3 0 j u n e 2 0 1 6 | V O L 5 3 4 | n A T u R e | 6 3 1 2016 Macmillan Publishers Limited. All rights reserved i PeRsPecTiVe doi10.1038/nature18307Paris Agreement climate proposals need a boost to keep warming well below 2 C joeri Rogelj1,2, Michel den elzen3, niklas Hhne4,5, Taryn Fransen6, Hanna Fekete4, Harald Winkler7, Roberto schaeffer8, Fu sha9, Keywan Riahi1,10 how much adaptation may be required given the current level of mitigation ambition; how ‘intended’ national pro- posals will be implemented; how they will be financed; and the extent to which the INDCs contribute to the achievement of other goals of the UNFCCC by building on institutions that can support adaptation to climate change, technology advancement, development path transfor- mation, sustainable development and enhanced awareness. Although these issues are important for many countries, they fall outside the scope of this analysis. In this Perspective, we assess the implications of the current INDCs for GHG emissions, including the main factors and uncertainties that influence the levels of GHG emissions in 2030the latest year covered by the vast majority of INDCsand we explore the consistency of these reductions with the objective of the Paris Agreement to keep warming well below 2 C and pursue efforts towards 1.5 C. This work updates and expands work undertaken in the framework of the 2015 United Nations Environment Programme UNEP Emissions Gap Report18an author- itative annual assessment that has tracked climate policy action over the past six years, and provides a synthesis of a wide range of INDC model- ling studies19–29 that are available in the public domain. The number of INDCs considered by the studies that we assess here ranges from the 118 INDCs ted by 1 October 2015 to the 160 INDCs ted by 12 December 2015 Supplementary Tables 1 and 2. These 118 to 160 INDCs cover emissions from 145 to 187 out of 195 Parties to the UNFCCC, which in turn were responsible for roughly 88 to more than 96 of global GHG emissions in 201230. We also look at projections of global-mean temperature increase over the twenty-first century that would be consistent with the INDCs, and at post-2030 implications for limiting warming to no more than 2 C. Finally, we discuss options to 1ENE Program, International Institute for Applied Systems Analysis IIASA, Laxenburg, Austria. 2Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland. 3PBL Netherlands Environmental Assessment Agency, Bilthoven, The Netherlands. 4NewClimate Institute, Cologne, Germany. 5Environmental Systems Analysis Group, Wageningen University, Wageningen, The Netherlands. 6World Resources Institute, Washington DC, USA. 7Energy Research Center, University of Cape Town, Cape Town, South Africa. 8Universidade Federal do Rio de Janeiro COPPE/UFRJ, Rio de Janeiro, Brazil. 9National Center for Climate Change Strategy and International Cooperation, Beijing, China. 10Graz University of Technology, Graz, Austria. 11Australian-German Climate and Energy College, School of Earth Sciences, The University of Melbourne, Melbourne, Victoria, Australia. 12PRIMAP Group, Potsdam Institute for Climate Impact Research PIK, Potsdam, Germany. 2 | n A T u R e | V O L 5 3 4 | 3 0 j u n e 2 0 1 6 2016 Macmillan Publishers Limited. All rights reserved reseArcH PersPective Box 1 scenario definitions Scenarios represent alternative images of the future, or “[stories] about what happened in the future”76. They are neither predictions nor forecasts77, but tools to understand how the future might unfold under a consistent set of assumptions. In this analysis, we use four types of scenarios, drawn from a wide variety of sources. No-policy baseline scenarios. These are emissions projections that assume that no new climate policies have been put into place from 2005 onwards. We select these scenarios from the IPCC AR5 Scenario Database16, which is hosted at the International Institute for Applied Systems Analysis IIASA, https//tntcat.iiasa.ac.at/AR5DB/. By design, these no-policy baseline scenarios exclude climate policies, but may include other policies that can influence emissions and are implemented for other reasons, such as energy efficiency or energy security policies. Current-policy scenarios. These consider the most recent estimates of global emissions and take into account implemented national policies. This is different from the INDC scenarios described below, which reflect international pledges and intended policies. Here, we draw these scenarios from three global analyses19,29,78. INDC scenarios. These project how global GHG emissions evolve under a successful implementation of the INDCs. These projections are based on ten global INDC analyses19–29 Supplementary Table 2 provides an overview, in which calculations can be based on official estimates from countries or on documents ted to the UNFCCC such as national GHG inventories, national communications, biennial reports or biennial update reports. INDCs were ted before the Paris summit; under the Paris Agreement, future mitigation contributions will be referred to as NDCs, without the ‘intended’. 