In the linked May 10, 2016 article by Rolf Schuttenhelm, entitled: "Real Global Temperature Trend, p18 – Now how high is climate sensitivity? Here’s the answer of the world’s 13 leading climate experts!", Schuttenhelm ask 13 highly regarded climate experts what their guts tell them about climate sensitivity.http://www.bitsofscience.org/real-global-temperature-trend-climate-sensitivity-leading-climate-experts-7106/
Extract: "Piers Forster, James Hansen, Gavin Schmidt, Alan Robock, Michael Mann, Ken Caldeira, Stefan Rahmstorf, Chris Forest, Gabriele Hegerl, Hans Joachim Schellnhuber, Jonathan Gregory, Drew Shindell and Andrei Sokolov share their thoughts, and gut feelings, on climate sensitivity."
While I respect all of these experts, the fact that they largely indicated that their guts supported the a range of 2 to 4.5C (which largely overlaps with the AR5 finding), indicates to me that even experts can be out of touch with the latest research and tend to revert to consensus values in order to guard their well-earned reputations. To support my position, I provide the following 20 references that either directly, or indirectly, indicate that climate sensitivity is most likely significantly higher than the range summarized by AR5:
1. The linked reference analyses the CMIP3&5 results to conclude the ECS is likely 3.9C +/- 0.45C:
Chengxing Zhai, Jonathan H. Jiang & Hui Su (2015), "Long-term cloud change imprinted in seasonal cloud variation: More evidence of high climate sensitivity", Geophysical Research Letters, DOI: 10.1002/2015GL065911http://onlinelibrary.wiley.com/doi/10.1002/2015GL065911/full
2. The linked reference provides findings from CMIP5 of the continued poleward expansion of the Hadley Cell with continued global warming; which in-turn supports the idea that ECS is greater than 3C:
Lijun Tao, Yongyun Hu & Jiping Liu (May 2016), "Anthropogenic forcing on the Hadley circulation in CMIP5 simulations", Climate Dynamics, Volume 46, Issue 9, pp 3337-3350 DOI: 10.1007/s00382-015-2772-1http://rd.springer.com/article/10.1007%2Fs00382-015-2772-1
3. The linked reference presents new paleo evidence about the Eocene. While the authors emphasize that their findings support the IPCC interpretation for climate sensitivity, when looking at the attached Fig 4 panel f, it appears to me that this is only the case if one averages ECS over the entire Eocene; while if one focuses on the Early Eocene Climate Optimum (EECO) which CO₂ levels were higher than in current modern times, it appear that ECS was higher (around 4C) than the IPCC AR5 assumes (considering that we are increasing CO2 concentrations faster now that during the EECO this gives me concern rather than reassurance).
Eleni Anagnostou, Eleanor H. John, Kirsty M. Edgar, Gavin L. Foster, Andy Ridgwell, Gordon N. Inglis, Richard D. Pancost, Daniel J. Lunt & Paul N. Pearson (2016), "Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate", Nature, doi:10.1038/nature17423http://www.nature.com/nature/journal/vaop/ncurrent/full/nature17423.html
4. Tan et al (2016) indicates that ECS may well be between 5.0 and 5.3C.
Ivy Tan, Trude Storelvmo & Mark D. Zelinka (08 Apr 2016), "Observational constraints on mixed-phase clouds imply higher climate sensitivity", Science, Vol. 352, Issue 6282, pp. 224-227, DOI: 10.1126/science.aad5300 http://science.sciencemag.org/content/352/6282/224
5. According to the IPCC AR5 report: "The transient climate response is likely in the range of 1.0°C to 2.5°C (high confidence) and extremely unlikely greater than 3°C"; however, the linked reference uses only observed data to indicate that TCR is 2.0 +/- 0.8C. Thus AR5 has once again erred on the side of least drama.
T. Storelvmo, T. Leirvik, U. Lohmann, P. C. B. Phillips & M. Wild (2016), "Disentangling greenhouse warming and aerosol cooling to reveal Earth’s climate sensitivity", Nature Geoscience, doi:10.1038/ngeo2670http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2670.html
6. The linked reference reassesses ECS from CMIP3 &5 and find an ensemble-mean of 3.9C, and I note that CMIP3&5 likely err on the side of least drama as they ignore several important non-linear slow feedbacks that could be accelerated by global warming:
Chengxing Zhai, Jonathan H. Jiang, Hui Su (2015), "Long-term cloud change imprinted in seasonal cloud variation: More evidence of high climate sensitivity", Geophysical Research Letters, DOI: 10.1002/2015GL065911http://onlinelibrary.wiley.com/doi/10.1002/2015GL065911/full
7. The linked reference could not make it more clear that paleo-evidence from inter-glacial periods indicates that ECS is meaningfully higher than 3C and that climate models are commonly under predicting the magnitude of coming climate change.
Dana L. Royer (2016), "Climate Sensitivity in the Geologic Past", Annual Review of Earth and Planetary Sciences, Vol. 44 http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-100815-024150?src=recsys
8. Thompson indicates that ECS has a 95%CL range of from 3C to 6.3C, with a best estimate of 4C, and Sherwood (2014) has a higher value still:
Climate sensitivity by Roy Thompson published by Earth and Environmental Science Transactions of the Royal Society of Edinburgh, DOI: 10.1017/S1755691015000213http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=10061758&fileId=S1755691015000213
9. Tian (2015) indicates that the double-ITCZ bias constrains ECS to its high end (around 4.0C):
Tian, B. (2015), "Spread of model climate sensitivity linked to double-Intertropical Convergence Zone bias", Geophys. Res. Lett., 42, doi:10.1002/2015GL064119.http://onlinelibrary.wiley.com/doi/10.1002/2015GL064119/abstract
10. Sherwood et al (2014), which found that ECS cannot be less than 3C, and is likely currently in the 4.1C range. Also, everyone should remember that the effective ECS is not a constant, and models project that following a BAU pathway will result in the effective ECS increasing this century:
Sherwood, S.C., Bony, S. and Dufresne, J.-L., (2014) "Spread in model climate sensitivity traced to atmospheric convective mixing", Nature; Volume: 505, pp 37–42, doi:10.1038/nature12829http://www.nature.com/nature/journal/v505/n7481/full/nature12829.html
11. The linked reference studies numerous climate models and finds that: "… those that simulate the present-day climate best even point to a best estimate of ECS in the range of 3–4.5°C."
