-ural hazards and eects on agricultural productivity and infrastructure (IPCC, 2014c).1.2 Engineering the climate as aproposed response to climate changeAgainst this background, various researchers, policy makers,andotherstakeholdershavebeguntocon-siderresponsestoclimatechangeviamethodsthat cannoteasilybesubsumedunderthecategoriesof mitigation and adaptation. The rst question that is often raised is: are there viable ways to remove large amounts of CO2 and other greenhouse gases from the atmosphere? Many ideas have been proposed for this, whichvaryconsiderablyintheirapproach,and include combining biomass use for energy generation withcarboncaptureandstorage(BiomassEnergy withCarbonCaptureandStorage,BECCS),large-scale aorestation, and fertilising the oceans in order to induce growth of phytoplankton and thus increase the uptake of CO2 from the atmosphere.Going beyond ideas for removing greenhouse gases, the question has also been raised: are there also pos-sibilities for directly cooling the Earth? Several ideas havebeenproposedthatcouldpotentiallydoso, most aiming to increase the planetary albedo, i.e., the amount of solar radiation that is reected (mostly by cloudsorattheEarthssurface)andthereforenot absorbedbytheEarth.Techniqueshavebeenpro-posed that would act at a range of altitudes, including whitening surfaces, making clouds brighter, injecting aerosolparticlesintothestratosphere,andplacing mirrors in space. Another proposed technique would involvemodifyingcirruscloudstoincreasethe amount of terrestrial radiation leaving the Earth. In thisreport,alloftheseapproachesaresubsumed 18_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEunderthetermalbedomodificationandrelated techniques.Taken together, ideas for greenhouse gas removal and foralbedomodicationareoftenreferredtobythe umbrellaterm,climateengineering.Bothofthese conceptswouldactontheglobalsurfaceatmos-phere radiation budget, but in very distinct ways, as depictedinFigure1.2.Greenhousegasremoval woulddecreasetheamountofoutgoingradiation thatistrappedbygreenhousegasesintheatmos-phere, thus decreasing the downward ux of infrared radiationattheEarthssurface.Planetaryalbedo modication, on the other hand, would increase the Earths natural reection of solar radiation at various possible altitudes, as noted above.Figure 1.2: Global surfaceatmos-phere solar and terrestri-al radiation budget; solar radiation (largely visible) components are shown on the left, terrestrial ra-diation (largely infrared) components are on the right, and sensible andlatent surfaceatmos-phere energy transfer are in the middle. Red-circled labels indicate the main foci of proposed climate engineering: removal of greenhouse gases, and increasing the planetary albedo, either at the surface, or via clouds or aerosol particles (space mirrors are not discussed in detail in this report and thus are not shown). Source: Adapted from Kiehl and Trenberth (1997).EuTRACE Report_191. Introduction107 Reected solarradiation 107 Wm-2Reected by clouds, aerosol and atmospheric gases77Reected bysurface 30168 Absorbed bysurface342 Incomingsolarradiation 342 Wm-2Absorbed byatmosphere6724Thermals78Evapo-transpiration390Surfaceradiation324Absorbed bysurface324Back radiation24Emitted by atmosphereOutgoinglongwaveradiation 235 Wm-2350Latentheat78Emitted by clouds16540Atmospheric windowGreenhouse gases2353040Althoughthisassessmentfocusesontherangeof ideas being discussed under climate engineering, it is importanttokeepinmindthattheyaregenerally being considered within the broader context of miti-gationandadaptationastheprimaryresponsesto climatechange.Mitigationandadaptationaredis-cussedextensivelyintheassessmentreportsofthe Intergovernmental Panel on Climate Change (IPCC). MitigationisdenedbytheIPCCastechnological changeandsubstitutionthatreduceresourceinputs and emissions per unit of output, further specifying thatalthoughseveralsocial,economicandtechno-logical policies would produce an emission reduction, withrespecttoclimatechange,mitigationmeans implementing policies to reduce greenhouse gas emis-sions and enhance sinks (IPCC, 2007a). This deni-tion implies that methods aiming at reducing natural sources or enhancing natural sinks of CO2 and other greenhouse gases can be considered to qualify as mit-igationpolicies,andisconsistentwiththeusageof thisterminologybytheUNFCCC.Therefore,tech-niques such as reforestation, aorestation, improved soil carbon sequestration, and enhanced weathering can, in principle, be classied as both mitigation and asclimateengineeringviagreenhousegasremoval, dependingonthedenitionofclimateengineering thatisbeingemployedand,whereappropriate,the scale of the intervention. Carbon capture and storage (CCS) usually refers to proposed mitigation technolo-gies that would reduce CO2 emissions directly at vari-oussources,e.g.,capturingCO2fromuegasesof powerplants,soisgenerallyclassiedasmitigation (IPCC, 2005). However, in the sense that CCS would cause substantial modication of geological reservoirs if implemented at a scale that had a signicant impact ontheglobalatmosphericCO2burden,itissome-times also classied as geoengineering (although usu-ally not as climate engineering). CCS combined with bio-energy generation (BECCS) would remove CO2 at theemissionsource,butcanalsobeconsideredan enhancement of a natural sink (through vegetation). It accordingly sits at the boundary between mitigation and greenhouse gas removal. Removing CO2 directly fromtheatmosphereiscommonlyreferredtoas direct air capture or free air capture, which is nor-mally considered to be a type of climate engineering by greenhouse gas removal, distinct from mitigation eorts. The fth IPCC assessment report denes adaptation astheprocessofadjustmenttoactualorexpected climate and its eects. In human systems, adaptation seeks to moderate or avoid harm or exploit benecial opportunities. In some natural systems, human inter-vention may facilitate adjustment to expected climate and its eects (IPCC, 2014c). In its fourth assessment report,theIPCC(2007b)specifiedthatvarious typesofadaptationexist,anddenedvariousaxes such as anticipatory and reactive, private and pub-lic,andautonomousandplanned.Keyexamples includeraisingandreinforcingdykesonriversor coasts, and the substitution of plants sensitive to tem-perature shocks with more resilient species. Central to the concept of adaptation is the idea of reducing the vulnerability of natural and human systems to climate change through modication of these systems. Here, approaches such as whitening the facades and roofs of buildingsaregenerallyconsideredtobeformsof adaption(tomoderatetheurbanheatislandeect), butifconductedonasufficientlylargescalethey couldalsobeclassifiedasclimateengineeringby modifyingtheplanetaryalbedo(Olesonetal.,2010; Akbari et al., 2009). Whether an intervention into the Earth system quali-es as climate engineering is often considered to be a matterofintentandscale.Whilstsometechniques canbeconsideredeithermitigationorclimateengi-neering (or both), usually depending on their scale, it has been argued that the classication is not a purely technicalmatter,ratherthattheumbrellatermcli-mateengineeringsigniesthatproposalsforlarge-scaledeliberateinterventionsintotheEarthsystem deservespecialscrutinyandattention(Jamieson, 2013).Inthiscontext,ageneraldenitionofclimate engineering is proposed here, along with other terms usedinthisreport,inBox1.1.Intheliterature,the