This Special issue of Climatic Change documents the main findings of Energy Modeling Forum Model Inter-comparison Project (MIP) number 27 (EMF 27) entitled “The EMF27 Study on Global Technology and Climate Policy Strategies”. This study focused on the development and cross model comparison of results from a new generation of comprehensive international climate policy intervention scenarios focusing on technology strategies for achieving climate policy objectives. These scenarios enabled the community to exercise enhanced modeling capabilities that were focused on in previous EMF studies on the international trade implications of climate policies; the representation of technological change; and the incorporation of multi-gas mitigation and land use emissions and mitigation policy alternatives.

This introduction has four objectives: (1) describe the motivation for the EMF 27 study, (2) put this study in the context of other past and current IAM inter-model comparison projects, (3) describe the structure of this special issue of Climatic Change, and (4) give a brief overview of the insights developed in the papers produced by the individual modeling teams that are included in this special issue.

EMF 27 focused on the interactions between climate change policy architectures and advanced energy technology availabilities at global scale. It followed on previous EMF climate change oriented Model Inter-comparison Projects (MIPs): EMF 12 on carbon emission limits (EMF 12 1993; Gaskins and Weyant 1993; Weyant 1993), EMF 14 on carbon concentration limits (EMF 14 1996; Haites et al. 1997), EMF 16 on the costs and energy system impacts of the Kyoto Protocol (Weyant 1999), EMF 19 on carbon constraints and advanced energy technologies (Weyant 2004), EMF 21 on non-CO2 Kyoto gas mitigation (de la Chesnaye and Weyant 2006), and EMF 22 on climate control scenarios (focusing on phased participation in a climate mitigation coalitions and the possibility overshooting long run climate targets (Clarke et al. 2009; Fawcett et al. 2009)). As such, this study was able to take advantage of all the significant model extensions and enhancements that have taken place over the last 20 years.

EMF 27 itself was the outgrowth of a study started in April of 2010 (which was then tentatively called EMF 24) and was set up to include three parallel model comparison exercises at the global, US and European Union (EU) levels as had been the case in the EMF 22 study. As the work progressed, however, that study became too large, including too many people, models, (over 40 models across the three domains) and interests to deal with efficiently in one large project and so the original project was split into three separate studies on constructing and interpreting the results of climate policy and technology scenarios at the global (EMF 27), US (EMF 24: Fawcett et al. 2014; Clarke et al. 2014) and EU levels (EMF 28, Knopf et al. 2013). At the same time there was great interest in doing a new model comparison study on the international trade dimensions of climate policy (following on an earlier attempt in EMF 18 2002) using a largely different set of (trade oriented) global models than those included in EMF 27, and a MIP focused on energy infra-structure transitions in Europe tied into the EMF 28 study. The trade interest lead to another working group which produced a trade oriented global model inter-comparison on leakage effects and border carbon adjustments (Böhringer et al. 2012), and the latter lead to an extension of the EMF 28 scenario analysis focusing on infrastructure constraints and opportunities (Holz and von Hirschhausen 2014). Thus, the reporting on this collective work is being communicated through five separate journal special issues. EMF 27 is by far the largest and most complex of these studies with 18 models, over 30 scenarios and 29 modeler, cross cut, and overview papers included in this volume.

Over the last 10 years, there has also been a steady and extremely valuable increase in model inter-comparison studies organized within the European Union and other parts of the world as well as a broadening of the types of exercises being conducted in the U.S. In fact, this trend, lead, in part, to the formation of the Integrated Assessment Modeling Consortium (IAMC 2014) 6 years ago to coordinate this work and make the studies truly global in scope and participation. The IAMC has now matured to the point that it has formal charter, a scientific steering committee, an annual research conference, and a world wide web site (IAMC 2014).

