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Beyond 'dangerous' climate change: emission
scenarios for a new world
Kevin Anderson and Alice Bows
Phil. Trans. R. Soc. A 2011 369, 20-44
doi: 10.1098/rsta.2010.0290

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Phil. Trans. R. Soc. A (2011) 369, 20–44
doi:10.1098/rsta.2010.0290

Beyond ‘dangerous’ climate change: emission
scenarios for a new world
BY KEVIN ANDERSON1,3 AND ALICE BOWS2, *
1 TyndallCentre for Climate Change Research, School of Mechanical,
Aerospace and Civil Engineering, and 2 Sustainable Consumption Institute,
School of Earth, Atmospheric and Environmental Sciences,
University of Manchester, PO Box 88, Manchester M60 1QD, UK
3 School of Environmental Sciences and School of Development,
University of East Anglia, Norwich NR4 7JT, UK

The Copenhagen Accord reiterates the international community’s commitment to ‘hold
the increase in global temperature below 2 degrees Celsius’. Yet its preferred focus
on global emission peak dates and longer-term reduction targets, without recourse to
cumulative emission budgets, belies seriously the scale and scope of mitigation necessary
to meet such a commitment. Moreover, the pivotal importance of emissions from non-
Annex 1 nations in shaping available space for Annex 1 emission pathways received,
and continues to receive, little attention. Building on previous studies, this paper uses
a cumulative emissions framing, broken down to Annex 1 and non-Annex 1 nations, to
understand the implications of rapid emission growth in nations such as China and India,
for mitigation rates elsewhere. The analysis suggests that despite high-level statements
to the contrary, there is now little to no chance of maintaining the global mean surface
temperature at or below 2◦ C. Moreover, the impacts associated with 2◦ C have been
revised upwards, sufficiently so that 2◦ C now more appropriately represents the threshold
between ‘dangerous’ and ‘extremely dangerous’ climate change. Ultimately, the science of
climate change allied with the emission scenarios for Annex 1 and non-Annex 1 nations
suggests a radically different framing of the mitigation and adaptation challenge from
that accompanying many other analyses, particularly those directly informing policy.
Keywords: emission scenarios; Annex 1; non-Annex 1; cumulative emissions; climate policy;
emission pathways

1. Introduction

The 2009 Copenhagen Accord [1] has received widespread criticism for
not including any binding emission targets. Nevertheless, it does reiterate
the international community’s commitment to ‘hold the increase in global
temperature below 2 degrees Celsius, and take action to meet this objective
*Author for correspondence (alice.bows@manchester.ac.uk).
Electronic supplementary material is available at http://dx.doi.org/10.1098/rsta.2010.0290 or via
http://rsta.royalsocietypublishing.org.

One contribution of 13 to a Theme Issue ‘Four degrees and beyond: the potential for a global
temperature increase of four degrees and its implications’.

20 This journal is © 2011 The Royal Society

org on December 27. as it stands. Total cumulative emissions produced by nations that underwent industrialization in the nineteenth century and first half of the twentieth century will be eclipsed if the five billion people currently resident in non-Annex 1 nations remain or become locked into a fossil fuel economy. [2. The importance of the distinction between Annex 1 and non-Annex 1 is also noted in the Accord. [11–13]). By considering global emission budgets alongside emission pathways for non-Annex 1 nations. Clearly. this dramatic potential for emissions growth within non-Annex 1 nations is typically neglected in global and national mitigation scenarios. However. Phil. Recent years have seen the development of an increasing number of global emissions scenarios. very significantly. Although included in non-mitigation energy scenarios (e. A (2011) . Building particularly on previous analyses by Anderson & Bows [2] and more recently by Macintosh [3]. Trans. such integration is rare with 1 For the purpose of this paper. [4. the Accord still falls short of acknowledging what the science makes absolutely clear—it is cumulative emissions that matter. 2. preferring instead to focus on the ‘peaking of global and national emissions as soon as possible’ and the need for ‘Annex I Parties to implement . Moreover. quantified economy-wide emissions targets for 2020’. quantify the degree of mitigation required to meet this commitment nor does it give an indication of whether it is still possible to do so. though this is not made clear in the Accord. Alongside these global analyses a growing range of ever more detailed national-level energy and emission scenarios are being developed (e. integrating national and global analyses is a prerequisite of understanding the scale and rate of mitigation. Soc. the paper translates earlier global assessments of cumulative emissions into emission pathways for Annex 1 and non-Annex 1 nations.6–10]).5]). 2010 Beyond dangerous climate change 21 consistent with science and on the basis of equity’ [1]. . the Accord recognizes ‘that the time frame for peaking will be longer in developing countries’ and also.royalsocietypublishing. and despite making reference to being guided by the ‘science’.1 The Accord does not. Specifically.3. that ‘social and economic development and poverty eradication are the first and overriding priorities of developing countries’. impacts and adaptation associated with differing levels of climate change. it is assumed that the ‘2 degrees Celsius’ relates to the temperature rise above pre-industrial levels. This paper takes both the Accord’s commitment to ‘hold the increase in global temperature below 2 degrees Celsius’ along with its focus on the nearer term targets.g. Downloaded from rsta. however. While the inclusion of nearer-term targets is certainly a welcome complement to targets for 2050.g. . the Accord makes no mention of cumulative emissions as providing the scientifically credible framing of mitigation. each with a differing quantity of cumulative emission over the twenty-first century and hence with different temperature implications (e. R. this paper illustrates the increasing relevance of the latter for the mitigation policies of Annex 1 nations. electricity and transport within Annex 1 nations coupled with the very rapid industrialization of many non-Annex 1 nations. Analysis framing The first decade of the new millennium has witnessed unprecedented increases in emissions reflecting ongoing high levels of energy usage for heat. in particular China and India.g. and considers these in light of post-2000 and recession-adjusted emission trends.

R. . By disaggregating selected global emission pathways into Annex 1 and non-Annex 1 nations. EU’s and UK Government’s 2 Similar but less-contextual analyses also illustrate the division of global emissions between Annex 1 and non-Annex 1 nations (e.royalsocietypublishing. The third considers how a slower uptake of mitigation measures combined with later emissions peaking impact on cumulative emissions and. 3 It is important to note that within non-Annex 1 nations there will be significant differences in peaking years and emission reduction rates between the rapidly industrializing nations (e. temperature.g. their per capita emissions are around one fifth of those for the USA. stating ‘we should limit climate change to a maximum of two degrees’ [18] (emphasis added). The first derives pathways for CO2 emissions consistent with reasonable-to-low probabilities of exceeding 2◦ C. the Accord’s. in July 2009. The second explores the implications of incorporating all greenhouse gases within the scenario pathways for a similar chance of exceeding 2◦ C. For example. p. However.3 The paper comprises three principal analyses. subsequently reiterated this commitment. p. Phil. Following this approach. in which it stated explicitly that ‘to avoid the most dangerous impacts of climate change. the UK Government published its UK Low Carbon Transition Plan. Bows little more than perfunctory correlation between national and global emission pathways. Downloaded from rsta. In the absence of such quantification. this paper provides an improved and more contextual understanding of the extent of the mitigation challenge specifically and the adaptation challenge more generally.g. the Accord. to ensure that global average temperature increases do not exceed preindustrial levels by more than 2◦ C’ [15] (emphasis added). average global temperatures must rise no more than 2◦ C’ [17.org on December 27. are unlikely to succeed those in the USA in the next two to three decades (see note 17 for a discussion on cumulative per capita emissions). hence. whereby a correlation is made between the language of likelihood and quantified probabilities [19.2 Such analysis cannot substitute for detailed national-level assessments. Ed Miliband. below 2 degrees Celsius’ reflects the clear and long-established stances of both the European Union (EU) Commission and the UK Government. China) and regions such as sub-Saharan Africa. whilst China’s total emissions are now higher than those from any other nation. and given current trends and agreements. 2010 22 K. Trans. 23]. Soc. the language of many Government statements suggests. (a) Determining the ‘appropriate’ probability for 2◦ C The framing of the Copenhagen Accord around the importance of ‘hold[ing] to . Anderson and A. but does offer clear guidance as to the scale and rate of mitigation necessary to avoid particular rises in temperature above pre-industrial levels. [14]). . Without such quantification it is not possible to derive the accompanying range of twenty-first century cumulative emissions budgets from which emission pathways can be derived. EU and the UK do not make explicit what quantitative ‘risk’ of exceeding 2◦ C is considered ‘acceptable’. A (2011) . The EU maintains it ‘must adopt the necessary domestic measures . if not a zero probability of exceeding 2◦ C. Within the UK. 5] (emphasis added). at least a very low one [16]. probabilities may be inferred based on the approach developed for the Intergovernmental Panel on Climate Change’s (IPCC’s) reports. . Although this language is qualitatively clear. The previous Secretary of State for Energy and Climate Change. .

