Recent studies have identified a near-linear relationship between

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1 PUBLISHED ONLINE: 21 SEPTEMBER 214 DOI: 1.138/NGEO2248 Persistent growth of CO 2 emissions nd implictions for reching climte trgets P. Friedlingstein 1 *, R. M. Andrew 2, J. Rogelj 3,4, G. P. Peters 2, J. G. Cndell, R. Knutti 3, G. Luderer 6, M. R. Rupch 7, M. Scheffer 8,9, D. P. vn Vuuren 1,11 nd C. Le Quéré 12 Efforts to limit climte chnge elow given temperture level require tht glol emissions of CO 2 cumulted over time remin elow limited quot. This quot vries depending on the temperture level, the desired proility of stying elow this level nd the contriutions of other gses. In spite of this restriction, glol emissions of CO 2 from fossil fuel comustion nd cement production hve continued to grow y 2.% per yer on verge over the pst decde. Two thirds of the CO 2 emission quot consistent with 2 C temperture limit hs lredy een used, nd the totl quot will likely e exhusted in further 3 yers t the 214 emissions rtes. We show tht CO 2 emissions trck the high end of the ltest genertion of emissions scenrios, due to lower thn nticipted cron intensity improvements of emerging economies nd higher glol gross domestic product growth. In the sence of more stringent mitigtion, these trends re set to continue nd further reduce the remining quot until the onset of potentil new climte greement in 22. Breking current emission trends in the short term is key to retining credile climte trgets within rpidly diminishing emission quot. Recent studies hve identified ner-liner reltionship etween glol men temperture chnge nd cumultive CO 2 emissions 1 9. This reltionship leds to n intuitive nd ppeling ppliction in climte policy. A glol quot on cumultive CO 2 emissions from ll sources (fossil fuel comustion, industril processes nd lnd-use chnge) cn e directly linked to nominted temperture threshold with specified proility of success. It cn e used regrdless of where, or to lrge degree when, the emissions occur 1. Despite the mny reservoirs nd timescles tht ffect the response of the climte nd cron cycle 11, the proportionlity etween temperture nd cumultive CO 2 emissions is remrkly roust cross models. The reltionship hs een clled the trnsient climte response to cumultive cron emissions (TCRE) nd ws highlighted in the fifth ssessment report (AR) of the Intergovernmentl Pnel on Climte Chnge (IPCC) 12. The nerliner reltionship hs strong theoreticl support: rditive forcing per emitted tonne of CO 2 decreses with higher CO 2 concentrtions, n effect tht is compensted y the wekening of the ocen nd iosphere cron sinks leding to lrger frction of emitted CO 2 remining in the tmosphere 13. The uncertinty in the TCRE, ccounted for here in the given proility 12,16, thus comes from the climte response to CO 2 nd the cron cycle feedcks 14, The ner-liner reltionship holds for cumultive CO 2 emissions less thn out 7, GtCO 2 nd until tempertures pek 16. Although CO 2 is the dominnt nthropogenic forcing of the climte system 2, non-co 2 greenhouse gses nd erosols lso contriute to climte chnge. However, unlike for CO 2, the forcing from short-lived gents is not relted to the cumultive emissions ut more directly determined y nnul emissions Therefore it is necessry to ccount for the dditionl wrming from non-co 2 gents seprtely when estimting CO 2 emission quots comptile with given temperture limit. The forcing from non-co 2 gents hs considerle rnge cross emissions scenrios in the recent IPCC Working Group III () dtse 24, reflecting expected development pthwys, coherently for CO 2 nd other forcing gents given the underlying climte nd other policies 2. Generlly, forcing from non-co 2 gents contriutes 1 3% of the totl forcing 9 (Supplementry Fig. 1). For 66% proility of stying elow temperture threshold of 2 C, CO 2 emissions would need to e kept