In addition to physical masking factors, the current AR5 climate projections are underestimated due to masking of both observed and paleo data, and by model bias as discussed in the following recent references. Thus is yet another example of human stupidity that our recent policies have not made sufficient provisions for such masking of relatively high climate sensitivity in the reported input data.
The first reference indicates that recent SST values have been systemically under-reported (see the attached image.
Zeke Hausfather, Kevin Cowtan, David C. Clarke, Peter Jacobs, Mark Richardson and Robert Rohde (04 Jan 2017), "Assessing recent warming using instrumentally homogeneous sea surface temperature records", Science Advances, Vol. 3, no. 1, e1601207, DOI: 10.1126/sciadv.1601207 http://advances.sciencemag.org/content/3/1/e1601207
The second linked article is entitled: "Fossil leaves suggest global warming will be harder to fight than scientists thought", and it presents new evidence about paleo-atmospheric CO₂ concentrations (indicating lower values than previously assumed) that indicate that climate sensitivity is likely higher than most current climate scientists assume:http://www.sciencemag.org/news/2017/01/fossil-leaves-suggest-global-warming-will-be-harder-fight-scientists-thought
Extract: "Now, scientists have developed a new method for wringing CO2 estimates from fossilized leaves—one that can go deeper into the past, and with more certainty. “At the moment, it’s very promising and it’s probably the best tool that we’ve got,” says David Beerling, a biogeochemist at the University of Sheffield in the United Kingdom who helped develop the so-called fossil leaf gas exchange technique. Already, it is solving ancient climate puzzles and delivering some unsettling news about the future.
… in applications of the method to times between 100 million and 400 million years ago, Franks finds hints of a foreboding message. During documented episodes of global warmth, he says, the method reveals relatively low CO2 values, nothing like the levels of 2000 ppm or more suggested by other proxies. If these downward revisions hold, Earth may be even more sensitive to injections of CO2 than current models predict. “Temperatures are going to climb further for less carbon and we better be mindful of that,” Franks says."
Lastly, the third linked reference and the associated fourth linked commentary by Rahmstorf entitled: "The underestimated danger of a breakdown of the Gulf Stream System", indicate that climate models over-estimate the stability of the AMOC; which increases the probability that Hansen's positive ice-climate feedback is reasonably accurate:
Wei Liu, Shang-Ping Xie, Zhengyu Liu and Jiang Zhu (04 Jan 2017), "Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate", Science Advances, Vol. 3, no. 1, e1601666, DOI: 10.1126/sciadv.1601666http://advances.sciencemag.org/content/3/1/e1601666
Extract: "According to the observational data, the AMOC is exporting freshwater, which is why freshwater will accumulate in the Atlantic when the AMOC breaks down. That is precisely the instability described by Stommel 1961 and Broecker 1987. In the models, on the other hand, the AMOC in most cases imports freshwater, so the flow is fundamentally stable there. The differences in AMOC stability between different models cannot be understood without the fundamental criterion of whether the AMOC imports or exports freshwater, and by what amount. Liu et al. 2014 have identified a known common bias in all coupled climate GCMs without flux adjustments, the “tropical bias”, which makes them import freshwater in contrast to what observations show for the real ocean. A model bias towards stability is also consistent with the fact that most models underestimate the cooling trend observed in the subpolar Atlantic, which is indicative of an ongoing significant AMOC weakening, as we have argued (Rahmstorf et al. 2015).
There are, therefore, two reasons why thus far we could have underestimated the risk of a breakdown of the Gulf Stream System. First, climate models probably have a systematic bias towards stable flow. Secondly, most of them do not take into account the melting ice of Greenland. As the new studies show, each of these factors alone can lead to a much stronger weakening of the Gulf Stream system. We now need to study how these two factors work together. I hope these worrying new results will encourage as many other research groups as possible to pursue this question with their own models!"