termsgeoengineeringandclimateengineeringare oftenusedinterchangeablywithonlysubtledier-ences(asnotedintheexampleabove);thetermcli-mate engineering is adopted here, as it is more specic and the intent is more immediately apparent (Caldeira andWood,2008,FeichterandLeisner,2009,GAO, 2011; Vaughan and Lenton, 2011; Rickels et al., 2011).20_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEThe broad denition of climate engineering has been widelyadoptedandthereisgeneralagreementon manyofthetechniquesthatitshouldencompass. However, it is important to realise that the application of the blanket term can sometimes be misleading, and thattherearelimitstotheapplicabilityofgeneral statements on climate engineering, since the eects, side eects,associated risks, ethical dimensions, and theeconomic,social,andpoliticalcontextsdiffer greatlyforeachofthevariousclimateengineering techniques (Heyward, 2013; Boucher et al., 2014). As a result, many arguments only apply to a sub-set of the techniquesortosingletechniques,andtheresearch community faces the challenge of carefully dierenti-ating between the various climate engineering tech-niques and their implications in their analyses, as well as elucidating commonalities that justify the judicious use of the blanket term. The individual techniques are distinguished carefully in the report, and are general-isedtoeitherclassesofclimateengineering(i.e., greenhousegasremovaloralbedomodicationand relatedtechniques)ortoclimateengineeringasa whole only where appropriate. In some contexts the term climate engineering is applied, but only refers to oneparticulartypeortechnique,inwhichcasean appropriatemodierisapplied(e.g.,climateengi-neering by greenhouse gas removal, or climate engi-neering by stratospheric aerosol injection).EuTRACE Report_211. IntroductionBox 1.1 EuTRACE term DenitionInitiatives and measures to reduce or prevent anthropogenic emissions of climate-forcing agents into the atmosphere. The process of adjustment to actual or expected climate; seeks to moder-ate or avoid harm or to exploit bene-cial opportunities.A collective term for a wide range of proposed techniques that could potentially be used to deliberately counteract climate change by either directly modifying the climate itself or by making targeted changes to the composition of the atmosphere, without seeking to reduce anthropo-genic emissions of greenhouse gases or other warming agents.Removal of atmospheric CO2 and other long-lived greenhouse gases.Deliberate modication of incoming solar or outgoing terrestrial radiation on a regional to global scale.MitigationAdaptationClimate Engineering (or Geoengineering) Greenhouse Gas RemovalAlbedo ModicationDenition of terms for responses to climate change In order to help elucidate several of the physical and societal considerations associated with climate engi-neeringmoreclearlyandconcretely,threeselected techniquesarediscussedingreaterdetail.Twoof thesearetechniquesforgreenhousegasremovalbioenergy with carbon capture and storage (BECCS) and ocean iron fertilisation (OIF) and the other is a techniqueformodifyingtheEarthsalbedostrat-osphericaerosolinjection(SAI).Thesetechniques were chosen for several reasons. They are among the most discussed techniques in the literature and in the broader socio-political context, including some of the mostadvancedgovernancediscussionsandespe-cially for OIF the most advanced actual governance developments,aswellasthemostextensivefield experimentation.Theyincludeoneland-based,one ocean-based,andoneatmosphere-basedtechnique. They encompass techniques that could potentially be connedtosmallareas(BECCS),andthusarenot alwaysconsideredtobeaclimateengineeringtech-nique,andothersthataretransboundaryinnature (OIFandSAI).Theyarecurrentlyatverydierent stagesofresearch,aswellastechnologicalandgov-ernancedevelopment,andtheirpresumedlevelsof eectiveness and potential risks also dier widely.BECCS is a technique that ts the denitions given above for both mitigation and climate engineering. It wasalsoincludedinthefutureclimatechangesce-narios of the IPCC Fifth Assessment Report (Moss et al.,2008).Ofparticularimportance,theonlyIPCC scenario with more than 50% probability of meeting the internationally agreed target of limiting mean glo-baltemperaturerisetolessthan2Cincludeswide-spreaduseofBECCSinthesecondhalfofthe21st century.OIFisagreenhousegasremovaltechniquethathas received attention since natural variations in oceanic iron supply were rst postulated to have played a role inglacialinterglacialchangesinatmosphericCO2 (Martinetal.,1990).Morethanadozeneldtests sincethe1990shaveconsistentlyshownthat,under specic circumstances, a small input of iron can have a large eect on iron-limited ocean ecosystems, pro-ducing large plankton blooms that might carry carbon todepth,althoughalargeandlong-termironinput would also perturb these ecosystems in ways that are diculttoforesee.Researchoverthepasttwodec-adeshasgenerallyshownthatOIFmayhaveonlya limitedeectonatmosphericCO2concentrations (Boydetal.,2007;Buesseleretal.,2008).However, OIF is still being considered and pursued by some as apossiblemeanstoremoveexcessCO2fromthe atmosphere,andisaninterestingcasestudy.Thisis especiallyrelevantfromtheperspectiveofgovern-ance, since examination of past developments on OIF mayyieldinsightsintomoregeneralgovernance aspectsofclimateengineering(inbothitsmain forms),sinceOIFhasreceivedthemostregulatory attention, especially through the London Convention and London Protocol (LC/LP), as described in Section 4.1.2.SAI is the albedo modication technique that is cur-rentlyreceivingthemostattention.Thegoalofthis technique is to create an eect roughly analogous to that of a large volcanic eruption, i.e., a cooling of the planet through the reection of sunlight by aerosols in thestratosphere(Crutzen,2006,Budyko,1974), although with dierent timing and geographical dis-tribution. If delivery and dispersal of particles were to prove technically feasible and politically implementa-ble, SAI could induce a rapid cooling eect on the cli-mate. It is thus often referred to as a high-leverage technique (Keith et al., 2010), which could have a large eect over a short period of time, potentially at a rela-tively low cost (Robock et al., 2010; McClellan et al., 2012). However, SAI and all other albedo modication techniquescouldnotreversetheeectsofelevated GHGconcentrations,butwouldinsteadchangethe climateinwaysthatmightreducesomeclimate impacts,notaectothers,andpotentiallyintroduce newrisks(Raschetal.,2008;Robocketal.,2008; Tilmes et al., 2009).1.3 Understanding climateengineering: the role of scenariosand numerical climate modelling Our understanding of most of the physical eects of climate engineering primarily comes from theoretical andmodellingstudies.Formosttechniques,dedi-cated eld tests have not been carried out. In addition, manydetailsoftheeectsoffull-scaledeployment cannotbescaleduporanticipatedfromsmall-scale eld tests. This section describes some of the model-ling tools used to understand the potential eects of 22_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEgreenhousegasremovalandalbedomodification techniques on the Earth system. CoupledAtmosphereOceanGeneralCirculation Models (AOGCMs), often simply called climate mod-els, have been the standard tool for studying climate variability and climate change since the 1990s. In the