8 The 63 per cent probability of exceeding 2◦ C is an outcome of the CCC’s modelling approach and relates to its global cumulative emissions budget. do not exceed their apportioned emissions budgets.4 If government responses to climate change are to be evidence-based or at least informed significantly by science. it is reasonable to assume. The first report of the UK’s Committee for Climate Change (CCC) [8] heralded a significant departure from a focus on end-point and typically long-term targets. at their highest 5–33% of exceeding 2◦ C). Consequently.g. Phil. more recently. even within nations such as the UK. in which case the importance of low probabilities of exceeding 2◦ C increases substantially. there are clearly major implications for all tiers of government. these have since been re-evaluated with the latest assessments suggesting a significant increase in the severity of some impacts for a 2◦ C temperature rise (e. 10 per cent as ‘very unlikely’.org on December 27. p. at least collectively. 2010 Beyond dangerous climate change 23 statements all clearly imply very low probabilities of exceeding 2◦ C. although the UK Government’s framing of its climate change legislation is the first to detail emission pathways. acknowledging explicitly the need to re-align policy with cumulative emissions rather than simplistic targets. 5 per cent as ‘extremely unlikely’ and 1 per cent as ‘exceptionally unlikely’. and wider public and private decision making in both bringing about the scale of mitigation accompanying 2◦ C and responding to the impacts and associated adaptation of a failure to significantly mitigate. but between dangerous and ‘extremely dangerous’ climate change.e. that 2◦ C now represents a threshold. Although the language of many high-level statements on climate change supports unequivocally the importance of not exceeding 2◦ C. A (2011) . ceteris paribus. As it stands the carbon budget and emission pathway now enshrined in legislation are underpinned by analysis assuming a 63 per cent probability of exceeding 2◦ C [8. 21]. not between acceptable and dangerous climate change. the argument for low probabilities is reinforced still further. the accompanying policies or absence of policies demonstrate a pivotal disjuncture between high level aspirations and the policy reality. and even a highly conservative judgement would suggest the statements represent no more than a 5–33% chance of exceeding 2◦ C.royalsocietypublishing. 5 Or at least the rate of increase associated with a 2◦ C rise by 2100. the report proceeds to describe an emissions pathway out to 2050.8 a position that cannot be reconciled with the probabilities implied repeatedly by Government statements (i. the IPCC categorizes a 33 per cent probability of missing or exceeding something as ‘unlikely’. Given the UK budget is premised on the CCC’s choice of regime for apportioning global emissions between nations.21]). it is reasonable to describe the UK’s budget as correlating with a 63 per cent chance of exceeding 2◦ C. However. albeit with the important caveat that other nations. the disjuncture remains. Nevertheless. 6 Although this paper explicitly steers away from issues of governance. Downloaded from rsta. it is still far removed from its and others’ high-level commitments to ‘limit climate change to a maximum of two degrees’ [18]. Complementing the UK’s 2050 emission-reduction target with short-term budgets. Soc. 7 Typically 2050 but also. R.6 In part this reflects the continued dominance of ‘end point’ targets7 rather than scientifically credible cumulative emission budgets and their accompanying emission pathways. The characterization of 2◦ C5 as the appropriate threshold between acceptable and ‘dangerous’ climate change is premised on an earlier assessment of the scope and scale of the accompanying impacts. 2020. Trans. where the relevant policy community (and recent legislation) align themselves closely with the science of climate change. [20. 4 At the ‘less likely’ end of the spectrum. However.

Anderson and A. institutional and economic factors if resilience to the impacts of climate change is to be embedded in development. impacts and adaptation. With regard to exploring the consistency of scenarios. Scenario pathway assumptions Scenario approaches are increasingly used within mitigation and adaptation research for visioning alternative futures. 2010 24 K.) as the principal route by which emissions reductions will be achieved. This 9 This is particularly evident in the continued recourse to the implementation of future and innovative low-carbon technologies (e. p. the relative simplicity of the analysis presented here permits the connection between temperature targets and emission reductions to be readily assessed. if guided quantitatively at all. However. R. The scenario pathways developed in this paper are explicitly ‘backcasting’ and quantitative.10 3. but rather are premised on a cumulative emissions framing of climate change for which richer and more qualitative scenarios could be developed in terms of mitigation. From a mitigation perspective.g. This is certainly important for the transition of Annex 1’s existing built environment and infrastructures. exploring consistency. etc. where an opportunity still exists for societies to locate in areas geographically less vulnerable to the impacts of climate change. in relation to climate change it suggests the scale of current emissions and their relationship to the cumulative nature of the issue is not adequately understood. the language of many policy statements suggests such implications are not either understood or accepted. it is appreciably more important for the development of new built environments. Soc. agricultural practices and water regimes etc. Downloaded from rsta. carbon capture and storage. As it stands and in keeping with the dominant policy discourse. the scenarios are internally consistent. the framing of much of the detailed research and practice around adaptation. In general there remains a common view that underperformance in relation to emissions now can be compensated with increased emission reductions in the future. is informed primarily by the 2◦ C characterization of dangerous climate change.org on December 27. adaptation must consider more extreme climate change futures than those associated with 2◦ C [22]. Bows While the climate specialists within the CCC are aware of the implications of their analysis and conclude explicitly that ‘it is not now possible to ensure with high likelihood that a temperature rise of more than 2◦ C is avoided’ [8. a position that cannot be reconciled with the rate of reductions implied in high-level statements on 2◦ C. A (2011) . Phil. 16]. and range from top-down and quantitative through to more bottom-up and qualitative assessments. nuclear power.royalsocietypublishing. These approaches vary in terms of ‘backcasting’ and ‘forecasting’. They are not vision-based. Trans. as the impacts of rising temperatures are unlikely to be linear and also given rising temperatures are increasingly likely to be accompanied by additional feedbacks and hence further temperature rises. within the non-Annex 1 nations. In that sense. the gap between the scientific and policy understanding of the challenge needs urgently to be addressed. What is perhaps less evident is the implication of this gap for adaptation. 10 Such geographical vulnerability will need to be considered alongside other cultural. marine-based biofuels. Yet.9 Although for some environmental concerns delaying action may be a legitimate policy response. infrastructures. assessing plausibility and providing policy guidance [23].

