Social Cost of Carbon: a Closer Look at Uncertainty
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SEI Oxford Office 266 Banbury Road, Suite 193 Oxford OX2 7DL Tel: 01865 426316 Fax: 01865 421898 [email protected] Social Cost of Carbon: A Closer Look at Uncertainty Final project report November 2005 Final revision August 2005 Authors Thomas E Downing Stockholm Environment Institute, Oxford Office David Anthoff Environmental Change Institute, Oxford Ruth Butterfield Stockholm Environment Institute Megan Ceronsky Environmental Change Institute, Oxford Michael Grubb Imperial College Jiehan Guo Environmental Change Institute, Oxford Cameron Hepburn University of Oxford Chris Hope University of Cambridge Alistair Hunt Metroeconomica, Bath Ada Li Environmental Change Institute, Oxford Anil Markandya Metroeconomica, Bath Scott Moss Centre for Policy Modelling, MMU Anthony Nyong University of Jos Richard S.J. Tol University of Hamburg Paul Watkiss AEA Technology Environment Department for Environment, Food and Rural Affairs Nobel House 17 Smith Square London SW1P 3JR Telephone 020 7238 6000 Website: www.defra.gov.uk © Queen’s Printer and Controller of HMSO 2005 This document/publication is value added. If you wish to re-use, please apply for a Click-Use Licence for value added material at http://www.opsi.gov.uk/click-use/system/online/pLogin.asp. Alternatively applications can be sent to: Office of Public Sector Information Information Policy Team St Clements House 2-16 Colegate Norwich NR3 1BQ Fax: 01603 723000 e-mail: [email protected] Information about this publication and further copies are available from: Environment Policy Economics Defra Ashdown House 123 Victoria Street London SW1E 6DE Tel: 020 7082 8593 This document is also available on the Defra website. Published by the Department for Environment, Food and Rural Affairs Social Cost of Carbon: A Closer Look at Uncertainty Page iii November 2005 Social Cost of Carbon: A Closer Look at Uncertainty Uncertainty is inherent in estimates of the social cost of carbon (SCC). The UK Department for Environment, Food and Rural Affairs (Defra) initiated this research to evaluate the sources of uncertainties, plausible ranges of estimates of the SCC and areas for further research and assessment. The analytical framework for the project is a risk assessment that brings together elements of uncertainty in climate change and its impacts with uncertainties in economic valuation; both are related to the context of decision making. This review of uncertainty in estimates of the social cost of carbon is summarised in key messages: Understanding of the social cost of carbon: • Our understanding of future climatic risks, spanning trends and surprises in the climate system, exposure to impacts, and adaptive capacity, is improving, but knowledge of the costs of climate change impacts is still poor. • The lack of adequate sectoral studies and understanding of local to regional interactions precludes establishing a central estimate of the social cost of carbon with any confidence. • The balance of benefits and damages in the social cost of carbon shifts markedly over time, with net damages increasing in later time periods. Estimates of the SCC are particularly sensitive to the choice of discount rates and the temporal profile of net damages • Vulnerability and adaptation to climate change impacts are dynamic processes responding to climatic signals, multiple stresses, and interactions among actors. Large scale impacts, such as migration, can be triggered by relatively modest climate changes in vulnerable regions. Uncertainty and risk: • Climate uncertainties and the climate sensitivity are key factors in larger estimates of the social cost of carbon. • Uncertainties in coverage, sectoral assessments and regional processes are likely to be significant, but are difficult to judge without further model development and inter-model comparison. • Decision variables such as the discount rate and equity weighting also are extremely important. The range of estimates of the social cost of carbon: • Estimates of the social cost of carbon span at least three orders of magnitude, from 0 to over 1000 £/tC, reflecting uncertainties in climate and impacts, coverage of sectors and extremes, and choices of decision variables. • A lower benchmark of 35 £/tC is reasonable for a global decision context committed to reducing the threat of dangerous climate change and includes a modest level of aversion to extreme risks, relatively low discount rates and equity weighting. • An upper benchmark of the SCC for global policy contexts is more difficult to deduce from the present state-of-the-art, but the risk of higher values for the social cost of carbon is significant. Uncertainty in the social cost of Discount rates, Equity weighting, risk ambiguity carbon: lines of evidence. Significant improvement in Meta-analysis of estimates of the SCC will require literature Expert well validated assessments at the knowledge Fund & Page regional scale of the dynamic elicitation processes of vulnerability and adaptation. Partnerships among researchers and stakeholders in Climate change Climate change Catastrophic Regional developing countries are essential. changes model Decision Framework Economic valuation Social Cost of Carbon: A Closer Look at Uncertainty Page iv November 2005 Social Cost of Carbon: A Closer Look at Uncertainty Page v November 2005 Contents 1 Introduction ..................................................................................................................................... 1 2 A methodology based on risk assessment ....................................................................................... 5 3 Understanding the social cost of carbon........................................................................................ 11 4 Uncertainty and risk ...................................................................................................................... 27 5 The range of estimates................................................................................................................... 33 6 Further research and next steps ..................................................................................................... 37 7 Conclusion..................................................................................................................................... 41 8 References and bibliography ......................................................................................................... 43 Annex: Description of integrated assessment models FUND and PAGE ............................................. 53 Annex: Sensitivity of the SCC to extreme events, equity weighting, discounting, and risk and ambiguity aversion ................................................................................................................................ 65 Annex: Understanding uncertainty through expert knowledge............................................................. 71 Annex: Regional studies of dynamic and multiple stresses .................................................................. 79 Social Cost of Carbon: A Closer Look at Uncertainty Page vi November 2005 Tables Table 1. Objectives and achievements of the scoping uncertainty project............................................. 2 Table 2. Measures of central tendency.................................................................................................. 10 Table 3. Locating the literature in a risk assessment framework ......................................................... 13 Table 4. Distribution of the SCC from a meta-analysis of the literature, GBP2000/tC ........................ 15 Table 5. Sensitivity of FUND estimates for different climate sensitivities.......................................... 29 Table 6. Summary of the probability that the SCC exceeds a given threshold in FUND .................... 34 Table 7. Summary of FUND results, GBP2000 /tC reference, averages and standard deviation ......... 34 Table 8. Selected estimates of the SCC from experts for a range of scenarios, £/tC ........................... 34 Table 9. Key messages .......................................................................................................................... 42 Figures Figure 1. An iterative framework for setting climate policy ................................................................... 3 Figure 2: Schematic mapping of multiple lines of evidence in understanding uncertainty in the social cost of carbon. ......................................................................................................................................... 5 Figure 3. A risk assessment framework ................................................................................................. 6 Figure 4. Expert confidence in estimates of the social cost of carbon for three groups of experts based on their overall expectation of the SCC ................................................................................................ 13 Figure 5. The composite cumulative density function of the marginal social cost of carbon. .............. 16 Figure 6. Probability distributions from FUND .................................................................................... 17 Figure 7. Distribution of the total climate damages ($2000) for the A2 reference scenario from the PAGE model ......................................................................................................................................... 17 Figure 8. Comparison of distributions of estimates