lastdecade,EarthSystemModels(ESMs)havealso become common; these add the treatment of the car-boncycleandotherlarge-scaleprocessestothe AOGCMs. These global models are used to make pro-jections of how the climate system will evolve in the coming century and beyond. The projections of these modelsaresupportedbyarangeofothermodelling tools,fromprocessmodelssuchascloud-resolving modelsthathelptoimprovetheunderstandingof cloud feedbacks, to impact models such as crop mod-els that evaluate the eects of climate change on crop yield. The communities working on modelling climate changearewelldevelopedandcoordinated,and throughprojectssuchastheCoupledModelInter-comparisonProject(CMIP),wheremanyAOGCMs arecomparedsystematically,theyworktogetherto betterunderstandthemodelprojectionsandtheir uncertainties,formingthebasisfortheassessments that are carried out in the WG1 contributions to the IPCC reports.Theevaluationofthepotentialclimateeffectsof greenhousegasremovalandalbedomodification techniquesisnotasmatureastheevaluationof anthropogenic climate change, but draws on the same toolsandknowledgebase.Greenhousegasremoval and albedo modication are fundamentally dierent in terms of their eects on the Earth system. Remov-inggreenhousegasesfromtheatmospherewould reducetheirconcentration,oratleasttheirrateof increase,indirectlyreducingtheamountofglobal warming,whereasmodifyingtheplanetaryalbedo would alter the climate directly. There has been little work on the detailed climate and Earth system conse-quencesofgreenhousegasremovalingeneraland only a few studies focused on specic techniques, e.g., afforestation(Ornsteinetal.,2009;Swannetal., 2010).Thisisinpartbecausetheeectsofgreen-house gas removal do not dier much from the eects of mitigation, as both approaches would alter the con-centrationsofgreenhousegasesintheatmosphere. However, greenhouse gas removal enables scenarios thatincludenegativenetglobalemissionsofCO2. ThiswouldallowconcentrationsofCO2todecline much faster than by means of natural processes. Some studies have investigated the climate consequences of such peak-and-decline scenarios (Boucher et al., 2012). Since implementing an albedo modication technique would constitute a direct modication of the climate with the intention of reducing the impacts of climate change, evaluating of the consequences for the climate and the Earth system is critical to understanding the potential utility and risks. This understanding and the relateddecision-makingprocesswilleventuallyrely on eective detection and attribution of the impacts of any albedo modication technique, which presents challengessuchasthosediscussedinBox1.2.The observational component of detection and attribution willalsodependonacomplementarycontribution from model analyses.Thus far, most modelling studies have not yet focused on the specic issue of detection and attribution, but rather on the range of consequences of various albedo modication techniques for the climate and Earth sys-tem. These comprise both idealised model simulations that improve our understanding of the basic response to albedo modication (Lunt et al., 2008; Irvine et al., 2011;Kravitzetal.,2013a)aswellasmorerealistic deploymentscenariostounderstandpotential impacts in context. This can be achieved, for example, by using the scenarios employed in the IPCC assess-ments as a baseline and then applying albedo modi-cationtoachieveaspecictemperatureorradiative forcingtarget(Kravitzetal.,2011;Niemeieretal., 2013).TheGeoengineeringModelIntercomparison Project (GeoMIP) (Kravitz et al., 2013a; Kravitz et al., 2011), and prior to that the EU FP7 Project IMPLICC (ImplicationsandRisksof Engineering Solar Radia-tion to Limit Climate Change (Schmidt et al., 2012b), attempted tosystematise this investigation, where a numberofalbedomodificationexperimentswere conducted in the same way by many modelling groups in order to develop a better understanding of the pro-jections and their uncertainties. These modelling eorts have also been supported by detailedprocessstudiesinvestigatingsmaller-scale processes, for example with detailed cloud-resolving modelsandaerosolmodels(Cirisanetal.,2013; EuTRACE Report_231. IntroductionJenkins et al., 2013). The understanding of the poten-tialclimateconsequencesofSAIandanumberof other albedo modication techniques is currently lim-ited by various uncertainties, such as how the small-scaleaerosolmicrophysicalprocesses,uponwhich SAIdepends,scaleuptotheglobalscale,especially since many global models involve relatively simplistic treatmentsoftheseprocesses.Additionally,todate, there has been no detailed and systematic evaluation oftherangeofimpactsofvariousformsofalbedo modication on other components of the Earth sys-tembesidesclimate.Thismakestheevaluationof thesetechniquesincomplete,althoughtherearea numberofnotablestudiesontheimpactsofalbedo modication on crop yields and sea level rise (Moore etal.,2010;Irvineetal.,2012;Pongratzetal.,2012). The results of these modelling eorts are assessed in Chapter 2.Theconsequencesofgreenhousegasremovaland albedomodicationtechniqueswilldependonthe manner and the context in which they might eventu-allybedeployed.Todeterminepossibleevolution pathwaysofpopulation,energydemand,andthe otheraspectsofthesocialandeconomicspheresas they relate to climate, future scenarios are often used, suchastheRepresentativeConcentrationPathways (RCPs) used in the IPCCs Fifth Assessment Report (AR5)(Meinshausenetal.,2011;vanVuurenetal., 2011a). The RCP scenarios were developed via broad, interdisciplinary collaboration and represent coherent scenariosforpolicyandtechnologydevelopment, constrainedbyanunderstandingofavailable resourcesthatoutlinepossiblefutures.Forthesce-nario with the lowest projected temperature increase by 2100, RCP2.6, large-scale aorestation and BECCS isassumedforthesecondhalfofthe21stcentury. These are a necessary part of the scenarios to achieve negative net global emissions of CO2 , making it pos-sible to reduce the atmospheric concentration of CO2 much more quickly than through natural processes.Box 1.2 Detection and attribution of albedo modication consequences One of the greatest challenges for climate science has been to robustly detect and attribute the consequences of human actions on the climate system(Barnett et al., 1999; Stone et al., 2009; Bindof et al., 2013). The role of an-thropogenic inuences on the observed changes in surface air temperature at the global and continental scales can now be clearly attributed (Bindof et al., 2013). However, explicitly detecting and then attributing changes at smaller spatial scales and for other climate variables has proven challenging, due to uncertainties in climate models as well as uncertainties in the magnitude of an-thropogenic inuences (e.g., emissions of various greenhouse gases and aero-sol particles), and most importantly due to the large internal variability of the climate system (Stott et al., 2010; Bindof et al., 2013).These same difculties would be faced when attempting to detect and attribute the consequences of an albedo modication intervention. This means that it could take years or even decades to detect and attribute the efect of albedo modication on global mean temperatures, and longer still for changes at small-er spatial scales and for more variable climate parameters such as precipitation patterns and extreme weather events (MacMynowski et al., 