elow 3,67 GtCO 2 if ccounting for forcing from CO 2 only (4,44 GtCO 2 for % proility) 12,26. When ccounting for oth CO 2 nd non-co 2 forcing s represented in the multiple scenrios ville in the IPCC dtse, the quot ssocited with 66% proility of keeping wrming elow 2 C reduces to 3,2 (2,9 3,6) GtCO 2 (3, (3,1 3,9) GtCO 2 for % proility) (Tle 1 nd Supplementry Tle 1). The estimte of cumultive udget cn vry slightly (y out %) with the set of scenrios used, due to vritions in the reltive contriution of non-co 2 rditive forcing (Supplementry Informtion). In recent yers, interest hs grown in using cumultive emissions more directly in climte policy 9,27 3. In the following we updte regionl nd glol emission estimtes up to 214 nd provide projections up to 219. The emission estimtes nd trends re used to updte the emission quot remining from 22, the potentil yer 1 College of Engineering, Mthemtics nd Physicl Sciences, University of Exeter, Exeter EX4 4QF, UK, 2 Center for Interntionl Climte nd Environmentl Reserch Oslo (CICERO), PO Box 1129 Blindern, 318 Oslo, Norwy, 3 Institute for Atmospheric nd Climte Science, ETH Zurich, CH-892 Zurich, Switzerlnd, 4 Energy Progrm, Interntionl Institute for Applied Systems Anlysis (IIASA), A-2361 Lxenurg, Austri, Glol Cron Project, CSIRO Ocen nd Atmospheric Flgship, Cnerr, ACT 261, Austrli, 6 Potsdm Institute for Climte Impct Reserch (PIK), PO Box 6123, Potsdm, Germny, 7 Climte Chnge Institute, Austrlin Ntionl University, Cnerr, ACT 2, Austrli, 8 Climte Anlytics, 1969 Berlin, Germny, 9 Environmentl Systems Anlysis Group, Wgeningen University, PO Box 47, 67 AA Wgeningen, The Netherlnds, 1 PBL Netherlnds Environmentl Assessment Agency, PO Box 33, 372 AH Bilthoven, The Netherlnds, 11 Copernicus Institute of Sustinle Development, Fculty of Geosciences, Utrecht University, Budpestln 4, 384 CD Utrecht, The Netherlnds, 12 Tyndll Centre for Climte Chnge Reserch, University of Est Angli, Norwich Reserch Prk, Norwich NR4 7TJ, UK. *e-mil: p.friedlingstein@exeter.c.uk NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION Mcmilln Pulishers Limited. 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2 NATURE GEOSCIENCE DOI: 1.138/NGEO2248 Tle 1 Cumultive cron udget (GtCO 2 ), remining emissions quots from 2 nd 22 (GtCO 2 ) nd equivlent emission-yers ssocited with 66% or % proility of glol-men wrming elow 2 C, 3 C nd 4 C (reltive to 18 19). Cumultive udget (since 187) 3,2 (2,9 3,6) From 2 Remining quot 1,2 (9 1,6) 2 C 3 C 4 C 66% % 66% % 66% % 3, (3,1 3,9) 1, (1,1 1,9) 4,9 (4,,7) 2,9 (2, 3,7),3 (, 6,2) 3,3 (3, 4,2) 6,4 (6,1 7,7) 4,4 (4,1,7) Emission yers 3 (22 4) 37 (27 47) 72 (62 92) 82 (74 14) - - From 22 Remining quot 1, (7 1,4) 1,3 (8 1,7) 2,7 (2,3 3,) 3,1 (2,8 4,) 4,2 (3,9,) Emission yers 22 ( 3) 28 (19 38) 8 (49 7) 67 (6 86) - - 7,1 (7, 8,),1 (, 6,) 4,9 (4,7 6,3) The equivlent emission-yers correspond to the emission quot divided y the lst ville yer of emissions, given for 2 C nd 3 C only. Cumultive emissions nd quots re shown with 9% rnge, rounded to the nerest 1. for the onset of new glol climte greement. We explore vrious uncertinties with cumultive emissions nd the consequences for the remining quot. We compre the emission trends nd remining emission quot with the emissions scenrios used in the recently pulished IPCC AR report tht re consistent with keeping the glol temperture increse elow 2 C ove pre-industril levels. This nlysis thus rings together currently disjointed perspectives: (1) the dependence etween cumultive emissions nd glol temperture chnges, (2) the decomposition of recent trends in emission nd (3) mitigtion pthwys from integrted ssessment modelling, nd nlyses their consistency with the 2 C climte trget. CO 2 emission updte The CO 2 emission quot comptile with given temperture limit encompsses oth pst nd future emissions. Since CO 2 is emitted ech yer, the remining quot decreses