Early EU sponsored inter-model comparison studies included “The Innovation Modeling Comparison Project” (IMCP: Edenhofer et al. 2006) which noted that in the first generations of global energy-economy modeling applied to climate change, emerging from the late 1980s roughly up until the mid-1990s, technology generally entered through a series of exogenous assumptions. In true ‘top-down’ models, supply side technologies were reflected in assumptions about the elasticity of substitution between generic carbon and non-carbon sources (if any), whilst an “autonomous energy efficiency improvement” (AEEI) parameter was often used to reflect an assumed degree of decoupling between GDP and energy consumption—a single, fixed parameter encompassing both structural change in the relationship between economy and energy and the development and diffusion of demand-side technologies. Another early EU model inter-comparison study was “The Economics of Low Stabilization Project” (Edenhofer et al. 2010) explored the economics of very low targets for stabilization of atmospheric concentrations of GHGs in the atmosphere. The objective of the United Nations Framework Convention on Climate Change (UNFCCC) is “stabilization of greenhouse gas (GHG) concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system” (UNFCCC 1992, Article 2). Reaching the target of climate stabilization at no more than 2 °C above pre-industrial levels by the end of this century—which is how the European Union (EU) interprets Article 2—is a historic challenge for humankind. To make it likely that this challenge will be met, greenhouse gas concentrations have to be limited to at no more than 450 ppm CO2 equivalent (for a 50 % likelihood) or below. The study showed that this goal requires a portfolio of mitigation options for very stringent emission reductions and requires taking globally coordinated action now.

A very important non-EMF, US based model inter-comparison study was Climate Change Science Program (CCSP) Product 2.1 (a). In the CCSP Product 2.1(a) study (Clarke et al. 2007) actively involved each of three modeling groups—MERGE, MIT-IGSM, Mini-CAM in the model comparison process. The study produced one reference scenario and four stabilization scenarios, for a total of 15 scenarios. The reference scenarios were developed under the assumption that no climate policy would be implemented beyond the set of policies currently in place (e.g., the Kyoto Protocol and the U.S. carbon intensity goal, each terminating in 2012 because goals beyond that date have not been identified). Each modeling group developed its own reference scenario. The Prospectus required only that each reference scenario be based on assumptions believed by the participating modeling groups to be meaningful and plausible. Each of the three reference scenarios is based on a different set of assumptions about how the future might unfold without additional climate policies. These assumptions were not intended as predictions or best-judgment forecasts of the future by the respective modeling groups. Rather, they represented possible paths that the future might follow to serve as a platform for examining how emissions might be reduced to achieve stabilization.

Another more recent U.S based non-EMF Inter-Model Comparison study was the “The Asian Modeling Exercise (AME).” This was originally an outreach and capacity building oriented model comparison exercise sponsored by the U.S.E.P.A. the U.S.A.I.D., the EMF and several other groups. It was launched by Jae Edmonds, Leon Clarke and Katherine Calvin of the Joint Global Research Institute (JGCRI) at the University of Maryland and Pacific Northwest National Laboratory. It engaged a large number of global Integrated Assessment models and Asian country/regional models in a comparison of baseline, carbon cap and carbon tax scenarios. A number of study groups were formed to interpret the model results in an innovative set of cross cutting papers: (1) base year data, (2) a base line projections, (3) urban and rural development, (4) low carbon societies, (5) technology, (6) regional mitigation comparability, and (7) national policies and measures. A special issue of Energy Economics documenting the results from this study was published in late 2012 (Calvin et al. 2012).

There are also a number of ongoing EU and US sponsored climate policy oriented model inter-comparison projects that are finishing or producing interim results during 2013. These include the RoSE project (Luderer et al. 2013a), the LIMITS project, (Tavoni et al. 2014; Kriegler et al. 2014c), and the AMPERE project (Kriegler et al. 2014b) coordinated within the European Union, as well as the PIAMDDI and LAMP projects in the United Sates. In the EU “Roadmaps towards Sustainable Energy Futures (RoSE)” project, a set of low-stabilization scenarios under a policy target of limiting atmospheric greenhouse gas concentrations at 450 ppm CO2eq by 2100 are analyzed. For comparison, another set of stabilization scenarios with a less stringent policy scenario of 550 ppm CO2 eq reached at 2100 was considered. This study focuses on a deep and systematic exploration of the importance of various scenario drivers like economic growth projections, energy resource base assumptions, and energy conversion technologies between primary and final energy for achieving such targets (Luderer et al. 2013b).