are more robustly incorporated than is possible through the coarser regimes reliant on global warming potential.royalsocietypublishing. The scenario pathways are all premised on a cumulative emission budget approach. This is not a criticism of existing bottom-up analyses. Downloaded from rsta. the budgets within Anderson & Bows’ analysis were for the basket of six Kyoto gases. if not all. (i) The CO2 -plus regime (C +) and twenty-first century budgets The budgetary regime used by Macintosh [3] separates CO2 emissions from non-CO2 greenhouse gases and aerosols by applying Meinshausen et al. the link between aerosol emissions and assumptions about fossil fuel combustion and rates of deforestation. given the analysis here illustrates pathways offering an ‘unlikely to extremely unlikely’4 chance of exceeding 2◦ C. However. both are considered in this paper. but a recognition that the range of impacts associated with different levels of climate change and differential impacts on temperature and precipitation make bottom-up analysis much more challenging. few if any energy scenarios addressing mitigation include reductions in efficiency of thermal power stations if the temperature of cooling water rises. including aerosols. if not impossible. Given there are merits and drawbacks for each of the budgetary regimes. the potential of culturally distinct migrants to embed alternative practices into established transport and housing energy use. how drought conditions may impact energy use for desalination and grey-water recycling or the impacts of changing precipitation and temperature on biomass yields. the scenario pathways demonstrate the disjuncture between such high-level statements and the emission pathways proposed by many policy-advisers and academics. A (2011) .org on December 27. although offering significant scientific merit. building particularly on the work of Macintosh [3] and Anderson & Bows [2. 11 Forexample. Soc.11 (a) Cumulative emission budget The scenario pathways developed in this paper illustrate quantitatively the scale of mitigation implied in high-level policy statements on 2◦ C. for a given temperature and assuming other factors remain unchanged. However.26].24] but also on a range of wider studies [7. While Macintosh [3] focused on CO2 -only emissions in correlating twenty- first century budgets with global mean temperatures (denoted by the CO2 plus regime).25. Macintosh’s high budget is excluded from the analysis. and with direct reference to Annex 1 and non-Annex 1 nations. Phil. The CO2 -only budgets considered in this paper are the same as the middle and lower estimates used by Macintosh [3]. the approach poorly represents the contextual framing of emission scenarios. The advantage of this regime is that non- CO2 emissions. with regard to achieving consistency. the cumulative budget for CO2 -only is lower than would be the equivalent CO2 e greenhouse gas value.’s [7] assumptions on the net radiative forcing of the non-CO2 components. Trans. bottom-up mitigation analyses where consistency is constrained to issues of mitigation with climate related impacts typically exogenous to the analyses. Macintosh also analysed a higher budget of 2055 GtCO2 (560 GtC) as an ‘outer marker’ of abatement necessary for avoiding a 2◦ C. for example. R. 2010 Beyond dangerous climate change 25 contrasts with most. Moreover. Consequently.

Within this paper data are aggregated for the latest year available from a number of different sources.royalsocietypublishing. A (2011) . 14 ‘Twenty-first century low-emission’ scenarios are premised on low fossil fuel combustion and low deforestation rates. it assumes the ‘CO2 equivalence’ of Kyoto gases reasonably captures the warming implications of non-CO2 emissions from producing food for an increasing and more affluent population. However. The first is informed by the Garnaut Climate Change Review’s 450 ppm CO2 e stabilization scenario [27]12 and according to Macintosh [3] provides an approximate 50 per cent chance of not exceeding 2◦ C. The two CO2 e budgets within this paper are those used within Anderson & Bows [2] and represent the low (1376 GtCO2 e) and high (2202 GtCO2 e) ends of the IPCC AR4 cumulative emission range for stabilization at 450 ppmv CO2 e [29]. 2010 26 K. it is necessary to use up-to-date and complete emissions data to construct future emission pathways. 1578 GtCO2 (430 GtC) and 1321 GtCO2 (360 GtC).’s [7] model assumptions can be calculated using the PRIMAP tool [28] if the 2000–2049 emissions are known. assumes the correlation between global mean temperature and cumulative emissions of the basket of six Kyoto gases as adequate for informing policy-makers of the scale of mitigation necessary. 15 Refers to the lowest level of annual emissions considered viable in the scenario. Bows The two remaining budgets. according to Macintosh reflects the ‘risk that climate-carbon cycle feedbacks respond earlier and more strongly than previously believed’ and corresponds with a higher probability of not exceeding 2◦ C. Consequently. are taken directly from Macintosh [3]. The probabilities related to 2◦ C in the B6 regime do however take into account the radiative forcing of the different emissions including aerosols based on assumptions embedded in Meinshausen’s et al. but it more appropriately captures the contextual implications of alternative emission scenario pathways.’s PRIMAP tool [28] does not permit a direct calculation of the probability of exceeding 2◦ C for emission pathways that maintain a substantial and long-term emission burden.’s PRIMAP tool for estimating probabilities of exceeding 2◦ C [28].13 (ii) The basket of six regime (B6) and twenty-first century budgets The B6 regime. the CO2 e regime is not as scientifically robust as the CO2 -only regime. 12 For more details see Macintosh [3]. Soc. In relation to aerosols it assumes they are both short-lived and sufficiently highly correlated with fossil fuel combustion and deforestation as to have little net impact on temperatures associated with twenty-first century low- emission scenario pathways [10]. The latter.14 Moreover. Currently Meinshausen’s et al. These emissions are typically related to agriculture and food production. R. Trans. Evidently. (b) Empirical data The continued and high level of current emissions is consuming the twenty- first century emission budget at a rapid rate. 13 Probabilities based on Meinshausen et al. Phil. Downloaded from rsta.org on December 27. Anderson and A. a coarse-level but nevertheless adequate estimate is possible if the long lived gases within the ‘emissions floor’15 are added to the 2000–2050 cumulative values. used previously by Anderson & Bows [2].

though inevitably coarse-level. However. Their figures are estimated based on deforestation statistics published by the United Nations Food and Agriculture Organization [42] and a bookkeeping method up until 2005. 2010 Beyond dangerous climate change 27 (i) Energy and industrial process emissions For energy and process related CO2 emissions from 2000 until 2008.royalsocietypublishing. This crude method of apportionment was used previously (e. (iii) Non-CO2 greenhouse gas data The non-CO2 greenhouse gas emission data for 2000–2005 are based on the US Environmental Protection Agency (EPA) estimates [44]. Non-Annex 1 international aviation CO2 data are taken from the International Energy Agency [33] as non-Annex 1 nations do not submit this information to the UNFCCC. the data are not disaggregated into national statistics.16 (ii) Deforestation and land-use change For deforestation and land-use change (hereafter referred to as ‘deforestation’) data between 2000 and 2008 carbon emissions are again taken from the Global Carbon Project Carbon Budget Update 2009 [41]. and given difficulties in apportioning international shipping emissions to nations [38. data are not taken from CDIAC as their bunker fuel CO2 emission data are not disaggregated between nations and global marine bunker emissions are based on sales records that currently underestimate significantly the global greenhouse gas emission burden [31]. international aviation CO2 for Annex 1 nations is taken from the memo submissions to the United Nations Framework Convention on Climate Change (UNFCCC) [32]. the data have previously been subject to high levels of uncertainty [34–36]. The 2006–2008 emissions are derived from estimates of fire emissions using satellite data from the Oak Ridge National Laboratory Distributed Active Archive Center Global Fire Emissions Database in combination with a biogeochemical model [43]. [36]). For 2006 and 2007. Downloaded from rsta. data are taken from an interpolation between the 2005 and 2010 EPA projections. division of emissions between Annex 1 and non-Annex 1 nations. A (2011) . The nationally constructed data from this source exclude CO2 emissions from international aviation and shipping. data are taken for each Annex 1 and non-Annex 1 nation from the Global Carbon Project using the Carbon Dioxide Information Analysis Centre (CDIAC) [30] and aggregated to produce an Annex 1 and non-Annex 1 total. Soc. These data provide a figure for the global aggregated CO2 and other greenhouse gas emissions between 1990 and 2007. However. Phil. R.org on December 27. Trans. To include these additional emissions. To estimate the proportion of international shipping CO2 emissions split between Annex 1 and non-Annex 1 nations. 16 Ahybrid approach for apportioning aviation emissions between regions may provide insights into potential apportionment regimes for shipping [40]. is also used here as an adequate.39]. For international marine bunkers.g. Instead. a recent study by the International Maritime Organisation [37] has produced a time series for greenhouse gas emissions associated with international shipping activity. an assumption is taken that shipping activity is directly proportional to each nation’s proportion of global GDP.

