National Academy of Sciences : Report Recommends New Framework for Estimating the Social Cost of Carbon

Jan. 11, 2017

FOR IMMEDIATE RELEASE

Report Recommends New Framework for Estimating the Social Cost of Carbon

WASHINGTON -- To estimate the social cost of carbon dioxide for use in regulatory impact analyses, the federal government should use a new framework that would strengthen the scientific basis, provide greater transparency, and improve characterization of the uncertainties of the estimates, says a new report by the National Academies of Sciences, Engineering, and Medicine. The report also identifies a number of near- and longer-term improvements that should be made for calculating the social cost of carbon.

The social cost of carbon (SC-CO) is an estimate, in dollars, of the net damages incurred by society from a 1 metric ton increase in carbon dioxide emissions in a given year. The SC-CO is intended to be a comprehensive estimate of the net damages from carbon emissions -that is, the net costs and benefits associated with climate change impacts such as changes in net agricultural productivity, risks to human health, and damage from such events as floods. As required by executive orders and a court ruling, government agencies use the SC-CO when analyzing the impacts of various regulations, including standards for vehicle emissions and fuel economy, regulation of emissions from power plants, and energy efficiency standards for appliances.

The federal Interagency Working Group on the Social Cost of Greenhouse Gases (IWG) developed in 2010 a methodology to estimate the SC-CO. The National Academies committee that authored the report was charged with examining potential approaches for a comprehensive update to this methodology to ensure that SC-CO estimates reflect the best available science. The committee was not asked to estimate a value for the social cost of carbon.

The IWG's methodology uses three distinct models to estimate the economic consequences of CO emissions. First, a baseline of CO emissions is defined along with projections of underlying socioeconomic factors -- global economic growth and population -- decades into the future. Then, a small increase in CO emissions is added to the baseline for each of the three models, which is translated into an increase in atmospheric CO and a resulting increase in global mean temperature. These results are used to estimate potential net damages in dollars, using discounting to convert future damages into present dollars. The final IWG analysis averages the results from the three models.

The report recommends that the IWG 'unbundle' this process and instead use a framework in which each step of the SC-CO calculation is developed as one of four separate but integrated 'modules': the socioeconomic module, which generates projections of greenhouse gas emissions based on its estimates of population and world economic output; the climate module, which translates changes in emissions into changes in temperature; the damages module, which estimates the net impact of temperature changes in dollar terms; and the discounting module. Data generated by the socioeconomic module would feed into each of the other three modules, and the temperature changes generated by the climate module would inform the damages module. Each module would be developed based on expertise in the relevant scientific disciplines to reflect the most up-to-date research. The report offers detailed recommendations about how the IWG should develop each of the modules and how the proposed framework could include feedbacks between and interactions within the modules.

The current SC-CO methodology uses constant discount rates of 2.5 percent, 3.0 percent, and 5.0 percent. The report notes that differences in the discount rates have large impacts on the estimates; the SC-CO estimates per metric ton emitted in 2020 is $62 using a 2.5 percent rate, $42 using a 3.0 percent rate, and $12 using the 5.0 percent rate (in 2007 dollars).

Instead of using fixed discount rates, the discounting module should incorporate the relationship between economic growth and discounting for calculating the rates, which would help account for uncertainty surrounding discount rates over long time periods, the committee said. The IWG should clearly state how the SC-CO estimates should be combined with other types of cost-benefit estimates in regulatory impact analyses.

The committee outlined several other recommendations that would be feasible to implement in the next two to three years and would improve the analysis:

• The socioeconomics module should use statistical methods and expert input for projecting distributions of economic activity, population growth, and emissions into the future.
• The climate module should employ a simple Earth system model that satisfies well-defined diagnostic tests to confirm that it properly captures the relationships over time between CO emissions, atmospheric CO concentrations, and global mean surface temperature change and sea-level rise.
• The damages module should improve and update existing formulations of climate change damages. This update should draw on recent scientific literature related to both empirical estimation and process-based modeling of damages.