Reto Knutti, Maria A. A. Rugenstein (2015), "Feedbacks, climate sensitivity and the limits of linear models", Philosophical Transactions of the Royal Society A, DOI: 10.1098/rsta.2015.0146http://rsta.royalsocietypublishing.org/content/373/2054/20150146
12. The linked reference indicates that the cloud feedback from tropical land is robustly positive. As AR5 did not know whether this contribution to climate sensitivity was positive or negative, this clearly indicates that AR5 errs on the side of least drama with regard to both TCR & ECS:
Youichi Kamae, Tomoo Ogura, Masahiro Watanabe, Shang-Ping Xie and Hiroaki Ueda (8 March 2016), "Robust cloud feedback over tropical land in a warming climate", Atmospheres, DOI: 10.1002/2015JD024525http://onlinelibrary.wiley.com/doi/10.1002/2015JD024525/abstract
13. Graeme L. Stephens, Brian H. Kahn and Mark Richardson (5 May, 2016), "The Super Greenhouse effect in a changing climate", Journal of Climate, DOI: http://dx.doi.org/10.1175/JCLI-D-15-0234.1 http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0234.1
14. The linked reference assumes different degrees of nonlinearity for climate feedback mechanisms and concludes that such nonlinearity for positive feedback represents a Black Swan risk that linear climate models cannot recognize:
Jonah Bloch-Johnson, Raymond T. Pierrehumbert & Dorian S. Abbot (24 June 2015), "Feedback temperature dependence determines the risk of high warming", Geophysical Research Letters, DOI: 10.1002/2015GL064240http://onlinelibrary.wiley.com/doi/10.1002/2015GL064240/full
15. While the linked (open access) reference has many appropriate qualifying statements and disclaimers, it notes that the AR5 paleo estimates of ECS were linear approximations that change when non-linear issues are considered. In particular the find for the specific ECS, S[CO2,LI], during the Pleistocence (ie the most recent 2 million years) that:
"During Pleistocene intermediate glaciated climates and interglacial periods, S[CO2,LI] is on average ~ 45 % larger than during Pleistocene full glacial conditions."
Therefore, researchers such as James Hansen who relied on paleo findings that during recent full glacial periods ECS was about 3.0C, did not know that during interglacial periods this value would be 45% larger, or 4.35C.
Köhler, P., de Boer, B., von der Heydt, A. S., Stap, L. B., and van de Wal, R. S. W. (2015), "On the state dependency of the equilibrium climate sensitivity during the last 5 million years", Clim. Past, 11, 1801-1823, doi:10.5194/cp-11-1801-2015.http://www.clim-past.net/11/1801/2015/cp-11-1801-2015.htmlhttp://www.clim-past.net/11/1801/2015/cp-11-1801-2015.pdf
16. The linked reference implies that climate sensitivity (ESS) could be much higher than previously assumed:
Jagniecki,Elliot A. et al. (2015), "Eocene atmospheric CO2from the nahcolite proxy", Geology, http://dx.doi.org/10.1130/G36886.1http://geology.gsapubs.org/content/early/2015/10/23/G36886.1
17. The linked open access reference identifies three constraints on low cloud formation that suggest that cloud feedback is more positive than previously thought. If verified this would mean that both TCR and ECS (and ESS) are larger than previously thought:
Stephen A. Klein and Alex Hall (26 October 2015), "Emergent Constraints for Cloud Feedbacks", Climate Feedbacks (M Zelinka, Section Editor), Current Climate Change Reports, pp 1-12, DOI 10.1007/s40641-015-0027-1http://link.springer.com/article/10.1007%2Fs40641-015-0027-1
18. The linked article indicates that values of TCR based on observed climate change are likely underestimated:
J. M. Gregory, T. Andrews and P. Good (5 October 2015), "The inconstancy of the transient climate response parameter under increasing CO₂", Philosophical Transactions of the Royal Society A, DOI: 10.1098/rsta.2014.0417 http://rsta.royalsocietypublishing.org/content/373/2054/20140417
19. The linked reference indicates that most current climate models underestimate climate sensitivity:
J. T. Fasullo, B. M. Sanderson & K. E. Trenberth (2015), "Recent Progress in Constraining Climate Sensitivity With Model Ensembles", Current Climate Change Reports, Volume 1, Issue 4, pp 268-275, DOI 10.1007/s40641-015-0021-7http://link.springer.com/article/10.1007/s40641-015-0021-7?wt_mc=email.event.1.SEM.ArticleAuthorOnlineFirst
20. The linked reference indicates that studies that assuming linearity of climate sensitivity likely underestimate the risk of high warming.
Jonah Bloch-Johnson, Raymond T. Pierrehumbert and Dorian S. Abbot (June 2015), "Feedback temperature dependence determines the risk of high warming", Geophysical Research Letters, DOI: 10.1002/2015GL064240http://onlinelibrary.wiley.com/doi/10.1002/2015GL064240/abstract