4. — Construct emission pathways for the Annex 1 nations for which the cumulative emissions. Anderson and A. Trans. Non-Annex 1 nations are assumed to exhibit 0. 2010 28 K.royalsocietypublishing. Annex 1 and non-Annex 1 non-CO2 greenhouse gas emissions are assumed to proportionally follow the percentage change in their CO2 counterparts. it is assumed that their growth is also impacted by the economic downturn. again half the recent decade’s average. after which they grow at 2 per cent in 2010.e. then the rate of growth for non-CO2 greenhouse gas emissions is also assumed to halve. if the rate of growth halves for Annex 1 CO2 emissions. In the absence of such data. Soc.org on December 27. Phil. the 2010 growth figure is assumed to be half the recent decade’s average (i. For the emissions of non-CO2 greenhouse gases. growth in global fossil fuel CO2 (excluding bunkers) is assumed to be −3. but given China is already reporting high levels of growth for early 2010. estimates draw on the work of Macintosh [3] and the Global Carbon Project [41]. it is estimated Annex 1 nations’ emissions declined by 2 per cent in 2008. Scenario pathway development Following a brief explanation of how historical and deforestation emissions are accounted for. when added to non-Annex 1 and deforestation emissions. data were not available for either 2008–2010 bunker fuel emissions or 2009–2010 domestic fossil fuel CO2 emissions. Downloaded from rsta.7%). A (2011) . particularly those associated with energy. — Decide on a global cumulative CO2 and greenhouse gas emission budget associated with a range of probabilities of exceeding 2◦ C.5 per cent decline in emissions in 2009. Given the international marine bunker figures are based on proportions of global GDP. In other words. For Annex 1 nations. remaining static in 2009 and 2010. the construction of the scenario pathways involves the following steps. do not exceed the global ‘2◦ C’ cumulative budget.0 per cent in 2009 rising to 1. 2. — Construct emission pathways for the non-Annex 1 nations with varying peak dates. Although the crisis is beginning to show within 2008 emissions inventories. R. the same percentage growth rates for national emission trends are applied as for Annex 1 and non-Annex 1 domestic CO2 emissions between 2008 and 2010. Bows This dataset is identical to the one used within Anderson & Bows [2] but in this case individual national statistics are aggregated to produce the Annex 1 and non-Annex 1 totals. Non-Annex 1 nation aviation bunkers are assumed to be stable at 2007 levels until 2009. For international aviation bunkers. However. Consequently. (c) Economic downturn The economic downturn of 2007–2009 had a direct impact on greenhouse gas emission growth rates.5 per cent in 2010. stabilizing at 0 per cent in 2010. Given the absence of recent data from the EPA. fossil fuel CO2 (excluding bunkers) is assumed to decline by 6 per cent in 2009. non-CO2 greenhouse gas emission growth rates are typically lower than those for global fossil fuel CO2 by approximately 1–2% per year.

However. As it stands. 18 Emissions not attributable to any specific geographical location.18 is a pragmatic decision that. the authors demonstrate how China’s historical cumulative emissions are only one-tenth of the average in industrial countries and one-twentieth that of the USA.org on December 27. the implications of including twentieth century emissions and the concept of emission debt may guide the scope and scale of climate-related financial transfers (arguably as reparation) between Annex 1 and non-Annex 1 nations. — Assess the potential future climate impact of these more ‘politically acceptable’ and ‘economic feasible’ pathways. Soc. Deforestation emissions are treated as a global overhead and thus removed from the available emission budgets prior to developing the pathways. (a) Historical emissions In developing emission pathways for Annex 1 and non-Annex 1 regions it is necessary to make explicit which region is deemed responsible for which emissions. Whilst such an outcome may have some moral legitimacy. the approach adopted for this paper in which historical (and deforestation) emissions are taken to be global overheads. R. it evidently would not provide for a politically consensual framing of emission apportionment. While the following analysis focuses specifically on the period 2000–2100.17 Getting an appropriate balance of responsibilities is a matter of judgement that inevitably will not satisfy all stakeholders and certainly will be open to challenge. Building on this cumulative emissions per capita approach. Phil.19 (b) Deforestation emissions To explore the constraints on emissions from Annex 1 nations of continued growth in emissions from non-Annex 1 nations. it is important to reflect briefly on the treatment of recent historical emissions. industrial processes and agriculture are split between Annex 1 and non-Annex 1. only data for fossil fuel combustion. given most Annex 1 countries have already deforested (emitting CO2 ) it could 17 Factoring twentieth century emissions from Annex 1 nations into calculations of the ‘fair’ emission space available for Annex 1 in the twenty-first century would leave Annex 1 nations already in ‘emission debt’. errs in favour of the Annex 1 nations. A (2011) . However.royalsocietypublishing. 2010 Beyond dangerous climate change 29 — Construct emission pathways for the non-Annex 1 nations with a 2030 peak date and more ‘orthodox’ annual reduction rates following the peak. 19 It is worth noting that a recent paper [45] based on analysis undertaken at Tsinghua University in Beijing makes the case that ‘reasonable rights and interests should be strived for. based on the equity principle. — Construct emission pathways for the Annex 1 nations with a 2015 peak date and ‘orthodox’ annual emission-reduction rates following the peak. if anything. However. the highly constrained emission-space now remaining for a 2–3◦ C rise in global mean surface temperature leaves little option but to explicitly neglect the responsibility of historical emissions in developing pragmatic twenty-first century emission profiles. Downloaded from rsta. reflected through cumulative emissions per capita’. a case could be made for considering the responsibility for twentieth century emissions in apportioning future twenty-first century emission-space between Annex 1 and non-Annex 1 regions. Trans. Given temperature correlates with cumulative emissions of greenhouse gases. Such an approach could be argued to unreasonably favour non-Annex 1 nations as deforestation emissions occur within their geographical boundaries.

dotted line. non-Annex 1. (c) C+3) are for the same cumulative twenty-first century CO2 budget of 1321 GtCO2 (blue line. also be considered unreasonable to ascribe all of the non-Annex 1 deforestation emissions solely to non-Annex 1 nations. 11% per year). (b) C+2. The second three (C+ pathways 4–6 in figure 2) use the mid-level CO2 budget from the same paper. CO2 emissions associated with international bunkers and deforestation are included. will be reduced as a consequence of the emissions from deforestation. global including deforestation). R. along with the budget for Annex 1 nations’ energy emissions. the updated estimate used for this paper (266 GtCO2 over the twenty-first century) continues in this optimistic vein. While non-CO2 greenhouse gases are not included in the C+ scenario pathway. Bows (a) 50 (b) (c) 40 GtCO2 yr–1 30 20 10 0 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 year year year Figure 1. C+1 assumes non-Annex 1 emission growth continues at lower than economic downturn rates from 2010 to 2015 (3% per year) and that emissions peak in 2020. Annex 1. Anderson and A. The first three scenario pathways (C+ pathways 1–3 shown in figure 1) use the lowest CO2 budget from Macintosh [3]. Non-Annex 1 nations increase their emissions to 71 per cent higher than Phil. CO2 scenarios for approximately 37% chance of not exceeding 2◦ C. The global overhead approach applied here does not absolve non-Annex 1 nations of responsibility for deforestation emissions. The deforestation scenario used throughout the paper is taken as an average of the two scenarios used within Anderson & Bows [2]. as their available budget for energy-related emissions. red line. All scenario pathways ((a) C+1. Despite such significant reductions in Annex 1 nations (approx. Soc. Downloaded from rsta. A (2011) . For global emissions to remain within the budget. but updated to include the most recent emission estimates provided by the Global Carbon Project [41]. (c) CO2 plus (C +) All C+ scenario pathways take the development of emissions within the non- Annex 1 nations as the starting point and then build a related Annex 1 emission pathway that holds CO2 emissions within the chosen budget.org on December 27. 2010 30 K. Trans. 98 per cent by 2050. Annex 1 nations are assumed to reduce their emissions from 2011 onwards towards virtually complete decarbonization by 2050. This scenario pathway results in a 56 per cent reduction from 1990 levels in emissions for Annex 1 nations by 2020. non-Annex 1 nations’ emissions still need to decline at 6 per cent per year following their peak in 2020 if global emissions are to remain within the cumulative budget. The original Anderson and Bows scenarios were optimistic compared with scenarios within the literature.royalsocietypublishing.