2 C scenarios. These are idealized global scenarios limiting warming to well below 2 C , keeping open the option of strengthening the global temperature target to 1.5 C . These scenarios are based on a subset of scenarios from the IPCC AR5 Scenario Database Supplementary Table 3 that meet the following criteria they have a greater than 66 probability of keeping warming to below 2 C by 2100 this probability does not drop below 60 at any point during the entire twenty-first century; until 2020, they assume that actions that were pledged earlier under the UNFCCC Cancun Agreement are fully implemented; and, after 2020, they distribute emission reductions across regions, gases and sectors so that the total discounted costs of the necessary global reductions are minimized. These scenarios distribute emissions reductions among regions in the most cost-optimal way, and are often referred to as least-cost or cost-optimal trajectories. However, this does not imply that the actual costs to achieve this cannot be distributed differently, for example, on the basis of other equity principles79. A separate set of scenarios is used to examine the post-2030 implications of current INDCs for 2 C Supplementary Table 4. All scenarios are expressed in terms of billions of tonnes of global annual CO2-equivalent emissions Gt CO2-eq yr−1. CO2 equivalence of GHGs has been calculated by means of 100-year global warming potentials80. further reduce global GHG emissions in 2030 from their INDC levels towards levels that are more consistent with a long-term global pathway that limits warming to well below 2 C. We use four scenario groups to frame the implications of the INDCs for global GHG emissions in 2030 no-policy baseline scenarios, cur- rent-policy scenarios, INDC scenarios and least-cost 2 C scenarios. Their definitions and descriptions are provided in Box 1. Aggregate emissions impact of INDCs A first, obvious question to ask is what the ted INDCs deliver in terms of GHG emissions out to 2030. What sounds like simple arith- metic turns out to be a more complicated accounting rcise with an array of possible outcomes. Some countries provide a range instead of a single number of emissions reductions in their INDCs. Many INDCs lack necessary details, such as clarity on sectors and gases covered, details on the impact of listed mitigation actions, different metrics to aggregate gases, details on base year or reference values from which reductions or improvements would be measured, or accounting practices related to land use and the use of specific market mechanisms31. This murkiness compli- cates a precise estimate of their impact on emissions. Finally, some of the actions listed in INDCs are, either implicitly or explicitly, conditional on other factors, such as the availability of financial or technological support. All these factors can be interpreted differently and influence the range of possible outcomes. In our assessment, we distinguish between a condi- tional and an unconditional INDC scenario, with associated uncertain- ties. Interestingly, the Paris Agreement does not adopt such distinction, and instead defers any discussion on features of countries’ contributions to further negotiations. Unconditionally, the INDCs are expected to result in global GHG emissions of about 55 52–58; 10th–90th percentile range over all studies unless otherwise stated billion metric tonnes of annual CO2-equivalent emissions Gt CO2-eq yr−1; Box 1, Fig. 1, Supplementary Text 1 in 2030. This is a reduction of around 9 7–13 Gt CO2-eq yr−1 by 2030 rela- tive to the median no-policy baseline scenario estimate and of around 4 2–8 Gt CO2-eq yr−1 relative to the median current-policy scenario estimate Supplementary Table 5. Putting this into context, global GHG emissions in 2010 are estimated at about 48 Gt CO2-eq yr−1 46– 50 Gt CO2-eq yr−1; range across studies, Supplementary Table 2, and our median no-policy baseline estimate reaches about 65 Gt CO2-eq yr−1 by 2030. A number of countries place conditionsfor example, the provision of international financeon all or part of their INDC. Some countries such as Mexico, Indonesia and Morocco included a range of reduction targets in their INDC and attach conditions to the implementation of the more ambitious end. Other countries indicate that their entire INDC is conditional. Of the INDCs ted by 12 December 2015, roughly 45 came with both conditional and unconditional components; about a third was conditional only; and the remainder did not specify conditions32. When we assume in our uation that all conditions are met and conditional INDCs are fully implemented, estimated global GHG emissions end up about 2.4 1.2−4.8 Gt CO2-eq yr−1 lower