While the "Conservative Scientists & its Consequences" thread has much more information on other recently identified positive feedbacks, and on recently identified negative feedbacks/forcings that are diminishing, I have extracted the following list of recent (since May 2016) references that explicitly indicates that ECS is likely well above that expected by AR5 (which can be added to those listed in Reply #13). Such research indicates that collectively we are foolish to continue using a nominal value of about 3C for ECS when calculating our likelihood of staying below the 2C Paris Pact target limit:
1. The linked reference indicates that new research (from PlioMIP2) demonstrates that the climate sensitivity for the Pliocene was higher than previously believed (from PlioMIP1):
Kamae, Y., Yoshida, K., and Ueda, H.: Sensitivity of Pliocene climate simulations in MRI-CGCM2.3 to respective boundary conditions, Clim. Past, 12, 1619-1634, doi:10.5194/cp-12-1619-2016, 2016.
http://www.clim-past.net/12/1619/2016/http://www.clim-past.net/12/1619/2016/cp-12-1619-2016.pdfAbstract. Accumulations of global proxy data are essential steps for improving reliability of climate model simulations for the Pliocene warming climate. In the Pliocene Model Intercomparison Project phase 2 (PlioMIP2), a part project of the Paleoclimate Modelling Intercomparison Project phase 4, boundary forcing data have been updated from the PlioMIP phase 1 due to recent advances in understanding of oceanic, terrestrial and cryospheric aspects of the Pliocene palaeoenvironment. In this study, sensitivities of Pliocene climate simulations to the newly archived boundary conditions are evaluated by a set of simulations using an atmosphere–ocean coupled general circulation model, MRI-CGCM2.3. The simulated Pliocene climate is warmer than pre-industrial conditions for 2.4 °C in global mean, corresponding to 0.6 °C warmer than the PlioMIP1 simulation by the identical climate model. Revised orography, lakes, and shrunk ice sheets compared with the PlioMIP1 lead to local and remote influences including snow and sea ice albedo feedback, and poleward heat transport due to the atmosphere and ocean that result in additional warming over middle and high latitudes. The amplified higher-latitude warming is supported qualitatively by the proxy evidences, but is still underestimated quantitatively. Physical processes responsible for the global and regional climate changes should be further addressed in future studies under systematic intermodel and data–model comparison frameworks.
2. The linked reference indicates that corrected recent observations indicate that the most likely value of ECS may be as high as 4.6C (see attached plot of the time dependent curve):
Kyle C. Armour (27 June 2016), "Projection and prediction: Climate sensitivity on the rise", Nature Climate Change, doi:10.1038/nclimate3079
http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3079.html3. The linked reference indicates that the climate responses (climate sensitivities) projected by advanced climate models generally match observations when apple to apple comparisons are made. This is a useful finding as advanced climate models generally indicate that climate sensitivity values are towards the high end of the IPCC climate sensitivity range:
Mark Richardson, Kevin Cowtan, Ed Hawkins & Martin B. Stolpe (2016), "Reconciled climate response estimates from climate models and the energy budget of Earth", Nature Climate Change, doi:10.1038/nclimate3066
http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3066.html4. The linked reference discusses paleodata to indicate that climate sensitivity increased from 3.3 - 5.6 (mean of 4.45k) at the beginning of the PETM up to 3.7 - 6.5 K (mean of 5.1K) near the peak of the PETM; and that if we burn only the easily accessible carbon reserves then GMST could increase by about 10C. I note these climate sensitivity values are much higher than those inherent in the CMIP5 projections:
Gary Shaffer, Matthew Huber, Roberto Rondanelli & Jens Olaf Pepke Pedersen (23 June 2016), "Deep-time evidence for climate sensitivity increase with warming", Geophysical Research Letters, DOI: 10.1002/2016GL069243
http://onlinelibrary.wiley.com/doi/10.1002/2016GL069243/full5. The linked Reuters article notes that NASA reported that a new satellite-based method have located 39 unreported sources of anthropogenic emissions that, when accounted for, increase our previously estimated amount of sulfur dioxide by about 12 percent of all such anthropogenic emissions from 2005 to 2014. This indicates that the CMIP5 projections also underestimated the impact of this negative forcing source; which raises the prospect that climate sensitivity (ECS) is likely higher than the CMIP5 models indicate, and the linked Zhai et al (2015) reference analyses of the CMIP3&5 results conclude that 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: 10.1002/2015GL065911
http://in.reuters.com/article/us-nasa-pollution-idINKCN0YO1PW6. The linked reference uses an information-theoretic weighting of climate models by how well they reproduce the satellite measured deseasonlized covariance of shortwave cloud reflection, indicates a most likely value of ECS of 4.0C; which indicates that AR5 errs on the side of least drama:
Florent Brient & Tapio Schneider (2016), "Constraints on climate sensitivity from space-based measurements of low-cloud reflection", Journal of Climate, DOI:
http://dx.doi.org/10.1175/JCLI-D-15-0897.1 http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0897.17. The linked article indicates that the contribution of sea-ice loss to Arctic Amplification is regulated by the PDO and that in positive PDO phases (like we are in now) there should be less Arctic Amplification. Thus the fact that we are currently experiencing high Arctic Amplification during a period of highly positive PDO values gives cause for concern that climate sensitivity may be higher than considered by AR5:
James A. Screen & Jennifer A. Francis (2016), "Contribution of sea-ice loss to Arctic amplification is regulated by Pacific Ocean decadal variability", Nature Climate Change, DOI: 10.1038/nclimate3011
http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3011.html