[7. Annex 1. CO2 scenarios for approximately 50% chance of not exceeding 2◦ C. Here.org on December 27. if this is the case. R. and peak in 2025. Downloaded from rsta. Phil. (a) 50 (b) 40 GtCO2e yr–1 30 20 10 0 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 year year Figure 3.royalsocietypublishing. probabilities may vary slightly for the same twenty-first century budget. global including deforestation). Following the same approach. Kyoto gas scenarios for approximately 39–48% chance of not exceeding 2◦ C ((a) B6 1 not viable). the cumulative twenty-first century CO2 e budget is 2202 GtCO2 e (blue line. 1990 levels by 2020 and then reduce them to 76 per cent below 1990 levels by 2050. total including deforestation). red line. Soc. Trans. 2010 Beyond dangerous climate change 31 (a) 50 (b) (c) 40 GtCO2 yr–1 30 20 10 0 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 year year year Figure 2. this scenario pathway is not compatible with the lower of the cumulative carbon budgets. For (b) B6 2. ‘best estimates’ are presented. However. non-Annex 1. (b) C+5. All scenario pathways ((a) C+4. non-Annex 1. Using the PRIMAP tool developed by Meinshausen et al. dotted line. Annex 1. red line. non-Annex 1 nations use the entire carbon budget and leave no emission budget for Annex 1 nations.21 20 PRIMAP provides a range of probabilities and a ‘best estimate’ using cumulative emissions between 2000 and 2049. C+2 has non-Annex 1 emissions continuing to grow at a lower than pre-economic downturn rate until 2020 (3% per year). owing to differences in the 2000–2049 emissions. Thus. 21 Given it is not possible to have an immediate cessation of emissions from all Annex 1 nations. A (2011) . (c) C+6) are for the same cumulative twenty-first century CO2 budget of 1578 GtCO2 (blue line. dotted line.28] this scenario pathway is estimated to have a 36 per cent20 probability of exceeding 2◦ C.

C+5 has a 52 per cent chance of exceeding 2◦ C. non-Annex 1 nations’ emissions must still reduce at 7–8% per year after the peak date in order for global emissions to remain within the cumulative budget. Both Annex 1 and non-Annex 1 emissions are assumed to decline post-peak at 5–6% per year. Downloaded from rsta. and as a result of a step-change in emission growth from non-Annex 1 nations. Soc. A (2011) . Within this scenario pathway. Bows To explore the potential of providing for more acceptable reduction rates while still offering a ‘reasonable’ chance of not exceeding 2◦ C. Global emissions are 67 per cent below 1990 by 2050.royalsocietypublishing.org on December 27. Annex 1’s emissions peak by 2010 and decline at 7–8% per year. but are assumed to reach a peak by 2015. and if Annex 1 emissions begin to reduce immediately (as in C+1). all B6 pathways (figures 3 and 4) take the development of emissions within the non-Annex 1 nations as the starting point and then build a related Annex 1 emission scenario pathway that must. (d) Basket of six scenario pathways (B6) In a similar approach to the C+ pathways. in addition to an immediate Annex 1 reduction. R. C+5 again uses the higher budget within Macintosh [3] but assumes non- Annex 1 emissions to continue to grow at 4 per cent per year rates until 2020. keep total greenhouse gas emissions within the chosen budget. In addition to considering the effect of non-CO2 greenhouse gases contributing to the overall budget. This plausible but highly unlikely scenario has a 37 per cent chance of exceeding 2◦ C according to PRIMAP [28]. This scenario pathway has an estimated 50 per cent chance of exceeding 2◦ C according to PRIMAP [28]. C+4 assumes non-Annex 1 nation emissions grow at 4 per cent per year until 2015 peaking in 2020. The Annex 1 nations have more room to grow in early years than in C+1. The significant difference between the B6 and the C+ pathways is that the given emission budget is assumed to apply to the full basket of 6 greenhouse gases mirroring the approach taken in Anderson & Bows [2]. 2010 32 K. Non-Annex 1 emissions are 186 per cent above 1990 levels in 2020 and 45 per cent below them by 2050. global emissions peak in 2011 and Annex 1 nations’ future emissions do not grow any higher than current levels. in this case. no emission space is available for Annex 1 nations. global emissions are broadly flat between 2014 and 2022. The next three scenario pathways (figure 2) use the higher budget of 1578 GtCO2 within Macintosh [3]. an essential difference is the requirement for significant emissions space post-2050 to allow for greenhouse gas emissions (specifically N2 O and CH4 ) associated with food production for an approximate 9. Global emissions thus peak in 2019 with Annex 1 emissions 6 per cent below 1990 by 2020 and 84 per cent by 2050. Given this. To remain within budget. C+6 uses the same higher budget but illustrates that if reductions are lower. although emissions are highest in 2020. Trans. at 4–5% per year for non-Annex 1 nations following the peak date. Thus the penalty for a five year delay in the non-Annex 1 peak date is an additional 2 per cent per year on top of the emission reduction rate for both Annex 1 and non-Annex nations. Anderson and A. C+3 assumes non- Annex 1 nations’ emissions grow at a much reduced rate (1% per year) until 2025. and peak in 2025 with a rapid decline to a maximum of 7–8% per year. Following the dip in emissions owing to the economic downturn.2 billion global population (based on UN Phil.

This is in line with the value chosen by the UK [8] and results in an estimated 0. Annex 1. 2010 Beyond dangerous climate change 33 (a) 50 (b) 40 GtCO2e yr–1 30 20 10 0 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 year year Figure 4. red line. Given the food-related non- CO2 greenhouse gases post-2050. Given that within the B6 scenario pathways there is a substantial cumulative emission total for the post- 2050 emissions (with at least 300 GtCO2 e from greenhouse gases associated with Phil. non-Annex 1. while for Annex 1 they gradually build from around 3 per cent per year for 2015 to 2020 to 6 per cent later in the century. Soc. B6 1 uses the IPCC ‘low’ emission budget and assumes that between 2010 and 2015 non-Annex 1 emissions grow at slightly lower (3% per year) than pre- economic downturn rates and peak by 2020 (figure 3).3 billion in Annex 1 nations [46].46]. and 1. The PRIMAP tool to estimate the probability of exceeding the 2◦ C threshold assumes the vast majority of emissions are released pre-2050 (fig.86 GtCO2 e per year for Annex 1 nations. while Annex 1 nation emissions decline from 2010 onwards.95 GtCO2 e per person for 2010 [44. R. median estimate for 2050 [46]). In other words.1 GtCO2 e as a minimum annual greenhouse gas emission for non-Annex 1 nations and 0. Assuming by 2050 there are 7. (b) B6 4). this would allow approximately 5. Trans. and assuming food consumption is more evenly balanced between Annex 1 and non-Annex 1 nations than currently. where non-Annex 1 emissions peak in 2020. emissions from 2017 onwards for non-Annex 1 nations must tend immediately towards the emissions floor of 5. A (2011) .org on December 27. The cumulative twenty-first century CO2 e budget is 2202 GtCO2 e.1 GtCO2 e. Blue line.67 tCO2 e per person from 2050 onwards for food-related non- CO2 greenhouse gases compared with an approximate figure of 0. Kyoto gas scenarios for approximately 38–48% chance of not exceeding 2◦ C ((a) B6 3. B6 2 makes identical assumptions to B6 1 but for the ‘high’ IPCC emission budget (figure 3). the assumed minimum level of greenhouse gas production related to food is more optimistic still at 6 GtCO2 e per year as opposed to 7. Downloaded from rsta.royalsocietypublishing.5 GtCO2 e per year. The additional space allowed leads to a viable scenario pathway. Emission reductions for non-Annex 1 nations in this case are 6 per cent per year.9 billion people in non- Annex 1 nations. this scenario pathway is not viable. total including deforestation. dotted line. In an update to the previous Anderson & Bows [2] study. 2 in [7]).