with time. Here, we first updte the remining emissions quot y providing updted estimtes of cumultive emissions through to 213 efore projecting emissions up to 219. CO 2 emissions from fossil fuel comustion nd cement production (E FF ) were estimted t 36.1 ( ) GtCO 2 in 213, 2.3% ove emissions in 212 (Fig. 1, Methods). Cumultive E FF from 187 to 213 were 1,43 ± 7 GtCO 2, with historicl estimtes sed on energy consumption sttistics 31 nd including uncertinties in the energy sttistics nd conversion rtes 31,32. Recent ttempts hve een mde to verify emissions from tmospheric mesurements nd modelling 33, ut their interprettion is hindered y the influence of the cron sinks 34,3. On short timescles, the chnges in CO 2 E FF re generlly driven y increses in economic ctivity s mesured y the gross domestic product (GDP) nd the decrese (improvement) in the cron intensity of the world economy (I FF ) 36,37. A decomposition of emissions into simplified Ky identity, E FF = GDP I FF, offers n effective wy to understnd short-term emissions trends This simple reltionship will e used throughout this rticle to understnd drivers of recent emission chnges nd provide short-term emission projections. In the pst decde (24 213) glol CO 2 emissions hve hd continued strong growth of 2.% yr 1. This growth rte ws elow the 3.3% yr 1 verged over 2 29 ecuse of the lower 2.4% yr 1 growth rte since 21 (Fig. 1). Using the simplified Ky identity, the decrese in the growth rte of glol CO 2 emissions in recent yers hs een due, in roughly equl prts, to slight decrese in GDP growth rte nd slightly stronger decrese in I FF (Supplementry Fig. 2). The positive decdl growth rte in glol emissions is due to strong growth in economic ctivity nd emissions in emerging economies, prtly due to the intensifiction of world trde 43,44, nd prtilly offsetting emissions in some lrge developed countries 44. These ptterns hve led to significnt regionl redistriution in emissions in ll key dimensions: solute, per-cpit, nd cumultive (Tle 2, Fig. 2). The top four emitters ply criticl role in emissions growth, Chin ccounted for 7% of the growth in glol emissions from , USA for 2%, Indi for 17%, while EU28 hd negtive contriution of 11%. The developed countries defined in Annex B of the Kyoto Protocol hd.4% increse in emissions in 213, reversing the trend of decresed emissions since 27. The USA s 2.9% growth in emissions in 213 reversed the ntion s trend of decresing CO 2 emissions (GtCO 2 yr 1 ) GDP (PPP; trillion 2USD) % yr % yr % % GtCO 2 ( ) GtCO 2 ( ) Emissions intensity (CO 2 /GDP; kgco 2 USD 1 ) Figure 1 Glol CO 2 emissions nd decomposition into GDP nd cron intensity.,, Glol CO 2 emissions from fossil fuel comustion nd cement production (; lck dots); nd glol GDP (lue dots) nd cron intensity of GDP (I FF, green dots) () over period nd estimtes to 219 (red dots). Historicl emissions re from CDIAC nd BP, while GDP re from IEA nd IMF (Methods). Uncertinty in CO 2 emissions is ± % (1σ) over the historicl period with n dditionl uncertinty for the projection sed on sensitivity nlysis of GDP nd I FF. PPP, purchsing power prity. 2 NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION Mcmilln Pulishers Limited. All rights reserved

3 NATURE GEOSCIENCE DOI: 1.138/NGEO2248 REVIEW ARTICLE Tle 2 Estimted CO 2 emissions from fossil fuel comustion nd cement production for 213, 214 nd 219, together with growth rtes. Totl (GtCO 2 yr 1 ) World 36.1 ( ) Per cpit (tco 2 p 1 ) Cumultive (GtCO 2 ). 