2011; Bindof et al., 2013). The difculty of attribution poses many challenges for governance, espe-cially in the context of compensation and liability (Svoboda and Irvine, 2014).24_EuTRACE ReportFinal report of the FP7 CSA project EuTRACE1.4 Historical context and overview of this report ThisreportpresentstheresultsofEuTRACE(the EuropeanTransdisciplinaryAssessmentofClimate Engineering),aprojectfundedbytheEuropean Unions 7th Framework Programme, assessing from a European perspective the current state of knowledge aboutthetechniquessubsumedundertheumbrella term climate engineering. It brings together scientists from14partnerinstitutionsacrossEurope,with expertiseindisciplinesrangingfromEarthsciences to economics, political science, law, and philosophy.Thisassessmentfollowsseveralotherassessments, startingwiththe2009assessmentreportbythe Royal Society (Shepherd et al., 2009). While some of thetechniquespresentlybeingdiscussedhave received some limited attention over several decades, thecurrentwaveofinterestwassparkedbyafew developments, including a series of open ocean exper-iments to examine the potential of ocean iron fertili-sation for reducing atmospheric CO2 , along with the 2006publicationofaspecialsectionofthejournal ClimaticChange,inwhichNobellaureatePaulCrut-zen contributed the lead essay (Crutzen, 2006). In the essay,Crutzenaskedwhetherintroducingreective particles into the stratosphere to cool the planet could contribute to resolving the policy dilemma that states facewhenreducingcertaintypesofpollution,espe-cially sulphate aerosol particles, which mask warming. Whiletheoceanironfertilisationexperimentsand the essays in Climatic Change clearly focused on par-ticular techniques, the discussion quickly broadened tocoverotherpossiblemeanstoachievedeliberate large-scalemanipulationoftheplanetaryenviron-menttocounteractanthropogenicclimatechange (Shepherd et al., 2009). This assessment report moves the discussion forward in several respects. For one, in a eld with such a rap-idly-evolvingliteraturebaseandglobaldiscussions, regular assessments are important for tying in the dif-ferent strands of literature and debate, and providing accessiblecompilationsofthestateoftheart.A number of recent activities have moved the eld for-ward,includingprogressintheGeoengineering ModelIntercomparisonProject(GeoMIP),building partly on the EU FP7 project IMPLICC; the advances madebytheLC/LPintheregulationofmarinecli-mate engineering activities; planning of the rst eld campaigns for atmospheric albedo modication tech-niques;publicationoftheIPCCsFifthAssessment Report;andahostofworkshops,mostlyinEurope andNorthAmerica,butalsoafewinotherpartsof the world, e.g., those organised by the Solar Radiation ManagementGovernanceInitiative(SRMGI). Through the large and interdisciplinary composition oftheEuTRACEprojectconsortium,thisreportis abletocaptureabroadrangeofperspectivesacross disciplinesandreflectonthefieldsdevelopment throughallofthem.Thereportisalsotherstto reect on the eld from a particularly European per-spective, especially in its analysis of existing govern-anceandpossiblepolicyoptions.Basedonastrong focusonethicalconsiderations,thereportanalyses researchneedsandpolicyoptionsatanimportant pointinthedevelopmentofindividualclimateengi-neering techniques and their governance.Withinthisbroadercontext,theEuTRACEassess-mentisintendedtoprovidevaluablesupporttothe EuropeanCommissionandthebroaderpolicyand researchcommunityintheassessmentofclimate engineering,includingthedevelopmentofgovern-anceforresearchandthepotentialdeploymentof varioustechniques.Thisrstchapteroftheassess-mentreporthasprovidedanoverviewofclimate engineering,particularlyplacingitinthecontextof climatechange.Chapter2describestheindividual techniquesthathavebeenproposedforgreenhouse gas removal and albedo modication. The state of sci-entic understanding and technology development is outlined, including a brief discussion of what is known aboutthepotentialoperationalcostsofindividual techniques,withconsiderationoftheuncertainties around all of these factors. Beyond the challenges of understanding and control-lingtheimpactsontheEarthsystem,thedierent techniques present great challenges in the social, ethi-cal,legal,andpoliticaldomains.Chapter3considers several of these issues that have informed this debate, such as: the possible inuence of climate engineering techniques on mitigation and adaptation eorts; how thesetechniquesareperceivedbythepublic;their conictpotential,economicaspects,distributional EuTRACE Report_251. Introductioneects,andcompensationissues;aswellasimplica-tionsforgovernance.Chapter4thenconsidersthe current regulatory and governance landscape with a particular focus on EU law, while taking into account and discussing the wider developments at the interna-tional level. Chapter 5 outlines major knowledge gaps andprovidesoptionsforfutureresearch,asaguide forhowtheEuropeanCommissionmightapproach funding decisions for future research on climate engi-neering.Finally,Chapter6illustrateshowpolicy options can be developed and justied, based on the principlesthatunderlieEUlawinitsapplicationto climateengineering(asidentiedinChapter4),the extensivebasicknowledgeofthescienceandtech-nologies that are fundamental to the various climate engineeringapproaches(asdescribedinChapter2), andthemultipleconcernsthatclimateengineering raises (as discussed in Chapter 3). Conscientious appli-cationofsuchanapproach,basedontheprinciples embodied in existing legal and regulatory structures, the scientic state of the art, and the concerns raised in connection with climate engineering, may help lead to the development of European policies, on research andthepotentialfutureimplementationofclimate engineering techniques, that are coherent and consist-entwiththebasicprinciplesuponwhichbroader Europeanresearchandenvironmentalpolicyare built. 26_EuTRACE ReportFinal report of the FP7 CSA project EuTRACE2. Characteristics of techniques to remove greenhouse gases or to modify planetary albedo Thischapterprovidesanoverviewofthecurrently mostdiscussedoptionstoremoveCO2fromthe atmosphereortoincreasetheplanetaryalbedoto reectmoresunlightbackintospace.Thechapter assessesthetechnicalfeasibility,potentialenviron-mental consequences, current knowledge of the oper-ational costs, and the various uncertainties associated with the main proposals for greenhouse gas removal (Section2.2)andalbedomodication(Section2.3). With regard to costs, it is worth noting that only the operational costs (installation and maintenance) will be discussed here; this is only one of three basic types ofcosts,alongwithpriceeectsandsocialcosts, which are discussed in more detail in Section 3.2.2.1. For each greenhouse gas removal and albedo modi-cationtechniquediscussed,thecurrentstateof knowledge of the eectiveness and impacts specic to thattechniqueissummarised,whilesomeimpacts thatarecommontoallofthesetechniquesaredis-cussed in sections 2.1.9 (for greenhouse gas removal) and 2.2.7 (for albedo modication). 