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A scientific critique of the two-degree climate change target

Nature Geoscience volume 9, pages 13–18 (2016)Cite this article

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Abstract

The world's governments agreed to limit global mean temperature change to below 2 °C compared with pre-industrial levels in the years following the 2009 climate conference in Copenhagen. This 2 °C warming target is perceived by the public as a universally accepted goal, identified by scientists as a safe limit that avoids dangerous climate change. This perception is incorrect: no scientific assessment has clearly justified or defended the 2 °C target as a safe level of warming, and indeed, this is not a problem that science alone can address. We argue that global temperature is the best climate target quantity, but it is unclear what level can be considered safe. The 2 °C target is useful for anchoring discussions, but has been ineffective in triggering the required emission reductions; debates on considering a lower target are strongly at odds with the current real-world level of action. These debates are moot, however, as the decisions that need to be taken now to limit warming to 1.5 or 2 °C are very similar. We need to agree how to start, not where to end mitigation.

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Figure 1: Projected climate change for different global mean temperature targets.
Figure 2: Dependence of different quantities on global mean surface temperature change.
Figure 3: Changes in the water cycle.
Figure 4: Historical and future global CO2 emissions from fossil-fuel burning and industry-related activities.

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References

  1. The Cancun Agreements: Outcome of the Work of the Ad Hoc Working Group on Long-term Cooperative Action Under the Convention FCCC/CP/2010/7/Add.1 Decision 1/CP.16 (UNFCCC, 2010).

  2. Rogelj, J. et al. Energy system transformations for limiting end-of-century warming to below 1.5 °C. Nature Clim. Change 5, 519–527 (2015).

    Google Scholar 

  3. Geden, O. & Beck, S. Renegotiating the global climate stabilization target. Nature Clim. Change 4, 747–748 (2014).

    Google Scholar 

  4. Victor, D.G. & Kennel, C.F. Climate policy: ditch the 2° C warming goal. Nature 514, 30 (2014).

    Google Scholar 

  5. United Nations Framework Convention on Climate Change (UN, 1992)..

  6. Report of the Conference of the Parties on its Eighteenth Session, Held in Doha from 26 November to 8 December 2012 - Addendum - Part Two: Action Taken by the Conference of the Parties at its Eighteenth Session FCCC/CP/2012/8/Add.1 (UNFCCC, 2012)..

  7. Stirling, A. Keep it complex. Nature 468, 1029–1031 (2010).

    Google Scholar 

  8. Schmidt, G.A. What should climate scientists advocate for? Bull. Atom Sci. 71, 70–74 (2015).

    Google Scholar 

  9. Steinacher, M., Joos, F. & Stocker, T.F. Allowable carbon emissions lowered by multiple climate targets. Nature 499, 197–201 (2013).

    Google Scholar 

  10. Leemans, R. & Eickhout, B. Another reason for concern: regional and global impacts on ecosystems for different levels of climate change. Glob. Environ. Change 14, 219–228 (2004).

    Google Scholar 

  11. LoPresti, A. et al. Rate and velocity of climate change caused by cumulative carbon emissions. Environ. Res. Lett. 10, 095001 (2015).

    Google Scholar 

  12. Deser, C., Knutti, R., Solomon, S. & Phillips, A.S. Communication of the role of natural variability in future North American climate. Nature Clim. Change 2, 775–779 (2012).

    Google Scholar 

  13. IPCC. Climate Change 2013: The Physical Science Basis (eds Stocker, T. F. et al.) (Cambridge Univ. Press, 2013).

  14. Tebaldi, C. & Arblaster, J. Pattern scaling: its strengths and limitations, and an update on the latest model simulations. Climatic Change 122, 459–471 (2014).

    Google Scholar 

  15. Herger, N., Sanderson, B.M. & Knutti,, R. Improved pattern scaling approaches for the use in climate impact studies. Geophys. Res. Lett. 42, 3486–3494 (2015).

    Google Scholar 

  16. Fischer, E.M. & Knutti, R. Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes. Nature Clim. Change 5, 560–564 (2015).