1,43 (1,36 1,) Growth (%)* Totl (GtCO 2 yr 1 ) ( ) Growth (%) 2. (1.3 3.) Totl (GtCO 2 yr 1 ) 43.2 ( ) Cumultive (GtCO 2 ) 1,67 (1,9 1,7) Chin US EU Indi *We mke lep-yer djustments to these growth rtes nd this cuses growth rtes to go up pproximtely.3% if the first yer is lep yer nd down.3% if the second yer is lep yer. Growth (% yr 1 ) 3.1 emissions since 27 s result of return to stronger economic growth rte (2.2%), nd n unusul increse in I FF (.7%) (Fig. 2c nd Supplementry Fig. 2c), lrgely ecuse col hs regined some mrket shre from nturl gs in the electric power sector 4. The EU28 s 1.8% decrese in emissions in 213 continued the persistent downwrd trend despite incresed col consumption in some EU countries (for exmple, Polnd, Germny nd Finlnd). The decrese in emissions in EU28 ws driven y reltively low GDP growth rte (.%) nd decrese in I FF (2.2%) (Fig. 2d nd Supplementry Fig. 2d), with the lrgest emission decreses occurring in Spin, Itly nd the United Kingdom, nd the lrgest increse in Germny. Developing countries nd emerging economies (tken s non- Annex B) hd 3.4% increse in emissions in 213, continuing previous trends 42. Chin s 4.2% growth in emissions in 213 continued its decelerting growth (Fig. 2) from 1% yr 1 for 2 29 to 6.1% yr 1 for The reduction of the emissions growth rte in Chin is due to decresing GDP growth comined with stronger decrese in I FF (Supplementry Fig. 2). It is too erly to sy whether the recent decline in I FF in 213 cn e ttriuted to dedicted mitigtion policies. Despite this strong decrese in I FF, the high solute I FF in Chin, comined with strong GDP growth, is the min reson for the wekening I FF t the glol level (Supplementry Fig. 3). Indi s.1% growth in emissions in 213 compres to growth rtes of.7% yr 1 from 2 29 nd 6.4% yr 1 from (Fig. 2e). The recent Indin emissions growth ws driven y roust economic growth nd y n increse in I FF (Supplementry Fig. 2e). Indi is the only mjor economy with sustined increse in I FF (croniztion of its economy) from (Fig. 2 nd Supplementry Fig. 2). The roust reltionship etween GDP nd E FF tht emerged in the pst (Figs 1 nd 2) is used here to estimte future emissions on short timescles using projected growth rtes of GDP y the Interntionl Monetry Fund (IMF) 46 comined with n ssumption of persistent trends in I FF 4,42. This method provides first-order estimtes of CO 2 emissions in the sence of dditionl emission mitigtion policies. Bsed on the forecst 3.4% increse of glol GDP in 214 (IMF) 47, nd the trend in I FF over of.7% yr 1, we estimte 214 E FF to e 37. ( ) GtCO 2, or 2.% (1.3% 3.%) ove 213 nd 6% ove 199 emissions (Fig. 1). The rnge tkes into ccount the uncertinty in IMF GDP projections nd vriility in I FF cused y rnge of socio-economic fctors 42 (Supplementry Informtion). Similr estimtes re mde t the ntionl level (Tle 2 nd Fig. 2). While strong inertil fctors mintin glol emissions growth within reltively smll rnge, t the regionl level significnt nd unexpected events cn led to strong devitions, nd regionl uncertinty is much more difficult to quntify. We therefore do not provide uncertinty estimtes t the regionl level, ut cknowledge tht they re potentilly lrge. Emissions from lnd-use chnges hve een stle or decresing in the pst decde 48 nd currently contriute out 8% of totl CO 2 emissions. We estimte lnd-use chnge emissions in 213 using the most recent glol cron udget 49 sed on comintion of ookkeeping estimte 48 nd fire emissions in deforested res (Methods). We estimte emissions of 3.2 ± 1.8 GtCO 2 yr 1 in 213 nd use the verge of 3.3 ± 1.8 GtCO 2 yr 1 for Thus, totl CO 2 emissions from ll sources re estimted to e 39.4 ( ) GtCO 2 in 213 nd 4.3 ( ) GtCO 2 in 214. d CO 2 emissions (GtCO 2 yr 1 ) GDP (PPP; trillion 2USD) GDP (PPP; trillion 2USD) c 2 CHN 2. 2 USA EU e IND CHN 12.7 USA.2 IND 3.4 EU Emissions Intensity (CO 2 /GDP; kgco 2 USD 1 ) Emissions Intensity (CO 2 /GDP; kgco 2 USD 1 ) Figure 2 Regionl CO 2 emissions nd decomposition into GDP nd cron intensity.,, The CO 2 emissions from the top four emitters (Chin, US, EU28, Indi) () nd the GDP nd I FF in ech region ( e) over the historicl ( ) nd future ( ) periods. See Fig. 1 cption for detils nd Supplementry Fig. 2 for n nnul decomposition of the trends. NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION Mcmilln Pulishers Limited. All rights reserved