2.1 Greenhouse gas removal Proposedmethodsfortheremovalandlong-term sequestrationofCO2andothergreenhousegases from the atmosphere range from those with primarily domesticinuencethathaveminorconsequences outsideagivendomain(exceptforthesmallglobal reduction in the atmospheric greenhouse gas concen-trations), to those with transboundary inuences on theenvironmentandonglobaleconomics,andthus on global societies. Inthissection,thefullrangeoftechniquesthatare primarily being discussed for greenhouse gas removal are considered, both terrestrial and marine, as well as biotic and chemical. Among the primarily terrestrial biotictechniquesareaorestation,BECCS,biochar, andadditionalbiomasstechniques;themainterres-trialchemicaltechniqueisdirectaircapture,while enhancedweatheringisbothterrestrialandmarine; nally, two techniques are considered that would aim toincreasetherateofcarbontransfertothedeep ocean, with ocean fertilization involving the biologi-calpump,andartificialupwellinginvolvingthe physicalpump.Theremainingchaptersfocus mainlyonBECCSandOIFastwoexampletech-niquesthathaveverydierentimplications.Inthe caseofBECCS,scaleplaysanimportantrole.For example,small-scaleapplicationofBECCSutilising waste biomass that is obtained from the same national jurisdictionlikelyhasfewtransboundaryconse-quences, whereas larger-scale applications using bio-massgrownspecicallyforthepurposeandpur-chased on the global market will have transboundary consequencesfortheglobaleconomy,evenifthe actual application remains domestically conned. In contrast,oceanfertilisationpurposelymodiessys-temsintheglobalcommons,andthusqualiesas EuTRACE Report_272. Characteristics of techniques to remove greenhouse gases or to modify planetary albedotransboundary regardless of the scale of application. Overall,theprocessesinvolvedinmanytechniques are by nature transboundary, or become transbound-ary if they involve global markets, and it is very likely that all methods of greenhouse gas removal will have at least some transboundary impacts (beyond the cli-mate and other impacts of reduced concentrations of greenhousegasessuchasCO2)ifappliedatasu-ciently large scale to have noticeable eects on global atmospheric greenhouse gas concentrations. For CO2, this would require a removal rate comparable to that ofcurrentglobalemissions,whichnowexceeds 30 Gt CO2/yr (IPCC, 2013a).2.1.1 AforestationDescription:RemovingCO2byaorestationwould enhance the terrestrial carbon sink by increasing for-est cover and/or density in unforested or deforested areas. Eectiveness:Estimatesofboththeannualandthe overall carbon uptake potential of aorestation vary widely.Anestimatebasedonthephysicalpotential mightconsiderregrowthofalldeforestedregions (around660290 Gt C)(IPCC,2014c).However, suchdeploymentisincompatiblewithcurrentand projectedland-usedemand(PowellandLenton, 2012), which reduces the estimates for global deploy-mentofafforestationtoaround1.53 Gt CO2/yr (Shepherdetal.,2009)includingareductioninthe rates of deforestation. Consideration of the payback period from any biomass or soil carbon loss during planting is also required (Jandl et al., 2007). Cost esti-mates in the literature vary widely as a consequence of diering and largely incomparable assessment cri-teriaandmethods.Aorestationis,technically,the simplestmethodofgreenhousegasremovalto undertake, if carried out in regions with favourable conditions;however,theresultingcarbonstorage would be temporary, lasting only as long as the aor-ested regions are continually protected or managed, and would therefore be vulnerable to changes in the environment(e.g.,fire,disease),climate(e.g., drought),andsociety(e.g.,demandsforwoodor land). Aorestation potential is principally limited by theavailabilityoflandthatisfertile,irrigated,and socially acceptable for aorestation. Impacts: The impacts of aorestation include land-use competition, environmental impacts (e.g., water con-sumption),possibleecosystemdegradationwhere commercialforestryapproachesareapplied(e.g., non-native species, pest-control), societal impacts of landscape and usage change (e.g., access to fuel), and climatic impacts such as reduced albedo (depending ongeographiclocation),whichcouldevenleadtoa netwarmingdespitetheadditionalsequesteringof CO2, along with evaporative eects on the hydrologi-cal cycle including cloud formation (Bonan, 2008).2.1.2 Biomass energy with carboncapture and storage (BECCS)Description:BECCSisaproposedgreenhousegas removal technique that would combine carbon cap-tureandstorage(CCS)technologywithbiomass burning.Thebiomass,producedusingenergyfrom sunlightforphotosynthesis,woulddrawitscarbon sourcefromCO2intheair.Anycarbonthatisthen capturedinthehigh-CO2-concentrationstreamof thebiomassburningexhaustgas,andsubsequently sequestered, would thus eectively remove CO2 from the atmosphere. BECCSalreadyguresprominentlyintheworkon future emissions scenarios, as depicted in Figure 2.1. However,ascanbeseeninthegure,varioussce-narios currently dier substantially in their assump-tions about the role of BECCs, which is in turn partly inuenced by diering assumptions about the role of CCS combined with conventional technologies such as coal, oil, and natural gas burning.28_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEFigure 2.1: Contributions of various technologies and chang-es in end-use to two miti-gation scenarios.A signif-icant role is assumed for BECCS in both scenarios (bright green in the top panel, turquoise in the bottom panel), especially in enabling net emissions to potentially go below zero in the second half of the century, but with notable differences in the relative role of BECCS in each scenario. Top panel: for the RCP2.6 scenario from the IPCC Fifth Assessment Report. Source: Van Vuuren et al. (2011b); Bottom panel: from a scenario for limiting long-term global mean CO2 mixing ratios to 450 ppm. Source: Luderer et al. (2012). EuTRACE Report_292. Characteristics of techniques to remove greenhouse gases or to modify planetary albedoNuclearRenewableBio-energy + CCSNat. gas + CCSOil + CCSCoal + CCSBio-energyNat. gasOilCoal150012507501000500250Primary energy use (EJ)1980 2000 2020 2040 2060 2080 2100ResidualBio + CCSBiomassRenewablesNuclearFossil + CCSFuel SwitchEnd-use804060200[Gt CO2/a]2005 2020 2040 2060 2080 2100-20Mitigation technologies: 450ppm World0Nevertheless, despite these dierences, many scenar-ios share the common feature of assuming some form ofgreenhousegasremoval.AstudybyFussetal. (2014) found that 87% (101 out of 116) of the scenarios that result in concentration levels of 430480 ppm CO2equivalent(CO2eq),consistentwithlimiting warming below 2C, require global net negative emis-sionsinthesecondhalfofthiscentury,asdomany scenarios (235 of 653) that reach between 480 and 720 ppm CO2eq in 2100. Accordingly, in these scenarios, limiting global mean warming to less than 2C would generallyrequiresomeformofwhatiscurrently thought of as climate engineering by greenhouse gas removal. Within this context, as can be seen in Figure 2.1, con-siderable hopes are being placed in BECCS as a sub-stantialcomponentofmanyscenariosforkeeping global mean warming below 2C. However, there are also strong doubts about BECCS; for instance, Fuss et al. (2014) prominently conclude that its credibility as a climate change mitigation option is unproven and itswidespreaddeploymentinclimatestabilization scenariosmightbecomeadangerousdistraction. Thus,duetothesecontrastingperspectivesandthe prominenceitalreadyhasincurrentscenariosof future emission pathways, BECCs warrants particular attention among the techniques being considered for climate engineering by greenhouse gas removal, and as noted in Section 1.2 it will be a particular focus of the following chapters. This section provides a basis for that later discussion by