then reducing at 22 Thisis an explicitly conservative assumption and results in slightly higher probabilities of not exceeding 2◦ C than would be the case if some allowance were to be made for post-2050 emissions of methane. figure is in brackets). — The shorter-lived nature of methane compared with N2 O results in a negligible impact on post-2050 warming from methane. emissions decrease at 6 per cent per year. B6 2 has at least a 39 per cent chance of exceeding 2◦ C and is potentially as high as 48 per cent.22 — N2 O and methane each account for approximately 50 per cent of the non- CO2 greenhouse gases post-2050 (Smith. Bows food production). Anderson and A. To account for this underestimate. R. — Thus 150 GtCO2 e of cumulative emissions are added to the pre-2050 emissions to estimate an alternative probability. following a levelling off of emissions until 2014. B6 3 has the same probability of exceeding 2◦ C as B6 2. — The amount of non-CO2 greenhouse gas emissions released per year possible post-2050 is 6 GtCO2 e (in line with assumptions made by the UK CCC [8]) of which 3 GtCO2 e per year is from N2 O. This scenario pathway has at least a 38 per cent probability of exceeding 2◦ C. B6 3 assumes non-Annex 1 nation emissions peak in 2025 following a growth of 3 per cent per year between 2010 and 2020 (figure 4). A (2011) .royalsocietypublishing.org on December 27. More gradual reductions in emissions from non-Annex 1 nations would render this scenario pathway impossible. The emission reductions post-peak for non-Annex 1 nations are 4–5% per year. an alternative probability is calculated assuming the following. 2010 34 K. Both B6 3 and B6 4 take the IPCC’s ‘high’ cumulative budget as a constraint. B6 4 mirrors the assumptions within C+3. (e) Orthodox scenario pathways The final scenario pathways developed are unconstrained by a particular emission budget (figure 5). With steep emission reductions for non-Annex 1 nations post-peak of 6 per cent per year. with considerably slower growth of 1 per cent per year until a peak date in 2025 (figure 4). and potentially as high as 47 per cent once post-2050 emissions are factored in. whereas for Annex 1 nations. inputting the 2000–2049 cumulative total for each B6 scenario into PRIMAP will result in an underestimate of the probability of exceeding 2◦ C. PRIMAP estimates an approximate ten percentage point increase in probability of exceeding 2◦ C (see table 1. Trans. If 150 GtCO2 e is added to the cumulative emission total for each scenario. Annex 1 nations would need to reduce emissions from 2010 onwards at more than 10 per cent per year to remain within the high IPCC cumulative budget. For both the C+ and B6 regimes. pathways are constructed that assume non-Annex 1 nations’ emissions continue to develop along their current trajectory until 2025. For example. Phil. peaking in 2030. Downloaded from rsta. Soc. personal communication).

royalsocietypublishing. If a different ‘emission floor’ were to be used. Table 1. 2010 [891] [2501] a These scenario pathways are not viable as they could not remain within the carbon budget prescribed. R.c [1376] 2007 2017 2017 95% (95%) 61% (61%) — — — — [265] [841] B6c 2 [2202] 2010 2020 2017 25% (82%) +135% (46%) 4–6% 5–6% 3% 39% (48%)d [639] [1293] B6c 3 [2202] 2007 2025 2013–2018 57% (95%) +154% (24%) 8–10% 6–7% 4–5% 39% (48%)d [429] [1503] Beyond dangerous climate change B6c 4 [2202] 2007 2025 2013 34% (90%) +111% (17%) 6% 4–5% 4–5% 38% (47%)d [552] [1380] orthodox C+ 2741 2015 2030 2027 2% (60%) +223% (+163%) 3% 3% 3% 88% [729] 1747 orthodox B6 [3662] 2015 2030 2028 5% (62%) +180% (128%) 3% 3% 3% 88% (92%)d Downloaded from rsta. Trans.org on December 27. A (2011) +1714% (54%) 313 742 C+2a 1321 2007 2025 2007 100% (100%) +193% (27%) — 6–7% 6–7% 139 916 C+3 1321 2007 2025 2007 56% (98%) +143% (54%) 10–11% 7–8% 7–8% 37% 313 742 C+4 1578 2007 2020 2019 6% (84%) +186% (45%) 5–6%b 5–6% 5–6% 50% 532 780 C+5 1578 2007 2025 2020 44% (95%) +220% (32%) 8% 7–8% 7–8% 52% 363 949 C+6a 1578 2007 2025 2024 100% (100%) +220% (+38%) — 4–5% 4–5% — 153 1159 B6 1a. Summary of scenario pathway characteristics. . emission reduction rates would be altered for the same cumulative values. Annex 1 peak non-Annex 1 approximate % global 21st century date/21st peak date/21st of exceeding CO2 or century century Annex 1 % non-Annex 1 % post-peak 2◦ C (based on greenhouse gas cumulative cumulative reduction on reduction on post-peak post-peak global rate of 2000–2049 budget in emissions emissions global 1990 levels 1990 levels Annex 1 non-Annex 1 reduction emissions scenario GtCO2 or in GtCO2 in GtCO2 peak by 2020 by 2020 rate of rate of (includes using pathway [GtCO2 e] or [GtCO2 e] or [GtCO2 e] date (2050) (2050) reduction reduction deforestation) PRIMAP) C+1 1321 2007 2020 2012 56% (98%) 10–11% 6–7% 6–7% 36% Phil. c All B6 scenario pathways assume an ‘emission floor’ of 6GtCO e for food-related emissions for an approximate 9 billion population post-2050 until 2 2100. b This is the reduction rate following the period of relatively stable emissions until 2016. d The figure in brackets illustrates a higher probability to take into account the ongoing emissions associated with food production as opposed 35 to greenhouse gas emissions tending to zero. Soc.

One exceeded the lower of the two chosen 23 The pathway of the CCC’s [8] most challenging scenario. thus a reasonable probability of exceeding 4◦ C. A (2011) . Stern concludes ‘it is likely to be difficult to reduce emissions faster than around 3 per cent per year’ [6. Trans. R.org on December 27. Orthodox B6 has cumulative emissions of greenhouse gases of 3662 GtCO2 e. dotted line. Cumulative emissions of approximately 2700 GtCO2 are associated with stabilization of 550 ppmv CO2 . ‘2016:4% low’. 2010 36 K. Blue line. The most stringent of the ADAM scenarios assumed emission reduction rates of approximately 3 per cent per year between a 2015 peak and 2050 [47 fig.741 GtCO2 . 32]. Bows (a) (b) 60 GtCO2e yr–1 40 20 0 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 year year Figure 5. non-Annex 1. Anderson and A. Stern [6] states ‘there is likely to be a maximum practical rate at which global emissions can be reduced’ pointing to ‘examples of sustained emissions cuts of up to 1 per cent per year associated with structural change in energy systems’ and that ‘cuts in emissions greater than this (1%) have historically been associated only with economic recession or upheaval’. Figures in excess of 3500 GtCO2 e can be assumed to be closer to the 750 ppmv range. pp. their twenty-first century cumulative budgets suggest the future temperature increase compared with pre-industrial times is more likely to be of the order of 4◦ C rather than 2◦ C. total including deforestation. two were rejected for exceeding the constraints on cumulative CO2 budgets related to the 2◦ C temperature threshold. emissions are assumed to be relatively stable with a peak in 2016 and subsequent emission reductions of 3 per cent per year. a rate considered politically and economically acceptable23 (3% reduction per year). 201–204]. These scenario pathways both result in an 88 per cent chance of exceeding the 2◦ C threshold (potentially 92% for orthodox B6). cumulative emissions of CO2 alone are 2.royalsocietypublishing. Phil.5 per cent per year from all sources. Soc. Both plots illustrate ‘orthodox’ mitigation pathways with (a) C+ for CO2 only (twenty-first century cumulative emissions: 2741 GtCO2 ) and (b) B6 for Kyoto gases (twenty-first century cumulative emissions: 3662 GtCO2 e). Discussion (a) CO2 plus (C+) Although six C+ pathways were developed in the previous section. Emission scenarios for approximately 88–92% chance of not exceeding 2◦ C. red line. For the Annex 1 nations. For orthodox C+. 5. p. Annex 1. Furthermore. 2. Downloaded from rsta. has post-peak emissions reducing at 3.