4 NATURE GEOSCIENCE DOI: 1.138/NGEO2248 Annul glol cron emissions from fossil fuel comustion nd cement production (GtCO 2 yr 1 ) % rnge min mx rnge IPCC AR emission dtse Bselines excl. climte policy Cost-effective 2 C scenrios (from 21 onwrd, 66% pro.) Updted estimtes nd projections Historicl estimtes GDP-sed projection Uncertinty 214 window 219 window Yer 214 c 219 Annul glol cron emissions from fossil fuel comustion nd cement production (GtCO 2 yr 1 ) 4 3 Annul glol cron emissions from fossil fuel comustion nd cement production (GtCO 2 yr 1 ) 4 3 n: 164 n: 81 n: 266 n: 14 2 GDP-sed projection selines optiml 2 C 2 GDP-sed projection selines optiml 2 C elow 3 C dely 2 C Figure 3 Consequences of current emissions nd projected ner-term trends. c, Comprison of nnul cron emissions from fossil fuel comustion nd cement production () together with time slices for 214 () nd 219 (c). The scenrios re from the IPCC AR emission dtse (coloured regions), nd updted estimtes (lck dots) nd projections (red dots) from this study. Horizontl lines show medin estimtes. Prt c includes dditionl 219 emission levels for the scenrios of the dtse tht remin elow 3 C (ornge), nd those resulting from full implementtion of Cncún pledges (eige). The vlues in pnel c indicte the numer of scenrios ville in the scenrio dtse for ech ctegory. Bsed on comined dt nd our 214 estimte, cumultive CO 2 emissions from ll sources during will rech 2, ± 2 GtCO 2. Aout 2% of this 14-yer period ws emitted over the lst yers lone (2 214). The cumultive emissions from 187 were 7% from fossil fuels nd cement production nd 2% from lnd-use chnge. Remining CO 2 quot Tking into ccount CO 2 emissions prior to 214, the remining emissions quot (from 2 onwrds) ssocited with 66% proility of keeping wrming elow 2 C is estimted to e 1,2 (9 1,6) GtCO 2. This 2 C quot will e exhusted in out 3 (22-4) equivlent emission-yers t the 214 emission level (4.3 GtCO 2 yr 1 ). Owing to inter-nnul nd decdl vriility 1 3, the ctul yer when 2 C will e reched is uncertin. The remining quot ssocited with % proility of committing to 2 C of wrming is estimted to e 1, (1,1 1,9) GtCO 2 (Tle 1), corresponding to 37 (27 47) equivlent emission-yers t the 214 emission level. The remining quot is significntly higher for 3 C (Tle 1), ut it is finite for even the highest wrming levels. The equivlent emission-yers indictor is simple nd trnsprent metric for communicting the size of the remining cron udget comptile with wrming level given our current emission levels. Mny of the low stiliztion scenrios in the literture, such s the representtive concentrtion pthwy (RCP) 2.6, rely on emissions elow zero (so-clled negtive emissions) in the second hlf of the century, in effect compensting for emissions tody 24,4. Most models chieve negtive emissions through intensive use of ioenergy coupled with cron cpture nd storge (BECCS) 7, nd the vilility of BECCS is importnt in cost-effective 2 C mitigtion pthwys,6. Negtive emissions t the glol level will led to pek nd decline in cumultive emissions 8. The vlidity of the TCRE in negtive emissions scenrio remins to e fully ssessed; nlyses with comprehensive Erth system models re required to fully explore the cron cycle nd climte response to negtive emission scenrios, though reserch hs strted in this re 1,9 61. There is lso need to fully explore the risks of relying on BECCS (currently unville t commercil scle) for 2 C mitigtion pthwys. Studies show tht explicitly limiting or eliminting the vilility of cron cpture nd storge 4 NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION Mcmilln Pulishers Limited. All rights reserved