summarizing the basic cur-rent understanding of the potentials and limitations of BECCs. InBECCS,theburningofbiomasswouldbeused either for electricity generation or in bio-ethanol pro-duction. Bio-ethanol production for transportation is welldeveloped,withanannualglobalproductionof 85billionlitres(InternationalEnergyAgency,2011). Bio-ethanolproductionbyfermentationproducesa relatively pure CO2 waste stream that can be captured and compressed for geological storage. The rst large-scale project combining bio-ethanol production with CCS is underway at the ADM Decatur ethanol plant in Illinois, USA (see Box 3.4). Electricity generation by co-firingasmallproportionofbiomass(typically around 3% of energy input) mixed into coal feedstock iswidespreadinlargecoalpowerplantswheresup-port incentives exist biomass accounted for 1.5% of globalelectricitygenerationinpowerplantsin2010 (InternationalEnergyAgency,2012).Biomasshasa lower energy density than coal (3080% less for wood pellets compared to steam coal) (International Energy Agency, 2012), although power plants using 100% bio-mass have been operated, such as RWE Tilbury (UK) (RWE,2012).Appropriatelyprocessedbio-methane can be added to the natural gas pipeline network for co-firingingaspowerplants(Weiland,2010). CO2capture from biomass burning (including co-r-ing) can be adapted to existing CO2 capture technolo-gies but introduces additional considerations such as impurityvariationandgreateruegasvolumefor post-combustion capture, managing tars released dur-ing gasication, and more variable combustion prop-ertiesinoxyfiring(InternationalEnergyAgency GHG R&D Programme, 2009).Eectiveness:Possiblescalesofdeploymentofboth bio-ethanol and biomass power generation with CCS are limited by three main factors: biomass availability and the sustainability of inten-sive, large-scale agricultural practices; theamountofinfrastructureavailableand/orthat couldbebuiltforCCS(Luckowetal.,2010),along withtheenergyrequirementsforrunningtheinfra-structure; and long-term CO2 storage availability, noting that geo-logicalstorageofcapturedCO2oerspossibleCO2 removal on an eectively permanent timescale. The availability of biomass for BECCS (as well as for otherbiomass-basedtechniques)issubjecttomany technical, climatic, and societal factors. Estimates of sustainable biomass supply dier by orders of magni-tude, resultingfromdieringassumptionsaround biomasstypes,bio-technologydevelopment,future climatic conditions and food demand, and the availa-bility of land, water, and nutrients, future climatic con-ditions, and food demand (Bauen et al., 2009; Berndes et al., 2003; Dornburg et al., 2010). Higher estimates of potentialremovalofatmosphericCO2byterrestrial biomass-based techniques likely require conversion of agriculturallandorcarbon-richecosystemsforbio-masscrops,and/ormassiveapplicationoffertiliser. 30_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEInadditiontoconsiderationsofthepaybackperiod and soil carbon availability, the purposeful increase in biomassforgreenhousegasremovalwilllikelyhave both environmental (for example, ecosystem change) and climatic eects (due to changes in regional albedo andinthehydrologicalcycle),whichmayconstrain deploymentandlimitthecoolingeectonthecli-mate. However, in the immediate future, the primary limitationsonterrestrialbiomassavailabilitywill likely result from dietary choices and the eciency of foodproduction(PowellandLenton,2012).Upper limit estimates of the potential of terrestrial biomass-basedmethodsgenerallypresumethatallbiomass thatispotentiallyavailableforCO2removalwillbe usedspecicallyforthatpurpose.Therefore,upper limitestimatesoftheamountofCO2thatcanbe removedbymethodsthatusebiomassarenotaddi-tive, although there is likely some potential for over-lap, for example turning waste from biomass for bio-fuelproductionintobiochar.Themostfeasibleand eective method will likely vary according to regional considerations.RegardingCCSinfrastructure,CO2capturetech-niquesarediverse(Fennelletal.,2014).Sometech-niques,suchasaminesolventcapture,havebeen understoodwellindustriallysincethe1920s,but require up to 25% of the energy output from a power plant.Suchapproachescanstillachieveanoverall carboncaptureeciencyof9095%,includingthe CO2emittedfromtheadditionalfuelrequiredto maintainthesametotalpoweroutput.Newertech-niques,suchasIGCC(integratedgasicationcom-binedcycle)haveanenergypenaltyofonly1%,and arenowstartingtobebuiltatcommercialscale. Research techniques such as oxycombustion, chemi-callooping,andcryogeniccyclesareunderactive development and hold the promise of capture penal-tiesofonlyafewpercent,alongwithpossible improvements in the eciency of CO2 capture. Issuesaroundthelong-termstorageofCO2aredis-cussed below, in Section 2.1.9.2. Finally,costestimatesrangewidelyduetodiering methodologies, assumptions around the value of the product (electricity, transport fuel), and the potential forcostreductionviaimprovedCO2capturetech-nologies. CCS deployment presently lags considerably behindthatenvisagedinCO2emissionreduction strategies (Scott et al., 2013). Estimates vary widely for the deployment timescale of BECCS and the potential for greenhouse gas removal (2.510 Gt CO2/yr), with highervaluesrequiringconsiderableconversionof land to grow feedstock (McGlashan et al., 2012).Impacts: The impacts include land-use and water sup-plycompetition,localenvironmentaldegradation associated with industrial agriculture and biofuel pro-ductionfacilities,andprolongeduseofcoalifthe power plants are co-red. 2.1.3 Biochar Description: Greenhouse gas removal through biochar aimstoincreasethelongevityofbiomasscarbon through conversion to a more stable form (char) com-bined with burial or ploughing into agricultural soils. Charisproducedbymedium-temperaturepyrolysis (>350C) or high-temperature gasication (~900C) of biomassinalow-oxygenenvironment.Thereare manyvarietiesofbiomassfeedstock(denedbythe US Department of Energy as any renewable, biologi-cal material that can be used directly as a fuel, or con-verted to another form of fuel or energy product; see www.energy.gov). Biomass feedstocks range from pel-letedorchippedwoodtoagriculturalresiduesand wastes, producing char with diering char yields and stable carbon fractions. Flammable syngas and bio-oil areco-producedwiththecharandmightinturnbe usedforinputenergytotheproductionprocess (Lenton and Vaughan, 2013).Eectiveness: Under current conditions, the potential for sustainable removal of CO2 by biochar is estimated at a maximum of 3.5 GtCO2/yr, equivalent to seques-tration of up to 350 Gt CO2 over a century (Woolf et al., 2010). Application to all agricultural and grassland areasgivesatechnicallong-termglobalpotentialof storing1500 Gt CO2(Lehmannetal.,2006).The greenhouse gas removal potential of biochar is inu-encedbymanyfactors,includingfeedstockproduc-tion,handlinglosses,energyinput,charyield,labile carbonfraction,andmeanresidencetimeofcarbon (Hammond et al., 2011). Under conservative estimates for biochar yield (25% dry feedstock mass) and stable carbon fraction (50% over 100 years), sequestration of 0.46 t CO2 (or 0.17 t C) per tonne of dry feedstock is EuTRACE Report_312. Characteristics of techniques to remove greenhouse gases or to modify planetary albedoattainable,withhighervaluespossibleusingbetter-optimised feedstock. Additional CO2 uptake may also occurfromincreasedvegetationproductivitybio-char