in 2030 compared to the unconditional INDC scenario case full range across six available estimates, Supplementary Text 1, Supplementary Table 5. Comparing the INDC scenario what countries propose as their con- tribution to the international agreement to the current-policy scenario what countries implement domestically provides lessons on the extent to which additional national policies are necessary to achieve the intended 2030 emissions reductions18. Projected emissions under current policies that match or are lower than those under the INDC can result either from a proactive and coordinated domestic policy response consistent with the INDC or from an INDC that is explicitly designed not to require PersPective reseArcH 3 0 j u n e 2 0 1 6 | V O L 5 3 4 | n A T u R e | 633 2016 Macmillan Publishers Limited. All rights reserved Global GHG emissions GtCO2-eqyr–1 70 GHG projections in the absence of climate policies 65 GHG reductions due 60 to current policies GHG reductions from implementing unconditional INDCs 55 Additional GHG reductions from 50 implementing conditional INDCs Additional GHG reductions 45 to embark on a least-cost pathway from 2020 onward for limiting warming to well below 2 C by 2100 Assessed modelling studies Climate Action Tracker PBL, The Netherlands IEA, France LSE, UK Univ. of Melbourne, Australia DEA, Denmark Climate Interactive, USA PNNL, USA JRC, European Union UNFCCC INDC Synthesis Estimated range per study 40 35 2010 2015 2020 Year 2025 2030 Figure 1 | Global greenhouse gas emissions as implied by INDCs compared to no-policy baseline, current-policy and 2 C scenarios. White lines show the median of each range. The white dashed line shows the median estimate of what the INDCs would deliver if all conditions are met. The 20th–80th-percentile ranges are shown for the no-policy baseline and 2 C scenarios. For current-policy and INDC scenarios, the minimum–maximum and 10th–90th-percentile range across all assessed studies are given, respectively. Symbols represent single studies, and are offset slightly to increase readability. Dashed brown lines connect data points for each study. References to all assessed studies are provided in Box 1. Scenarios are also described in Box 1. further policy effort. Likewise, projected emissions under current policy that exceed those under the INDC can result from a relatively ambitious INDC, from a lack of domestic climate policy, or a combination thereof. Therefore, this comparison alone cannot adequately reflect the overall level of ambition. For a number of countries such as Russia and Ukraine, the INDC targets suggest that emission levels above their estimated no-policy base- line or current-policy scenario will be reached. These countries are thus expected to overachieve their INDC targets by default. Under the rules of the Kyoto Protocol, over-delivery on a target would have generated surplus emission allowances by the quantity the target level is overa- chieved. These allowances can then be traded with other countries, who apply them to achieve their own GHG reduction target. Such a system could also be developed under the Paris Agreement, which allows for the voluntary use of “internationally transferred mitigation outcomes”. However, the extent to which such a mechanism will ultimately be devel- oped and used remains unclear, because it will require features, infor- mation and accounting of contributions to become much more precise than they are now. Different modelling teams treat these surpluses in different ways, which adds an uncertainty of about 1 Gt CO2-eq yr−1 to the estimates presented here. Confounding factors The literature synthesized in this assessment reveals a wide range of esti- mates of future emissions under nominally similar scenarios see small symbols in Fig. 1. These differences can stem from a number of factors, including modelling s, data and assumptions regarding country intent. Our review identifies four key factors that contribute to the discrepancies and differences between the various 2030 emissions estimates. Incomplete coverage Several global and national sectors as well as countries are not covered by INDCs. Often, emissions estimates for sectors that are not included under INDCs range widely. This is the case for, for example, global emissions from international aviation despite an industry pledge out- side the UNFCCC33 and maritime transport, or the national non-CO2 GHG emissions from China. Subtracting national sectors that are not covered, INDCs cover at least 8 percentage points less of global emis- sions than the 96 indicated earlier Supplementary Text 2. Under the Paris Agreement, developing countries are encouraged to move over time to economy-wide targets, so that future analyses should become more comprehensive. Countries that are not a UNFCCC Party or have not yet put forward an INDC are also studied in less depth, but represent only a