The “Low Climate Impact Scenarios And The Implications Of Required Tight Emission Control Strategies (LIMITS)” project is aimed at generating insight into how 2 °C compatible targets can be really made implementable, including a heavy focus on financial flows (from country to country and industry to industry) and infrastructure required to convert today’s energy systems to those required to achieve these targets in the future. This study is also examining the relationships between individual country or region action and global outcome (Tavoni et al. 2014; Kriegler et al. 2014b).

The EU sponsored “Assessment of Climate Change Mitigation Pathways and Evaluation of the Robustness of Mitigation Cost Estimates (AMPERE: Kriegler et al. 2014a)” project explores a broad range of mitigation pathways and associated mitigation costs under various real world limitations, while at the same time generating a better understanding about the differences across models, and the relation to historical trends. Uncertainties about the costs of mitigation originate from the entire causal chain ranging from economic activity, to emissions and related technologies, and the response of the carbon cycle and climate system to greenhouse gas emissions. AMPERE is using a sizable ensemble of state-of-the-art energy-economy and integrated assessment models to analyze mitigation pathways and associated mitigation costs in a series of multi-model inter-comparisons. It is focusing on four central areas: (i) The role of uncertainty about the climate response to anthropogenic forcing on the remaining carbon budget for supplying societies around the globe with energy, (ii) the role of technology availability, innovation and myopia in the energy sector, (iii) the role of policy imperfections like limited regional or sectoral participation in climate policy regimes, and (iv) the implications for de-carbonization scenarios and policies for Europe. This project is due to be completed by early 2014.

The U.S. Department of Energy sponsored “Program on Integrated Assessment Modeling Development, Diagnostic and Inter-Comparisons (PIAMDDI),” is an integrated assessment modeling (IAM) community research program on IAM model development; inter-comparisons and diagnostic testing; and multi-model “ensemble-like” analyses. The five cutting edge IAM research areas included in the program are: science and technology; impacts and adaptation; regional scale IA modeling; key intersecting energy-relevant systems; and uncertainty. The program is dedicated to improving the science of integrated assessment by doing cutting edge research in five critical areas of IAM development and integrating that research with a program of model inter-comparisons and ensemble-like activities. This program is linked closely to other climate change research programs in the U.S. and abroad. Progress on the scientific research areas is informing the model comparison and scenario ensemble tasks, and the comparisons and ensemble activities are helping set priorities for the research areas. Each research area as well as the model comparison and ensemble construction work is continually being broken down systematically, back down to fundamental first principals to help assess the state of the art and set focused priorities for the individual research efforts. A series of expert community workshops are facilitating this process. This project is due to be completed by the end of 2016 and is being closely coordinated with the EU sponsored AMPERE project described above.

The “Latin American Modeling Project (LAMP)” is a relatively new project patterned after the AME project but focusing on Latin American. This study was initiated by Katherine Calvin and Leon Clarke of the Joint Global Change Research Institute and is again sponsored by the U.S. Environmental protection Agency and U.S. Agency for International Development. A novel part of this study will be consideration of integrated climate change impacts assessment in major Latin American countries. LAMP is scheduled for completion by early 2015.

The wealth of results from the EMF27 study is presented in several layers in this volume. After a high level overview of the study design and results, (Kriegler et al. 2013), the technology and policy dimensions of the study are explored in greater depth in two separate overview papers (Krey et al. 2013; Blanford et al. 2014a). These overviews are augmented by a set of comparative analyses of model results on topics of interest. Those include the role of individual technology options for climate mitigation (Energy Efficiency: Sugiyama et al. 2013; Nuclear power, Kim et al. 2014; Renewable energy with a focus on the electricity sector, Luderer et al. 2013a; carbon capture and sequestration, Koelbl et al. 2014; and bioenergy, Rose et al. 2013a), the implications of fossil resource availability on the energy transition (McCollum et al. 2013a), the implications of the transition pathways for land use (Popp et al. 2013) and the forcing from aerosols and short-lived GHGs (Rose et al. 2013b). In addition, 17 of the 18 modeling teams that provided model results for this study developed individual modeling team papers, summarizing their experiences running the study scenarios and developing unique insights from the application of their individual modeling platforms. These insights are summarizing briefly here.