can increase soil retention of moisture and plant-availablenitrogen.Quanticationofenhancedpro-ductivity,char-typespecificinf luences,and accounting remain uncertain but are an active area of research.Costestimatesintheliteraturearewithin therange$30100pertonneofCO2(Shackleyand Sohi, 2011; McGlashan et al., 2012). Biochar potential is primarily limited by feedstock availability and logisti-calconstraintsonapplication.Withacarbonresi-dence time of decades to centuries, maintenance (re-application) is required. Char prior to burial could also be appropriated for use as fuel.Impacts:Theimpactsincludeland-usecompetition, possible health risks from associated dust production, andthepotentialtoreducealbedobyupto80%on application and 2026% post-harvest (Genesio et al., 2011), reducing the climate change mitigation eect by 1322% (Meyer et al., 2012).2.1.4 Additional biomass-basedprocesses: non-forest, burial, use in construction, and algal CO2 captureDescriptionandeectiveness:Whileforestshavethe greatestabove-groundcarbondensity,severalother techniques have signicant potential for using terres-trialbiomassforgreenhousegasremoval,including modiedagriculturalpracticesandpeatlandcarbon sinkenhancement(Worralletal.,2010;Freemanet al.,2012).Introductionoforganicmaterial(crop wastes,compost,manure)toagriculturallandis widely practiced and might be increased to enhance soil carbon, but would only achieve a limited uptake, not exceeding 2 Gt CO2/yr and the residence time of thecarbonwouldbeshort(yearstodecades).The naturalformationofpeatlandisslow(verticalaccu-mulation of approximately 1mm/yr). This sink might beenhancedbyburyingtimberbiomassinanoxic wetlands,butsuchapproachesarelogisticallycom-plex and the plausible scale is unknown (Freeman et al.,2012).Arelatedpossibility,withsimilarlackof knowledge about the plausible scale, is waste biomass burial in the deep ocean (Strand and Benford, 2009). Afurthersinkforterrestrialbiomassinvolveswide-spread use in construction, with a carbon residence of decadestocenturies,includingbothstructuraluse (timber)andforinsulation(e.g.,straw);thishasthe additionalbenetofdisplacingsomeCO2emissions fromcementproduction(Gustavssonetal.,2006). The potential overall contribution, limited primarily by demand, is likely to be small (10,000 years) pri-marily geological or geochemical storage have the potential to mitigate climate change on a permanent basis (Scott et al., 2013). Shorter-term CO2 removal or xation options eventually release the stored carbon back into the atmosphere as CO2, so that the removal processwouldneedtobecontinuallymaintainedin order to have the eect of long-term storage. Shorter-term storage is also potentially vulnerable to climatic andsocietalchanges,e.g.,subsequentclearanceof aorested regions. As such, these shorter-term oppor-tunitieshavebeenposedasbuyingtimewhile longer-term alternatives are being developed (Dorn-burg and Marland, 2008). These considerations would applysimilarlytotheremovalofothergreenhouse gases.Captured CO2 can potentially be used as a feedstock forchemicalmanufacturingprocesses,e.g.,liquid fuels, carbonate, methane, methanol, and formic acid. However, net CO2 removal is not achieved if it is used toformaproductthatissubsequentlycombusted (e.g.,liquidfuels)orsubjectedtoreactionsthatpro-duceCO2,eitherindeliberateprocessingorinthe natural environment. Demand for long-term, chemi-callystableCO2-basedproductsisverylikelyto remain extremely small compared to current anthro-pogenic emissions of CO2 (Kember et al., 2011). Con-sequently,carboncaptureandutilisation(CCU) projects may be important as a step towards develop-ing closed-cycle perspectives in the private sector and generalpublic,andtowardsaddingvaluetosomeof the CO2 that is captured by various processes, but are not likely to have a large impact on global atmospheric CO2 concentrations. Using CO2 to increase recovery fromoilandgasreservoirs(enhancedoilrecovery), which is sometimes considered as a form of CCU plus storage, does result in geological CO2 storage but the full lifecycle of the process, particularly the net eect ontheatmosphericcarbonbudgetoncetheoiland gasarecombusted,needstobeconsideredinmore detail (Jaramillo et al., 2009). Thecumulativecapacitiesofthevariousproposed CO2storageoptionsareuncertain,withmostesti-mates based on limited data and desk-based studies. Nonetheless, as discussed in Scott et al. (2015), useful insights can be gained by comparing these estimates with the estimated availability of fossil carbon reserves (very high condence in quantity and extractability) andthemuchlargertotalavailableresources(identi-ed, but without cost estimates for extraction). These are depicted, along with storage capacities, in Figure 2.3. The total storage capacity on the global land sur-face (in biomass and soils) is at least an order of mag-nitudesmallerthanavailablefossilcarbonreserves. Theoceanhasmuchgreaterstoragecapacity,theo-retically in excess of all known fossil carbon resources, but methods to access this storage and the timescales of such storage have not yet been established, and it is unclear if it will ever be possible to establish appropri-ate long-term storage methods for the oceans.38_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEAmongthepossiblestoragereservoirs,depleted hydrocarboneldsandsalineaquifersarethebest understood, proven, and quantied, providing secure CO2storageongeologicaltimescales(Scottetal., 2013; Sathaye et al., 2014). Estimates suggest that they havesucientglobalcapacitytocontaintheCO2 resultingfromtheuseofallcurrentfossilcarbon reserves,withplausibleengineeringinterventions, such as pumping out formation waters to reduce pres-sures, being able to further increase individual reser-voir capacities. At the continental scale, sedimentary basinswithpotentialasgeologicalCO2storagesites are relatively well distributed. However, storage sites are not necessarily co-located with major emissions sites, so that CCS sourcesink matchingwouldrequirethedevelopmentofsigni-cant CO2 transportation infrastructures (Metz et al., 2005;Stewartetal.,2013),andfactorssuchassocial acceptability (see example cases in Section 3.1.4) can limit the practical availability of storage sites. Reloca-tionofCO2-emittingfacilitiesorimplementationof direct air capture might oer improved co-location to storage, but would require appropriate above-ground conditions(e.g.,labourandmarketsforproducts,or suitablemeteorologicalconditionsforCO2aircap-ture).Figure 2.3: Sizes of fossil carbon supply (reserves and resources) and potential carbon stores divided between temporary (1,000 yr) and per-manent (geological timescales 100,000 yr) in Gt CO2. Oil and gas reservoirs are both a fossil carbon supply and potential CO2 storage resource. Question marks indicate undeveloped and/or unestablished (at the scale of MtCO2or more) CO2 storage mech-anisms. Global mean temperature increases associated with various amounts of cumula-tive CO2 emissions are indicated. Source: Scott et al. (2015). ResourcesReservesEmissions100,000 10,000 1,000 100 10 0 10 100 1,000 10,000 100,000CO2 (Gt)Permanent stores Temporary stores Supply Storage6C 2C 2C 6C4C 4C CO2 emissions to present CO2 emissions to budgetCoal UnminedOilGasUnminedCO2 swap ?CoalOilGasGas hydrates?Deep coal seams?Depleted oilDepleted gasSaline aquifersBasalts?Enhanced weathering?Sea-bed sediments?Ocean