Those pathways remaining within the lower budget required an immediate and rapid decline in Annex 1 emissions and an early peak in non-Annex 1 emissions.e. post-peak emission reductions of 4–5% per year were insufficient to stay within the cumulative budget. this probability is likely to be a significant underestimate. Moreover. given a significant portion of emissions are attributable to food production post-2050. it is more Phil. Even if non-Annex 1 emissions grow at much slower rates to a 2025 peak. it became apparent immediately that the ‘low’ IPCC cumulative emission value was not viable if non-Annex 1 emissions peak as late as 2020. Consequently. to 2025). In all cases. The other surpassed the higher of the two cumulative budgets (approx. For the IPCC’s ‘high’ end of the range. no emissions space remains for Annex 1 nations. there continues to be an absence of any meaningful global action to mitigate emissions or set binding targets. This result is in line with the scenario pathway analysis within Anderson & Bows [2]. the rates of reduction for CO2 presented in table 1 illustrate the change necessary within the energy system primarily.org on December 27. to 1%.royalsocietypublishing. with a more probable figure closer to 48 per cent. emission reductions of 6 per cent per year for both Annex 1 and non-Annex 1 nations are necessary if non- Annex 1 emissions continue at recent growth rates and peak in 2020. figure 2 and table 1 illustrate that a 5 year delay in the peak year for non-Annex 1 nations (from 2020 to 2025) forces a 2 per cent increase in reduction rates globally in addition to an immediate emission reduction for Annex 1 nations. 50%) chance of not exceeding 2◦ C. all viable scenario pathways exhibit emission reduction rates well in excess of those typically considered to be politically and economically feasible. under the IPCC’s higher budget. Furthermore. in addition to the emissions growth and peak years. if these rates are sustained for a further five years (i. Trans. Given these pathways explicitly exclude non-CO2 greenhouse gas emissions. unless the latter’s emission growth was constrained to rates much lower than historical trends (i. R. 50% of not exceeding 2◦ C) because. 37% of not exceeding 2◦ C) owing to non-Annex 1 emissions both continuing with recent growth rates out to 2020 and peaking in 2025. 2010 Beyond dangerous climate change 37 budgets (approx. post-peak emission reductions of 4–5 per cent per year are still needed from the aggregate of all nations. Soc. compared with current growth of 3–4% per year [48]).e. Downloaded from rsta. A (2011) . For the higher (approx. figures 1 and 2 illustrate the need for complete decarbonization of the Annex 1 energy system by around 2050. the minimum probability of not exceeding the 2◦ C threshold achievable in this scenario pathway set is 38 per cent. (c) Orthodox In light of the recent Copenhagen negotiations. and non- Annex 1 post-peak reductions are 6 per cent per year or less. (b) Basket of six (B6) When developing the B6 pathways (figures 3 and 4). Annex 1 emissions continue to decline following the current economic downturn at rates in excess of 5 per cent per year. Therefore. Even if Annex 1 nations agree on the scale of necessary emission reductions. However.

Trans.org on December 27. the overarching data and trend lines underpinning Köne and Büke’s analysis were available at the time many of the scenarios were developed. 25 Bottom-up and built on typically idealized inputs with only limited regard for ‘real-world’ constraints. it is perhaps surprising they are central to so many scenarios. yet were without detailed analysis of potentially significant constraints on storage capacity. included as a ‘key energy supply technology’ in all but one of the 450ppmv scenarios reviewed. and then sustained emission reductions at rates considered politically and economically feasible.26 Over a third factored in negative emissions through the inclusion of geo-engineering in the form of ‘biomass with carbon capture and storage’ (CCS) technologies. A (2011) . significant ramping up of nuclear capacity is likely to require fast breeder reactors with major challenges associated with their widespread introduction. This non-contextual approach to technology extends to nuclear power. here too the integrated assessment modelling approach typically treats these wider concerns as exogenous and resolvable. (d) Simple and complex scenarios The scenarios developed in this paper are relatively contextual24 and as such complement the wealth of scenarios from more non-contextual integrated assessment models. no CCS power plants have yet being built and consequently large-scale CCS remains a theoretical possibility with no operating experience of capture rates (though many of the component processes have undergone testing). Phil. Whilst sufficient uranium exists for moderate increases in conventionally fuelled reactors. while it may be argued that the latter approach benefits from greater internal consistency and more theoretically coherent parameters. slow action to mitigate emissions on the part of Annex 1 nations. Although these will go some way towards addressing future high-carbon lock-in.25 However. the majority had a global emissions peak in 2010. are more closely aligned with much higher climate change futures associated with at least 3–4◦ C of warming. deforestation. Given the many unknowns around bio-CCS. Downloaded from rsta. the outputs are typically removed from the political and empirical reality within which responses to climate change are developed.royalsocietypublishing. Aside from the considerable bio-energy debate surrounding the sustainability of biofuels. 26 While Köne & Büke’s [48] paper was published after many of the scenarios referred to. 2010 38 K.27 At least half of the scenarios relied on significant levels of ‘overshoot’ (between 500 and 590 ppmv CO2 e)28 and 24 Though constrained explicitly to consider top-down emissions only with coarse high-level divisions between food. Soc. For example. Resulting cumulative emissions. R. the two orthodox scenario pathways paint a picture of ongoing non- Annex 1 emission growth. 27 The inclusion of bio-CCS also demonstrates a degree of non-contextual engagement with technology. To explore the implications of this. recent overviews of scenarios generated by a range of different international integrated assessment modelling communities [10. energy and industrial processes. Anderson and A.14] illustrate the non-contextual framing that typifies much of this form of analysis. Bows probable that non-Annex 1 nations will set targets based on levels of carbon or energy intensity improvements. this despite irrefutable evidence to the contrary [48]. while still within the bounds of possibility of not exceeding the 2◦ C threshold (8–12% chance). it is unlikely that emission growth rates will be significantly moderated during the coming decade. These bio-CCS scenarios all included wider CCS facilities. Of the principal 450 ppmv scenarios reviewed.