5 NATURE GEOSCIENCE DOI: 1.138/NGEO2248 (CCS) nd cron dioxide removl (CDR) technologies in mitigtion scenrios does not necessrily rule out the fesiility of 2 C limit, ut does increse the need for deep emission reductions in the short term,6,62. The few studies tht explored 2 C pthwys without CCS nd CDR from emission levels tht re in line with the current emission reduction y 22 pledges of countries found these to e either unfesile or extremely costly 64, Emission projections nd climte trgets Current emission growth rtes re twice s lrge s in the 199s despite 2 yers of interntionl climte negotitions under the United Ntions Frmework Convention on Climte Chnge (UNFCCC). For illustrtion, we expnd our GDP-sed emissions projections to 219 using GDP projections from the IMF 46,47 nd continued trends in I FF. Assuming continution of pst climte policy trends through to 219 (the lst ville yer of IMF s GDP projections), we project n verge growth of fossil fuel nd cement emissions of 3.1% yr 1 to rech 43.2 ( ) GtCO 2 yr 1 in 219. The uncertinty rnge ccounts for the uncertinty in GDP nd fossil fuel intensity projections (Supplementry Informtion), ut does not ccount for unforeseen events, for exmple glol finncil crisis 4. Policies or trends tht further reduce I FF, or would lower GDP growth rtes, would directly reduce these emission estimtes. The recent US policy nnouncements on power plnt emissions or Chin s energy efficiency nd renewle trgets would t lest continue existing I FF trends, ut it is uncler t present if they would led to stronger decreses in I FF. Emission projections ccounting for current policies such s those from the Interntionl Energy Agency 7 nd seline projections ville in the literture nd summrized in the IPCC dtse often show lower growth rte thn our GDP-sed projection (Figs 3 nd 4), either sed on n ssumption of slower GDP growth or stronger decrese in I FF. We dditionlly extend these projections to the regionl level using the sme methods. Figure 2 shows the regionl trends in GDP, I FF nd hence E FF. In generl, nticipted GDP growth is offset y decreses in I FF (Supplementry Fig. 2). We find tht emissions from Chin would continue to grow t 3.9% yr 1 over , USA emissions t.2% yr 1 similr to recent estimtes y the US Energy Informtion Administrtion 71, EU28 emissions reduce y.9% yr 1 nd Indin emissions grow t.9% yr 1 (Tle 2, Fig. 2 nd Supplementry Fig. 2). Bsed on these projections, the cumultive fossil fuel nd cement emissions over re estimted to e 2 (19 21) GtCO 2. Assuming stle lnd-use-chnge emissions, we expect these to contriute n dditionl 16 (8 24) GtCO 2 during tht period. This rings totl cumultive emissions for to 22 (2 24) GtCO 2, nd the remining emission quot from 22, ssocited with 66% proility of limiting wrming elow 2 C, down to 1, (7 1,4) GtCO 2, or 22 ( 3) equivlent emission-yers from 22. The remining quots nd equivlent emission-yers from 22 onwrds for 3 C nd 4 C limits re given in Tle 1. Our GDP-sed emission estimtes re higher thn ll costeffective 2 C scenrios in the literture (Fig. 3) for In fct, current IPCC scenrios tht ttempt to keep wrming elow 2 C, show lower emissions for 214 thn our projection (Fig. 3), mostly ecuse these scenrios were pulished efore 214 nd ssumed cost-optiml mitigtion pthwy strting in 21. In 219, the discrepncy etween our GDP-sed estimtes nd the cost-effective mitigtion pthwys is even more excerted, with the GDP-sed emissions projections eing out 4% higher thn the levels suggested y cost-effective 2 C scenrios (Fig. 3c). This indictes tht without rpid nd cler rek in the historicl trends of I FF or GDP the opportunity to follow cost-effective 2 C mitigtion pthwys in the ner-term, s reported y the IPCC, hs pssed, nd the chllenges to mitigtion would Frequency in AR scenrio dtse Frequency in AR scenrio dtse c Frequency in AR scenrio dtse Cron emissions from fossil fuel nd industry growth rte (% yr 1 ) World gross domestic product (GDP) growth (%GDP yr 1 ) Cron intensity growth (%CI yr 1 ) IPCC AR emission dtse Bselines excl. climte policy Cost-effective 2 C