Efforts by the IWG to estimate the SC-CO focus primarily on total global damages because the impacts of CO emissions are global regardless of where they originate. While estimating net damages per ton of CO emissions to the United States alone is 'feasible in principle,' the report says, these efforts are limited by existing SC-CO methodologies. Thorough estimates of U.S.-specific damages would need to consider how climate change and CO reductions in other parts of the world could also impact the United States - for example, through increased migration because of economic or political destabilization, and through reciprocal actions by other countries in response to U.S. emission reductions.

The IWG should update the SC-CO roughly every five years following a regular, three-step process. This process will ensure that for each update, the components of each module, module feedbacks and interactions, and the SC-CO framework itself are consistent with the current state of scientific knowledge as reflected in peer-reviewed literature. Key uncertainties and sensitivities should be adequately identified and represented in technical support documentation, and uncertainties that cannot be or have not been quantified should also be identified. In addition, documentation should explain and justify choices, and the presentation of results should be transparent.

The study was funded by the U.S. Department of Energy. The National Academies of Sciences, Engineering, and Medicine are private, nonprofit institutions that provide independent, objective analysis and advice to the nation to solve complex problems and inform public policy decisions related to science, technology, and medicine. The Academies operate under an 1863 congressional charter to the National Academy of Sciences, signed by President Lincoln. For more information, visit www.national-academies.org.

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Copies of Valuing Climate Damages: Updating Estimation of the Social Cost of Carbon Dioxide are available at www.nap.edu or by calling 202-334-3313 or 1-800-624-6242. Reporters may obtain a copy from the Office of News and Public Information (contacts listed above).

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THE NATIONAL ACADEMIES OF SCIENCES, ENGINEERING, AND MEDICINE

Division of Behavioral and Social Sciences and Education

Board on Environmental Change and Society

Committee on Assessing Approaches to Updating the Social Cost of Carbon

Maureen L. Cropper* (co-chair)

Distinguished University Professor of Economics and Chair

Department of Economics

University of Maryland

College Park

Richard G. Newell (co-chair)

President and CEO

Resources for the Future

Washington, D.C.

Myles Allen

Professor of Geosystem Science

Environmental Change Institute, School of Geography and the Environment, and the Department of Physics

University of Oxford

Oxford, England

Maximilian Auffhammer

George Pardee Jr. Professor of International Sustainable Development and Associate Dean of Social Sciences

Department of Agricultural and Resource Economics

University of California

Berkeley

Chris E. Forest

Associate Professor of Climate Dynamics

Departments of Meteorology and Atmospheric Science and Geosciences, and

Associate

Earth and Environmental Systems Institute

Pennsylvania State University

University Park

Inez Y. Fung*

Professor of Atmospheric Science

Department of Earth and Planetary Science and Department of Environmental Science, Policy, and Management

University of California

Berkeley

James K. Hammitt

Professor of Economics and Decision Sciences

Harvard T.H. Chan School of Public Health, and

Director

Harvard Center for Risk Analysis

Boston

Henry D. Jacoby

William F. Pounds Professor of Management (Emeritus)

Sloan School of Management, and

Former Co-Director

Joint Program on the Science and Policy of Global Change

Massachusetts Institute of Technology

Cambridge

Robert E. Kopp

Associate Professor

Department of Earth and Planetary Sciences, and

Associate Director

Rutgers Energy Institute

Rutgers University

New Brunswick,, N.J.

William Pizer

Professor

Sanford School of Public Policy

Duke University

Durham, N.C.

Steven K. Rose

Senior Research Economist

Energy and Environmental Analysis Research Group

Electric Power Research Institute

Washington, D.C.

Richard Schmalensee

Howard W. Johnson Professor of Management (Emeritus)

Sloan School of Management

Massachusetts Institute of Technology

Cambridge

John P. Weyant

Research Professor of Management Science and Engineering,

Director of the Energy Modeling Forum, and

Deputy Director of the Precourt Institute for Energy Efficiency

Stanford University

Stanford, Calif.

STAFF

Jennifer Heimberg

Study Director

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*Member, National Academy of Sciences