However. leaves space for the simpler. appears to be the same question: what emission- reduction profiles are compatible with avoiding ‘dangerous’ climate change? However. the conclusions arising from this paper are significantly bleaker than those of the authors’ 2008 paper. Conclusions Over the past five years a wealth of analyses have described very different responses to what. Soc. For all practical purposes aggregated emissions related to Annex 1 are the same as those for Annex B. on closer investigation.29 The non-contextual framing of many complex modelling approaches (including integrated assessment modelling). despite the actual date being around 2006. Trans. [49. with Annex 1 and non-Annex 1 nations returning rapidly to their earlier economic and emissions trajectories and with the failure of Copenhagen to achieve a binding agreement to reduce emissions in line with 2◦ C. the prospects for avoiding dangerous climate change.50]). (1) What delineates dangerous from acceptable climate change? (2) What risk of entering dangerous climate change is acceptable? (3) When is it reasonable to assume global emissions will peak? (4) What reduction rates in post-peak emissions is it reasonable to consider? (5) Can the primacy of economic growth be questioned in attempts to avoid dangerous climate change? 28 Overshoot scenarios remain characterized by considerable uncertainty and are the subject of substantive ongoing research (e. only the global economic slump has had any significant impact in reversing the trend of rising emissions. (e) Development on the authors’ 2008 paper Two years on from earlier analysis by Anderson & Bows [2]. In both these regards and with the continued high-level reluctance to face the real scale and urgency of the mitigation challenge. are increasingly slim. R. Downloaded from rsta. A (2011) . 29 Within the integrated assessment modelling scenarios referred to. Furthermore. allied with their inevitable opaqueness and often abstract and implicit assumptions. if they exist at all. disaggregating global into Annex 1 and non- Annex 1 emission pathways only serves to exacerbate the scale of this disjuncture between the rhetoric and reality of mitigation. 2010 Beyond dangerous climate change 39 several assumed fossil fuel CO2 emissions from non-Annex 1 nations would exceed those from Annex 1 as late as 2013–2025 [14]. more transparent and contextual approach to scenarios presented in this paper. Phil. but can also provide ‘contextual’ parameters and constraints to more complex modelling approaches.royalsocietypublishing. the division related to Annex B regions.g. the difference in responses is related less to different interpretations of the science underpinning climate change and much more to differing assumptions related to five fundamental and contextual issues. Making explicit the implications of particular assumptions (such as peak emission dates or very low probabilities of exceeding 2◦ C) provides insights that not only are intelligible to wider stakeholders and decision-makers. 6. at first sight.org on December 27.

However. although the implications of their more nuanced analyses are rarely communicated adequately to policy makers. but given many impacts (and the responsibility for them) extend well beyond such boundaries any regional assessment needs to be within the context of a more global perspective. Bows While (1) and. 160]. Anderson and A. while the rhetoric of policy is to reduce emissions in line with avoiding dangerous climate change. to a lesser extent. given that it is a ‘political’ interpretation of the severity of impacts that informs where the threshold between acceptable and dangerous climate change resides. Downloaded from rsta. given the CCC acknowledge ‘it is not now possible to ensure with high likelihood that a temperature rise of more than 2◦ C is avoided’ and given the view that reductions in emissions in excess of 3–4% per year are not compatible with economic growth. Within global emission scenarios. In relation to the first two issues. at their most crude level. annual rates of emission reduction beyond the peak years are constrained to levels thought to be compatible with economic growth—normally 3 per cent to 4 per cent per year.royalsocietypublishing. p. can be understood in relation to Annex 1 and non-Annex 1 emission profiles. in effect.21]. many academics and wider policy advisers undertake their analyses of mitigation with relatively high probabilities of exceeding 2◦ C and consequently risk entering a prolonged period of what can now reasonably be described as extremely dangerous climate change. Phil. 30 Regions can evidently identify what may constitute dangerous within their geographical boundaries. 31 If the impacts are to remain the principal determinant of what constitutes dangerous. in discussing arguments for and against carbon markets the CCC state ‘rich developed economies need to start demonstrating that a low-carbon economy is possible and compatible with economic prosperity’ [8. Moreover. the CCC are.32 Put bluntly. conceding that avoiding dangerous (and even extremely dangerous) climate change is no longer compatible with economic prosperity. Soc. then would it be more reasonable to characterize ‘1◦ C as the new 2◦ C’ ? 32 Assuming the logic for the 2◦ C characterization of what constitutes dangerous still holds. R. Nevertheless.30 the latter three have pivotal regional dimensions that. most policy advice is to accept a high probability of extremely dangerous climate change rather than propose radical and immediate emission reductions.31 there is little political appetite and limited academic support for such a revision. the recent reassessment of these impacts upwards suggests current analyses of mitigation significantly underestimate what is necessary to avoid dangerous climate change [20. 2010 40 K.33 This already demanding conclusion becomes even more challenging when assumptions about the rates of viable emission reductions are considered alongside an upgrading of the severity of impacts for 2◦ C. such as those developed by Stern [6]. 33 With policies themselves lagging even further behind in terms of both actual reductions achieved or planned for. those providing policy advice frequently take a much less categorical position. A (2011) . the Copenhagen Accord and many other high- level policy statements are unequivocal in both their recognition of 2◦ C as the appropriate delineator between acceptable and dangerous climate change and the need to remain at or below 2◦ C. However. (2) are issues for international consideration.org on December 27. Despite such clarity. on closer examination these analyses suggest such reduction rates are no longer sufficient to avoid dangerous climate change. For example. In stark contrast. Trans. the CCC [8] and ADAM [47]. and despite the evident logic for revising the 2◦ C threshold.

and with tentative signs of global emissions returning to their earlier levels of growth. but rather a bare and perhaps brutal assessment of where our ‘rose-tinted’ and well intentioned (though ultimately ineffective) approach to climate change has brought us. [9. as the scenario pathways developed within this paper demonstrate. Stern. for a period of planned austerity within Annex 1 nations36 and a rapid transition away from fossil-fuelled development within non-Annex 1 nations. remains untried for a large scale power station. the approach they adopt in relation to peaking dates is very different. at least temporarily. Moreover. which. 36 In essence. Consequently.21]). Moreover. The scale of this departure is further emphasized when disaggregating global emissions into Annex 1 and non-Annex 1 nations. 2010 represents a political tipping point. By contrast. the CCC. if it is to arise at all. Real hope and opportunity. Downloaded from rsta.10]) will have increased cumulative emissions and hence further increased probabilities of exceeding 2◦ C (see also footnote 27). Trans. R. will do so from a raw 34 The reference to ‘feasible’ technologies typically extends to carbon capture and storage.g. Soc. 35 With the only exception being C+4 where Annex 1 emissions are stable until 2016. Without such negative emissions. ADAM and similar analyses suggest they are guided by what is feasible. do global emissions peak by 2020. Phil. do not reflect any orthodox ‘feasibility’. a profound departure from the more ‘feasible’ assumptions framing the majority of such reports.g. Consequently. Stern and ADAM analyses. All premise their principal analyses and economic assessments on the ‘infeasible’ assumption of global emissions peaking between 2010 and 2016. The analysis within this paper offers a stark and unremitting assessment of the climate change challenge facing the global community. the impacts associated with 2◦ C have been revised upwards (e. The science of climate change allied with emission pathways for Annex 1 and non-Annex 1 nations suggests a profound departure in the scale and scope of the mitigation and adaption challenge from that detailed in many other analyses. reducing thereafter.org on December 27.34 However. sufficiently so that 2◦ C now more appropriately represents the threshold between dangerous and extremely dangerous climate change. in 2010. particularly those directly informing policy. the 2010 global peak central to many integrated assessment model scenarios as well as the 2015–2016 date enshrined in the CCC. 2010 Beyond dangerous climate change 41 In prioritizing such economic prosperity over avoiding extremely dangerous climate change. while in terms of emission reduction rates their analyses favour the ‘challenging though still feasible’ end of orthodox assessments. However. [20. it is often allied with biomass combustion to provide ‘negative’ emissions (§5d). the logic of such studies suggests (extremely) dangerous climate change can only be avoided if economic growth is exchanged. Only if Annex 1 nations reduce emissions immediately35 at rates far beyond those typically countenanced and only then if non-Annex 1 emissions peak between 2020 and 2025 before reducing at unprecedented rates. several of the major analyses (e. a planned economic contraction to bring about the almost immediate and radical reductions necessary to avoid the 2◦ C characterization of dangerous climate change whilst allowing time for the almost complete penetration of all economic sectors with zero or very low carbon technologies. A (2011) . this paper is not intended as a message of futility. There is now little to no chance of maintaining the rise in global mean surface temperature at below 2◦ C.royalsocietypublishing. despite repeated high-level statements to the contrary.

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