Letter

Less than 2 °C warming by 2100 unlikely

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Abstract

The recently published Intergovernmental Panel on Climate Change (IPCC) projections to 2100 give likely ranges of global temperature increase in four scenarios for population, economic growth and carbon use1. However, these projections are not based on a fully statistical approach. Here we use a country-specific version of Kaya’s identity to develop a statistically based probabilistic forecast of CO2 emissions and temperature change to 2100. Using data for 1960–2010, including the UN’s probabilistic population projections for all countries2,3,4, we develop a joint Bayesian hierarchical model for Gross Domestic Product (GDP) per capita and carbon intensity. We find that the 90% interval for cumulative CO2 emissions includes the IPCC’s two middle scenarios but not the extreme ones. The likely range of global temperature increase is 2.0–4.9 °C, with median 3.2 °C and a 5% (1%) chance that it will be less than 2 °C (1.5 °C). Population growth is not a major contributing factor. Our model is not a ‘business as usual’ scenario, but rather is based on data which already show the effect of emission mitigation policies. Achieving the goal of less than 1.5 °C warming will require carbon intensity to decline much faster than in the recent past.

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Acknowledgements

This work was supported by NIH grants R01 HD054511 and R01 HD070936.

Author information

Affiliations

  1. Department of Statistics, University of Washington, Box 354322, Seattle, Washington 98195-4322, USA

    • Adrian E. Raftery
    •  & Peiran Liu
  2. Upstart, PO Box 1503, San Carlos, California 94070, USA

    • Alec Zimmer
  3. Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, Washington 98195-1640, USA

    • Dargan M. W. Frierson
  4. Department of Economics, University of California, Santa Barbara, California 93106-9210, USA

    • Richard Startz

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Contributions

The first four authors wrote the manuscript and developed the statistical model. A.E.R. and D.M.W.F. designed the study. A.Z. and P.L. compiled and analysed data and wrote computer code.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Adrian E. Raftery.

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