scenrios (from 21 onwrd, 66% pro.) 2 C scenrios with explicit dely of cost-effective mitigtion until 22 REVIEW ARTICLE Updted estimtes nd projections GDP-sed projection Uncertinty Figure 4 Comprison of trends in the IPCC AR scenrio dtse nd projected ner-term trends. c, Histogrm of growth rtes in glol CO 2 emissions (), world GDP () nd cron intensity of GDP (c). Colours differentite the seline scenrios (red) nd mitigtion scenrios without dely (lue) nd with it (rown), nd our GDP-sed projections (red verticl lines) with their uncertinty (grey). need to e frmed round the more costly scenrios tht ssume dely in comprehensive mitigtion 64, The IPCC mitigtion scenrios consistent with the 2 C limit show reduction or even reversl in the CO 2 emissions growth due to rdicl decreses of I FF (Fig. 4c). While GDP growth rtes re similr to our estimte (Fig. 4), they show cron intensity decresing y 2 to % per yer, s opposed to our estimte of.8% per yer sed on recent trends 64,66 68 (Fig. 4c). The rpidly chnging structure of the world economy with growing contriution from emerging economies nd developing countries with high cronintensity drives increses in I FF t the glol level (Supplementry Fig. 3) nd further excertes the mitigtion chllenge. For emerging economies nd developing countries, the recent cron intensity decreses, which we use for our ner-term projections, hve een significntly smller thn the ner-term trends nticipted y most NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION Mcmilln Pulishers Limited. All rights reserved

6 emission scenrios, even seline scenrios in sence of climte policy (see Supplementry Figs 4 8 for comprison of regionl trends in GDP nd I FF with IPCC emission scenrios). Climte policy implictions Climte policy discussions hve progressed since 21 nd mny countries hve pledged to limit or reduce their greenhouse gs emissions y While GDP-sed projections of emissions re considerly higher thn those of the cost-optiml 2 C scenrios, recent studies hve shown tht even from such high emission levels in 22, options exist to limit wrming to elow 2 C 64, However, following such trjectories hs importnt consequences nd entils risks. Five min chllenges nd trde-offs must e overcome 64,66 68,7 77 : (1) higher emissions in the ner term require stronger emission reductions therefter trde-off tht hs ecome trivilly understndle since the introduction of the TCRE concept nd the quntifiction of 2 C consistent cron emission quot; (2) n incresed lock-in into cron-intensive nd energyintensive infrstructure 66,67,78,79 the recent trends discussed ove provide rel-world support for this concern; (3) reduced societl choices for future genertions modest ner-term emission reductions increse the dependence on specific mitigtion technologies nd therewith foreclose choices nd options of future genertions,64,66 69,79 (dependence on negtive emissions technologies is one exmple); (4) higher overll costs nd economic chllenges; nd () higher climte risks, for exmple through higher ner-term rtes of chnge, higher cumultive climte impct dmges, or n incresed proility of rupt or irreversile chnges 64,68,77,8. Stiliztion of glol temperture rise t ny level requires glol cron emissions to ecome eventully virtully zero 81. The existence of limited glol emissions quot rises mny issues of how to shre remining emissions, including how to tke into ccount historicl responsiilities nd development needs. These issues re discussed in compnion pper 82. Irrespective of the difficulty of how to shre the remining quot, our review of recent emission trends nd the mitigtion scenrio literture shows tht, if keeping wrming elow 2 C reltive to pre-industril levels is to e mintined s n overrching ojective, rek in current emission trends is urgently needed in the short term. Methods Dt. Glol nd regionl CO 2 emissions from fossil fuel comustion nd cement emissions re sed