    Google Scholar 

  17. IPCC. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects (eds Field, C. B. et al.) (Cambridge Univ. Press, 2014).

  18. Mahlstein, I. & Knutti, R. September Arctic sea ice predicted to disappear near 2 °C global warming above present. J. Geophys. Res. 117, D06104 (2012).

    Google Scholar 

  19. Nordhaus, W.D. A Question of Balance: Economic Modelling of Global Warming (Yale Univ. Press, 2008).

  20. Rockstrom, J. et al. A safe operating space for humanity. Nature 461, 472–475 (2009).

    Google Scholar 

  21. Steffen, W. et al. Planetary boundaries: guiding human development on a changing planet. Science 347, 1259855 (2015).

    Google Scholar 

  22. Knutti, R. & Sedláček, J. Robustness and uncertaintiesin the new CMIP5 climate model projections. Nature Clim. Change 3, 369–373 (2013).

    Google Scholar 

  23. Fischer, E.M., Sedláček, J., Hawkins, E. & Knutti, R. Models agree on forced response pattern of precipitation and temperature extremes. Geophys. Res. Lett. 41, 8554–8562 (2014).

    Google Scholar 

  24. Sedláček, J. & Knutti, R. Half of the world's population experience robust changes in the water cycle for a 2°C warmer world. Environ. Res. Lett. 9, 044008 (2014).

    Google Scholar 

  25. Tschackert, P. 1.5°C or 2°C: a conduit's view from the science-policy interface at COP20 in Lima, Peru. Clim. Change Resp. 2, 1–11 (2015).

    Google Scholar 

  26. Knutti, R. & Hegerl, G.C. The equilibrium sensitivity of the Earth's temperature to radiation changes. Nature Geosci. 1, 735–743 (2008).

    Google Scholar 

  27. Hansen, J., Sato, M., Russell, G. & Kharecha, P. Climate sensitivity, sea level and atmospheric carbon dioxide. Phil. Trans. R. Soc. A 371, 20120294 (2013).

    Google Scholar 

  28. Solomon, S., Plattner, G., Knutti, R. & Friedlingstein, P. Irreversible climate change due to carbon dioxide emissions. Proc. Natl Acad. Sci. USA 106, 1704–1709 (2009).

    Google Scholar 

  29. Bouttes, N., Gregory, J.M. & Lowe, J.A. The reversibility of sea level rise. J. Clim. 26, 2502–2513 (2013).

    Google Scholar 

  30. Davis, S., Caldeira, K. & Matthews, H. Future CO2 emissions and climate change from existing energy infrastructure. Science 329, 1330–1333 (2010).

    Google Scholar 

  31. Matthews, H.D. & Solomon, S. Irreversible does not mean unavoidable. Science 340, 438–439 (2013).

    Google Scholar 

  32. Ricke, K.L. & Caldeira, K. Maximum warming occurs about one decade after a carbon dioxide emission. Environ. Res. Lett. 9, 124002 (2014).

    Google Scholar 

  33. MacDougall, A.H, Avis, C.A. & Weaver, A.J. Significant contribution to climate warming from the permafrost carbon feedback. Nature Geosci. 5, 719–721 (2012).

    Google Scholar 

  34. Erickson, P., Kartha, S., Lazarus, M. & Tempest, K. Assessing carbon lock-in. Environ. Res. Lett. 10, 084023 (2015).

    Google Scholar 

  35. Baer, P. in Climate Change Policy: A Survey (eds Schneider, S. H., Rosencranz, A. & Niles, J.O.) Ch. 19, 393–408 (Island Press, 2002).

  36. Knutti, R. & Rogelj, J. The legacy of our CO2 emissions: a clash of scientific facts, politics and ethics. Climatic Change http://doi.org/896 (2015).

