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Jurisdictions throughout the world are contemplating greenhouse gas (GHG) mitigation strategies that will enable meeting long-term GHG targets. Many jurisdictions are now focusing on the 2020–2050 timeframe. We conduct an inter-model comparison of nine California statewide energy models with GHG mitigation scenarios to 2050 to better understand common insights across models, ranges of intermediate GHG targets (i.e., for 2030), necessary technology deployment rates, and future modeling needs for the state. The models are diverse in their representation of the California economy: across scenarios with deep reductions in GHGs, annual statewide GHG emissions are 8–46 % lower than 1990 levels by 2030 and 59–84 % lower by 2050 (not including the Wind-Water-Solar model); the largest cumulative reductions occur in scenarios that favor early mitigation; non-hydroelectric renewables account for 30–58 % of electricity generated for the state in 2030 and 30–89 % by 2050 (not including the Wind-Water-Solar model) ; the transportation sector is decarbonized using a mix of energy efficiency gains and alternative-fueled vehicles; and bioenergy is directed almost exclusively towards the transportation sector, accounting for a maximum of 40 % of transportation energy by 2050. Models suggest that without new policies, emissions from non-energy sectors and from high-global-warming-potential gases may alone exceed California’s 2050 GHG goal. Finally, future modeling efforts should focus on the: economic impacts and logistical feasibility of given scenarios, interactive effects between two or more climate policies, role of uncertainty in the state’s long-term energy planning, and identification of pathways that achieve the dual goals of criteria pollutant and GHG emission reduction.

To halt climate change this century, we must reduce carbon dioxide (CO 2 ) emissions from human activities to net zero. Any emission sources, such as in the energy or land-use sectors, must be balanced by natural or technological carbon sinks that facilitate CO 2 removal (CDR) from the atmosphere. Projections of demand for large-scale CDR are based on an integrated scenario framework for emission scenarios composed of emission profiles as well as alternative socio-economic development trends and social values consistent with them. The framework, however, was developed years before systematic reviews of CDR entered the literature. This primer provides an overview of the purposes of scenarios in climate-change research and how they are used. It also introduces the integrated scenario framework and why it came about. CDR studies using the scenario framework, as well as its limitations, are discussed. Possible future developments for the scenario framework are highlighted, especially in relation to CDR., (© 2020 The Authors.)

This paper is an introduction to, "The EMF 32 Study on U.S. Carbon Tax Scenarios," part of the Stanford Energy Modeling Forum (EMF) Model Inter-comparison Project (MIP) number 32. Eleven modeling teams participated in this study examining the economic and environmental impacts of various carbon tax trajectories and differing uses of carbon tax revenues. This special issue of Climate Change Economics documents the results of this study with four crosscutting papers that summarize results across models, and ten papers from individual modeling teams.

A number of analyses, meta-analyses, and assessments, including those performed by the Intergovernmental Panel on Climate Change, the National Oceanic and Atmospheric Administration, the National Renewable Energy Laboratory, and the International Energy Agency, have concluded that deployment of a diverse portfolio of clean energy technologies makes a transition to a low-carbon-emission energy system both more feasible and less costly than other pathways. In contrast, Jacobson et al. [Jacobson MZ, Delucchi MA, Cameron MA, Frew BA (2015) Proc Natl Acad Sci USA 112(49):15060-15065] argue that it is feasible to provide "low-cost solutions to the grid reliability problem with 100% penetration of WWS [wind, water and solar power] across all energy sectors in the continental United States between 2050 and 2055", with only electricity and hydrogen as energy carriers. In this paper, we evaluate that study and find significant shortcomings in the analysis. In particular, we point out that this work used invalid modeling tools, contained modeling errors, and made implausible and inadequately supported assumptions. Policy makers should treat with caution any visions of a rapid, reliable, and low-cost transition to entire energy systems that relies almost exclusively on wind, solar, and hydroelectric power., Competing Interests: Conflict of interest statement: The authors declare no conflict of interest, and with the exception of S.A.Q., none received support from sources other than normal salary from their employers for work on the preparation of this paper. With the exception of M.B. and J.C.S.L., all of the authors have recently received outside support for more general research on energy systems and renewable energy. C.T.M.C. received support in the past from NOAA. S.A.Q. was supported for analysis that supported this paper by the Rodel Foundation of Delaware and has received more general faculty funding from Uppsala University. J.A. and M.G.M. have received support from the National Science Foundation (NSF), EPRI, the Doris Duke Charitable Foundation, and members of the Carnegie Mellon Electricity Industry Center. A.R.B. has received support from the California Air Resources Board, the Carnegie Endowment for International Peace, Argonne National Laboratory, Sandia National Laboratory, NREL, Ford Motor Company, and Saudi Aramco. K.C. has received support from the Carnegie Institution for Science endowment and the Fund for Innovative Climate and Energy Research. S.J.D. has received support from the NSF. V.D. has received support from NREL. M.A.H. has received support from the NSF and DOE. P.D.H.H. has received support from the NSF and DOE. P.J. has received support from the NSF, EPA, and NOAA. D.M.K. has received support from the NSF and the Zaffaroni and Karsten Family Foundations. A.R. has received support from the NSF. V.S. has received support from the Sloan Foundation. J.S. has received funding from Jay Precourt, Bloom Energy, EPA, ExxonMobil Corporation, California Energy Commission, and DOE. G.R.T. has received support from DOE and the University of California, San Diego (UC San Diego) Deep Decarbonization Initiative. D.G.V. has received support from EPRI, the UC San Diego Deep Decarbonization Initiative, and the Brookings Institution. J.P.W. has received support from DOE, EPA, and industry affiliates of the Energy Modeling Forum. J.F.W. has received support from the NSF, DOE, DOD, Toyota, and Aquion Energy.

Advances in the science and observation of climate change are providing a clearer understanding of the inherent variability of Earth's climate system and its likely response to human and natural influences. The implications of climate change for the environment and society will depend not only on the response of the Earth system to changes in radiative forcings, but also on how humankind responds through changes in technology, economies, lifestyle and policy. Extensive uncertainties exist in future forcings of and responses to climate change, necessitating the use of scenarios of the future to explore the potential consequences of different response options. To date, such scenarios have not adequately examined crucial possibilities, such as climate change mitigation and adaptation, and have relied on research processes that slowed the exchange of information among physical, biological and social scientists. Here we describe a new process for creating plausible scenarios to investigate some of the most challenging and important questions about climate change confronting the global community.