on emissions estimtes from the Cron Dioxide Informtion Anlysis Center 83 (CDIAC), extended to 213 using nomlies in energy sttistics from BP (ref. 84) following the methodology nd country definitions used in the Glol Cron Budget 42. CO 2 emissions from lnd-use chnge re estimted using ookkeeping method 48 from nd then supplemented nd extended from using stellite-sed fire emissions in deforesttion res, following the methodology in the most recent Glol Cron Budget 49. GDP dt is from the Interntionl Energy Agency 8 up until 211 nd extended to 219 using the growth rtes from two editions of the IMF s World Economic Outlook 46,47. The IPCC scenrios re otined from the scenrio dtse 24,86. Uncertinty. We plce n uncertinty of ± % (1σ) on the fossil fuel nd cement emissions 31,42 consistent with recent detiled nlysis of uncertinty 32 nd pply the sme uncertinty for the cumultive emissions (Supplementry Informtion). The uncertinty in emissions projections includes the uncertinty in future GDP estimtes nd different time periods for estimting I FF, nd consecutive emissions estimtes re ssumed to e uncorrelted (Supplementry Informtion). The llowle cumultive emissions quot is derived with certin modelled likelihood (% of model runs) tht specified wrming level is exceeded (for exmple, 2 C ove the verge over 18 19) including non-co 2 forcing (Supplementry Informtion). Quots re shown with % 9% rnge, rounded to the nerest 1. The rnge in equivlent emission-yers is otined tking the rnge in remining udget, neglecting the reltively smll uncertinty due to glol nnul emissions uncertinty. Growth rtes. Growth rtes etween two yers (for exmple, ) re sed on the percentge increse over the first yer. To prevent invlid interprettions of nnul chnge we mke lep-yer djustments to nnul growth rtes, such tht growth rtes go up pproximtely.3% if the first yer is lep yer nd down.3% if the second yer is lep yer. Growth rtes over more thn two consecutive yers re computed y tking the first derivtive of the liner regression of the logrithm of ll vriles ville in this time period (Supplementry Informtion). 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J. Glol, Regionl, nd Ntionl Fossil- Fuel CO 2 Emissions in Trends (Cron Dioxide Informtion Anlysis Center, Ok Ridge Ntionl Lortory, US Deprtment of Energy, 213); ornl.gov/trends/emis/em_cont.html 84. Sttisticl Review of World Energy June 213 (BP, 213). 8. CO 2 emissions from fuel comustion 213 (IEA, 213). 86. AR Scenrio Dtse (IIASA, 214); ARDB/ Acknowledgements P.F. ws supported y the Europen Commission s 7th Frmework Progrmme (EU/FP7) under Grnt Agreements (EMBRACE) nd (HELIX). G.P.P. nd R.M.A. were supported y the Norwegin Reserch Council (236296). J.G.C. cknowledges the support from the Austrlin Climte Chnge Science Progrm. C.L.Q. ws supported y the UK Nturl Environment Reserch Council (NERC) s Interntionl Opportunities Fund (project NE/132X/1) nd EU/FP7 project 2838 (GEOCron). This work is collortive effort of the Glol Cron Project ( Author contriutions P.F., G.P.P., J.G.C., J.R. nd C.L.Q. designed the study. P.F. coordinted the conception nd writing of the pper. R.M.A. nd G.P.P. provided dt nd nlysis on historicl nd ner-term projections of emissions, GDP nd cron intensity. M.S. provided ll dt on cumultive emission udgets comptile with wrming levels from the IPCC WGII scenrios dtse. J.R. nd G.L. coordinted the ssessment of trde-offs in delyed ction scenrios. R.M.A. produced Figs 1 nd 2. J.R. produced Figs 3 nd 4. All uthors contriuted to the writing of the pper. Additionl informtion Supplementry informtion is ville in the online version of the pper. Correspondence nd requests for mterils should e ddressed to P.F. Competing finncil interests The uthors declre no competing finncil interests. REVIEW ARTICLE NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION Mcmilln Pulishers Limited. All rights reserved