  37. Raupach, M.R. et al. Sharing a quota on cumulative carbon emissions. Nature Clim. Change 4, 873–879 (2014).

    Google Scholar 

  38. Nordhaus, W.D. Economic growth and climate: the carbon dioxide problem. Am. Econ. Rev. 67, 341–346 (1977).

    Google Scholar 

  39. Siegenthaler, U. & Oeschger, H. Predicting future atmospheric carbon dioxide levels. Science 199, 388–395 (1978).

    Google Scholar 

  40. Randalls, S. History of the 2 °C climate target. WIREs Clim. Change 1, 598–605 (2010).

    Google Scholar 

  41. Mann, M.E. Defining dangerous anthropogenic interference. Proc. Natl Acad. Sci. USA 106, 4065–4066 (2009).

    Google Scholar 

  42. Kahan, D.M. et al. The polarizing impact of science literacy and numeracy on perceived climate change risks. Nature Clim. Change 2, 732–735 (2012).

    Google Scholar 

  43. Jaeger, C. & Jaeger, J. Three views of two degrees. Clim. Change Econ. 3, 145–166 (2010).

    Google Scholar 

  44. Van der Sluijs, J., Van Eijndhoven, J., Shackley, S. & Wynne, B. Anchoring devices in science and policy: the case of consensus around climate sensitivity. Soc. Stud. Sci. 28, 291–323 (1998).

    Google Scholar 

  45. Report on the Structured Expert Dialogue on the 2013–2015 Review FCCC/SB/2015/INF.1 (UNFCCC, 2015)..

  46. Ad Hoc Working Group on the Durban Platform for Enhanced Action: Agenda item 3—Implementation of All the Elements of Decision 1/CP.17 Negotiating text. FCCC/ADP/2015/1 (UNFCCC, 2015)..

  47. Rogelj, J., McCollum, D.L., O'Neill, B.C. & Riahi, K. 2020 emissions levels required to limit warming to below 2 °C. Nature Clim. Change 3, 405–412 (2013).

    Google Scholar 

  48. Friedlingstein, P. et al. Persistent growth of CO2 emissions and implications for reaching climate targets. Nature Geosci. 7, 709–715 (2014).

    Google Scholar 

  49. Blanco, G. et al. in Climate Change 2014: Mitigation of Climate Change (eds Edenhofer, O. et al.) 351–411 (IPCC, Cambridge Univ. Press,, 2014).

  50. How Close are INDCs to 2 and 1.5°C pathways? Climate Action Tracker Update (PIK, ClimateAnalytics, NewClimate & Ecofys, 2015).

  51. Rogelj, J., McCollum, D.L., Reisinger, A., Meinshausen, M. & Riahi, K. Probabilistic cost estimates for climate change mitigation. Nature 493, 79–83 (2013).

    Google Scholar 

  52. Wigley, T.M.L., Richels, R. & Edmonds, J.A. Economic and environmental choices in the stabilization of atmospheric CO2 concentrations. Nature 379, 240–243 (1996).

    Google Scholar 

  53. Hoffert, M.I. et al. Energy implications of future stabilization of atmospheric CO2 content. Nature 395, 881–884 (1998).

    Google Scholar 

  54. Hansen, J. et al. Target atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J. 2, 217–231 (2008).

    Google Scholar 

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Author information

Authors and Affiliations

  1. Institute for Atmospheric and Climate Science, ETH Zürich, Universitätstrasse 16, CH-8092 Zürich, Switzerland,

    Reto Knutti, Joeri Rogelj, Jan Sedláček & Erich M. Fischer

  2. Energy Program, International Institute for Applied Systems Analysis (IIASA), Schlossplatz 1, A-2361 Laxenburg, Austria,

    Joeri Rogelj

Authors
  1. Reto Knutti
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  2. Joeri Rogelj
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  3. Jan Sedláček
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  4. Erich M. Fischer
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Contributions

All authors jointly wrote the paper. J.S. produced Figs 1–3. J.R. produced Fig. 4.

Corresponding author

Correspondence to Reto Knutti.

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The authors declare no competing financial interests.

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Knutti, R., Rogelj, J., Sedláček, J. et al. A scientific critique of the two-degree climate change target. Nature Geosci 9, 13–18 (2016). https://doi.org/10.1038/ngeo2595

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