The Paris Agreement1 aims to ‘pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels.’ However, it has been suggested that temperature targets alone are insufficient to limit the risks associated with anthropogenic emissions2,3. Here, using an ensemble of model simulations, we show that atmospheric CO2 increase—an even more predictable consequence of emissions than global temperature increase—has a significant direct impact on Northern Hemisphere summer temperature, heat stress, and tropical precipitation extremes. Hence in an iterative climate mitigation regime aiming solely for a specific temperature goal, an unexpectedly low climate response may have corresponding ‘dangerous’ changes in extreme events. The direct impact of higher CO2 concentrations on climate extremes therefore substantially reduces the upper bound of the carbon budget, and highlights the need to explicitly limit atmospheric CO2 concentration when formulating allowable emissions. Thus, complementing global mean temperature goals with explicit limits on atmospheric CO2 concentrations in future climate policy would limit the adverse effects of high-impact weather extremes.

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H.S.B. is supported by Natural Environment Research Council grant NE/L002612/1. R.J.M. was supported by Natural Environment Research Council grant NE/P014844/1. D.M. is supported by a NERC independent fellowship. H.S. was supported by the Integrated Research Program for Advancing Climate Models (TOUGOU program) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and ERTDF 2–1702 of Environmental Restoration and Conservation Agency, Japan. The authors thank the HAPPI project team and the modelling centres who contributed simulations. This research used science gateway resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors are grateful to F. Otto for making available results from event attribution experiments that provided the original motivation for this study. We would like to thank our colleagues at the Oxford eResearch Centre: A. Bowery, M. Rashid, P. Uhe and D. Wallom for their technical expertise. We would like to thank the Met Office Hadley Centre PRECIS team for their technical and scientific support for the development and application of weather@Home. Finally, we would like to thank all of the volunteers who have donated their computing time to climateprediction.net and weather@home.

Author information


  1. Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK

    • Hugh S. Baker
    • , Tim Woollings
    •  & Myles R. Allen
  2. Environmental Change Institute, School of Geography and the Environment, University of Oxford, Oxford, UK

    • Richard J. Millar
    • , David J. Karoly
    • , Benoit P. Guillod
    •  & Myles R. Allen
  3. School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia

    • David J. Karoly
  4. Australian Research Council Centre of Excellence for Climate System Science, Melbourne, Victoria, Australia

    • David J. Karoly
  5. Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology (ETH Zurich), Zurich, Switzerland

    • Urs Beyerle
    •  & Benoit P. Guillod
  6. Institute for Environmental Decisions, Swiss Federal Institute of Technology (ETH Zurich), Zurich, Switzerland

    • Benoit P. Guillod
  7. School of Geographical Sciences, University of Bristol, Bristol, UK

    • Dann Mitchell
  8. Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan

    • Hideo Shiogama
  9. Oxford e-Research Centre, Department of Engineering Science, University of Oxford, Oxford, UK

    • Sarah Sparrow


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M.R.A. conceived the experiment. H.S.B. and B.P.G. set up the HadAM3P simulations, S.S ran the HadAM3P simulations, H.S. ran the MIROC5 simulations and U.B. ran the CAM4 simulations. H.S.B. performed the analysis with input from D.J.K., R.J.M., B.P.G., T.W., D.M. and M.R.A. R.J.M. contributed to one of the figures. H.S.B. wrote the paper with comments from all the authors.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Hugh S. Baker.

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