Several modeling teams explored the interactions of different combinations of technology restrictions in greater detail to illustrate important tradeoffs. Using the POLES model, (Griffin et al. 2013) illustrate a sharp increase in mitigation costs when Carbon Capture and Sequestration (CCS) is not available, especially when the growth of nuclear power generation is also restricted. Similarly, using TIAM-World (Kanudia et al. 2013) show the high value of bio-fuels in all scenarios and synergy between high bio-fuels availability and having either CCS, nuclear or wind and solar unrestricted using their system. In addition, (van Vliet et al. 2013) illustrate a dramatic increase in mitigation cots hen both CCS and bio-fuels production are limited.

Another group of modeling teams investigated possible constraints and opportunities involved in a rapid expansion of renewable energy production. For example using the DNE 21+ model, Sano et al. (2013), show that the share of electric generation can rise to high levels (in the 50–70 % range globally) when other generation sources are restricted and no grid integration costs are considered, but that this share may be reduced by 30 % or more when current grid integration costs are considered. Three teams focused on issues involved in large global expansion in the use of bio-energy. In GCAM, Calvin et al. (2013) illustrate the tradeoffs involved in land use/land cover oriented policies, like protection of forest lands or food security concerns, and the use of these lands for bio-energy production. In FARM (Sands et al. 2013) the emphasis is on productivity improvements in bio-fuels production as a means to moderate some of the cost increases that might otherwise occur as competition for land for this purpose accelerates. Using ReMIND/MAgPIE, Klein et al. (2013) show that biofuels used in combination with CCS can lead to much higher values for carbon storage than for fuel use from this combination of technologies.

Another set of models focus on specific types of technology innovation that could ease the transition to low GHG futures. This is accomplished by either by considered changes in model structures and input data or endogenous response functions. McCollum et al. (2013b) modify the MESSAGE model to include more favorable costs and constraints on use of biofuels as a transportation fuel directly or bio-energy based electrification of the transportation sector to show the dramatic reduction in mitigation costs this could produce. Similarly, using the BET model, Yamamoto et al. (2013) show dramatic reductions is mitigation costs that could result from significant improvements in end use technologies, especially advanced electric based end-use technologies. Similarly, Chaturvedi et al. (2013) focus on the climate benefits, as well as co-benefits, of aggressive energy efficiency policies and measures in India. In addition, Akashi et al. (2013) show similar positive effects results from improved (higher energy efficiency) materials use and recycling policies.

In De Cian et al. (2013) show that increases in energy technology R&D and technology learning that would result from a restriction on nuclear power expansion could largely offset the additional costs resulting from this restriction over time. In IMACLIM (Bibas and Méjean 2013) earlier mitigation action can lead to somewhat higher short run mitigation cost and much lower longer run mitigation cost under some not implausible conditions.

Two modeling teams focused on fossil fuel issues. Using ENV-Linkages, the OECD team (Magne et al. 2013) shows a large portion of the cost of a nuclear phase out could be offset by removal of fossil fuel subsidies around the world, while the Phoenix team (Daenzer et al. 2014) investigates the fate of the coal industry under the full range of study scenarios.

Two modeling teams (MERGE Blanford et al. 2014b; EC: Zhu et al. 2014) investigated the interplay of technology assumptions and policy architectures in more detail showing that inflexible week policies, tight targets and heavy technology restrictions can lead to very high mitigation costs and even technical infeasibility in some cases.

The steering committee for all of the current round of EMF climate studies was instrumental in the completion of all five model comparison projects it has generated, but all ten members were particularly active in this EMF 27 effort with Elmar Kriegler in the lead coordination role and Geoff Blanford particularly active in the scenario construction and definition phase, as well as the development of the policy overview paper and Volker Krey and Leon Clarke leading the technology overview paper.

The EMF 27 Steering Committee

Geoff Blanford (Vice Chair)

Leon E Clarke

Jae Edmonds

Allen Fawcett

Elmar Kriegler (Chair)

Volker Krey (Vice Chair)

Keywan Riahi

Richard Richels

Max Tavoni

John Weyant