water?Biochar?Afforestation?EuTRACE Report_392. Characteristics of techniques to remove greenhouse gases or to modify planetary albedoWhile secure storage capacity in geological reservoirs is estimated to be sucient to match known fossil car-bon reserves, it is not established whether it would be sucient to accommodate all estimated fossil carbon resources(Scottetal.,2015).Verylargetheoretical storage CO2 potentials have been identied in basalts (McGrailetal.,2006;MatterandKelemen,2009), seabedsediments(Houseetal.,2006;Levineetal., 2007), and through acceleration of mineral weather-ing (Hartmann et al., 2013), but their feasibility at scale is eectively unknown at the present time. 2.2 Albedo modication and related techniquesTheEarthsclimatedependsuponthebalance between absorbed solar radiation and emitted terres-trialradiation(seeFigure1.2forreference).Albedo modication refers to deliberate, large-scale changes of the Earths energy balance, with the aim of reduc-ing global mean temperatures. The proposed methods are designed to increase the reection of solar (short-wave) radiation from Earth. Suggestions for increas-ingtheEarthsreflectivityinclude:enhancingthe reectivityoftheEarthssurface;injectingparticles intotheatmosphere,eitherathighaltitudesinthe stratosphere to directly reect sunlight or at low alti-tudesovertheoceantoincreasecloudreectivity; and placing reective mirrors in space.Albedomodificationtechniquesaredistinctfrom mitigationandfrommostgreenhousegasremoval techniques, in three key ways: their operational costs are potentially low; their eects are potentially rapid and large; their evaluation is better characterised as a risk-risk trade-o (Goes et al., 2011). In light of this distinction, various potential roles for albedo modication have been proposed: employing albedo modication on a large scale with the goal of reducing climate risks as much as possible, potentially substituting for some degree of mitigation (Telleretal.,2003;Carlin,2007;BickelandLane, 2009); employing albedo modication as a stopgap meas-uretoallowtimeforreducingemissions(Wigley, 2006); reserving albedo modication for use in a potential climate emergency, such as the large-scale release of methane from permafrost and ocean deposits (Black-stock et al.; 2009, see also Box 3.7). Of course, it is also possible that albedo modication techniqueswillhavenoroleinfutureresponsesto climatechange,ifitisdecidednottoemployanyof them at all, given that they do not address the funda-mentalcauseofglobalwarming,namelyemissions, andthustheincreasingatmosphericconcentrations ofgreenhousegases(MatthewsandTurner,2009). Discussions on the potential role of albedo modica-tion frequently focus on three key drawbacks. Firstly, sincealbedomodicationimpactstheclimateina manner that is physically dierent from the impact of greenhouse gases, it would not be possible to simply reversetheeectsofglobalwarming.Thus,whilst albedo modication may reduce some risks associated withclimatechange,itmayinturnincreaseothers. The way in which an albedo modication technique is deployed would aect the distribution of benets and harms(Irvineetal.,2010;Rickeetal.,2010a;Mac Martin et al., 2013). Secondly, albedo modication car-riestheriskofaterminationshock:ifitwere deployed for some decades at large scale and thereaf-ter terminated, there would be a rapid warming glo-bally,backtowardsthetemperaturesthattheEarth wouldhavealreadyreachedintheabsenceofa deploymentofalbedomodication(Matthewsand Caldeira,2007;Irvineetal.,2012).Suchanevent wouldlikelybeparticularlydamaging,giventhat there are indications that the impact of climate change onhumanpopulationsandecosystemsdepends strongly on not only the amount but also especially on the rate of climate change (Goes et al., 2011). Thirdly, albedomodificationdoesnotaddressthedirect eectsofCO2ontheenvironment,suchasocean acidicationandimpactsonterrestrialvegetation (Matthews and Caldeira, 2007). Finally,beyondthesephysicalrisks,itisalsoimpor-tanttonotethatthepotentialfutureroleofalbedo modication, if any, will also depend on how the ini-tial scientic results are interpreted, framed, and com-40_EuTRACE ReportFinal report of the FP7 CSA project EuTRACEmunicated, as well as on how the socio-technical con-text, into which discussions of climate engineering are emerging,shapesthesetechniquesandtheirusage. This is discussed further in Chapter 3.The Sections below (2.2.12.2.5) assess several of the key methods that are currently being discussed, which mostly involve increasing the planetary albedo, either at the Earths surface, or in the atmosphere via modi-fying low-level clouds or stratospheric aerosol parti-cles.Usingspacemirrorsforclimateengineeringis not discussed in detail, since the technological devel-opment,materialandenergeticrequirements,and associatedoperationalcostswouldatpresentbeso prohibitivethatthistechniqueisnotrealistically being considered for implementation in the mid-term future, although it is used as a form of thought exper-iment for idealised climate model simulations, as dis-cussed in Section 2.2.6. While the majority of studies have concentrated on albedo modication, a few other relatedtechniqueshavebeenproposedthatwould altertheEarthsenergybalancebyincreasingthe amountofterrestrialradiationemittedfromthe planet (see Section 2.2.5). Numerous additional meth-ods beyond those discussed below have also been pro-posed.Anoverarchingdescriptionoftheresearch ndings on the responses of the climate to the various methods is presented at the end of this section.2.2.1 Stratospheric aerosol injection (SAI)Description:Stratosphericaerosolinjectioninvolves increasing the amount of aerosol particles in the lower stratosphere(ataltitudesaboveabout20km)asa meanstoincreasethereectionofsunlightbeyond whatisreectedbythenaturally-occurringstrat-ospheric aerosol layer (Niemeier et al., 2011; Rasch et al., 2008). Particles could either be injected directly or formedviainjectionofprecursorgasessuchassul-phurdioxide(SO2),whicharethenconvertedinto particles. SAI is currently the most discussed albedo modication technique. It was rst proposed by Bud-yko (1974), but was not widely discussed until the idea wasreiteratedbyCrutzen(2006);sincethenithas been heavily investigated, including numerous model-basedstudiesandrstproposalsandplansforeld experiments,aswellasstudiesonsocietalaspects such as perception, ethical concerns, and governance. Effectiveness:Analysesoftheglobaltemperature recordsubsequenttolargevolcaniceruptionsthat inject millions of tonnes of SO2 into the stratosphere leave little doubt that the introduction of aerosols into the stratosphere cools the climate (Robock, 2000). It has been suggested that the technical feasibility, eec-tiveness, aordability, and timeliness of stratospheric climateengineeringtechniquescouldmakethema possibleoptionforcounteractingglobalwarming (Robocketal.,2009),butmanyconcernshavealso been raised, including geographically inhomogeneous climateeectsandpotentialsideeects(Robock, 2008). Candidategasesforinjectionintothestratosphere includeSO2orhydrogensulphide(H2S)(Robocket al.,2008;Shepherdetal.,2009;Raschetal.,2008), which are oxidised to form small sulphuric acid aero-solparticles(Robocketal.,2009).H2Shasalower molecularmassandisaroundtwiceaseectiveas SO2increatingmoleculesofstratosphericsulphuric acid per kg of gas, but is highly toxic and thus may be problematic for transport to the stratos