Nevertheless, its conclusions essentially match the more sophisticated paper by Tian (2015) that 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.
The operative word from that study is that the sensitivity might be in the high end. Models that represent various cloud and tropical processes seem to have a higher sensitivity. But models that represent historical global temperatures have a moderate sensitivity.
And what if sensitivity is at the high end? Out of CMIP5 the second highest climate sensitivity is HADGEM2-ES with a sensitivity of 4.59. The temperature projection for this model suggests a 2016 temp of 1.11 above baseline, compared to 0. 96 for all models, and for 2100 under scenario 8.5 a temp of 5.47, compared to 4.56 for all models.
The projected warming for high sensitivity models is not nearly as high as you get by scaling the projection according to sensitivity (e.g. 4.5 high sensitivity/3 avg sensitivity * 4.5 temp in 2100 = 6.8 temp in 2100). The models can only warm things up over decades to century or two time frames just a little bit faster than the model average, otherwise the gap between model and observed temperature history becomes very large.
MH,
I have to begin by stating that I admire your (& Steven's) command of this topic; and I freely admit that there are ranges on all of the pertinent values in this discussion; including that the Tian (2015) only states that “... ECS might be in the higher end of its range (~4.0
oC) ...”.
That said, we started this line of discussion when I stated that exceeding the 2C limit was plausible by 2030, and in this line the linked Kevin Anderson article notes that to avoid the 2C limit our remaining CO2 budget might be about 450 Gt; however, this ignores the risks that climate sensitivity may well be higher than assumed by AR5:
http://www.skepticalscience.com/Anderson.htmlExtract: "Therefore, instead of a 1000 Gt CO2 budget, we might only have 450 Gt available for fossil-fuel energy emissions.”
I note that we are currently emitting more than 53Gt of CO2-e a year (see attached image), so Anderson is indicating that we might only have 8.5 more years (pulse an effective lag time of say 5.5 to 7.5 years for the emission period from 2016 to 2030) until 2030 to 2032 when we exceed the 2C limit (assuming an AR5 climate sensitivity, and assuming a GWP100 for methane of 34, while GWP20 for methane is 105). Furthermore, I note that about 1.75 years ago we were at about 0.85C above pre-industrial levels and now we are above 1.1C, and Steven thinks that we could reach 1.2C above pre-industrial by the end of this year.
Also, 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 they 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 Furthermore, the linked Knutti & Rogenstein (2015) open access reference addresses the limits of linear climate models (w.r.t. accessing climate sensitivity) and states: "But all comprehensive climate models indicate sensitivities above 2°C, and 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.2015.0146
http://rsta.royalsocietypublishing.org/content/373/2054/20150146Also, the linked Bloch-Johnson, Pierrehumbert & Abbot (2015) 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: 10.1002/2015GL064240
http://onlinelibrary.wiley.com/doi/10.1002/2015GL064240/full Finally, w.r.t. Black Swan risks I note that there are numerous factors that could have recently masked the impacts of relatively high ECS values including: (a) more negative anthropogenic & natural aerosol forcing & feedbacks than previously assumed, (b) poor understanding of Earth System states (like the PDO phase, etc), (c) the recent [but temporary] surge in plant growth related to our high CO2 concentrations. and (d) certain nonlinear positive feedbacks may be now accelerating due to global warming.
Best,
ASLR