High Climate Sensitivity
IPCC computer model probability distribution of of climate sensitivity.
Latest research

Oct 2013Discovery that agricultural practices help form clouds could change the way we calculate global warming- climate is more sensitive. 
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2012 Climate Sensitivity Estimated From Earth's Climate History
James E. Hansen and Makiko Sato
Fast feedback only                3.0 +- 0.5C​
CO2 Earth system sens​         6.0C​
All GHGs                               8.0C​

Present sensitivity much higher than 3C​

If non-CO2 trace gases are counted
as a fast feedback, the fast-feedback sensitivity becomes 4°C for doubled CO2, and the Earth  system sensitivity becomes 8°C for doubled CO2 with the surface albedo feedback included.
​The Earth system sensitivity relevant to humanity now is the sensitivity of the present  climate state to a positive (warming) forcing. That sensitivity is not as great as for a negative forcing, but it is much larger than the 3°C fast-feedback climate sensitivity.
​The lifetime of fossil fuel carbon inserted
into the climate system is millennia, so the Earth system sensitivity has relevance to the eventual  climate response (ie 8C).
​Moreover, the rapidity with which the human-caused positive forcing is being
introduced has no known analog in Earth's history. 

Feb 2013 ​Time-dependent climate sensitivity(higher) & legacy of GHG emissions ​​(last (23,000–165,000 yrs)
18 Dec 2015 NASA research Examination of Earth's recent history key to predicting global temperatures. The sensitivity is 70% higher than IPCC estimates used
Jan 2014 Drew Shindell Inhomogeneous forcing and transient climate sensitivity Transient C Sens ​is higher than IPCC estimates
7 April 2016 Global warming may be far worse than thought, cloud analysis suggests. Climate change projections have vastly underestimated the role that clouds play,
31 Dec 2013 Solution to cloud riddle reveals hotter future: Global temperatures to rise at least 4 degrees C by 2100
June 2015 Deep time evidence for climate sensitivity increase with warming
June 2016 Projection and prediction: Climate sensitivity on the rise
Nov 2016 Nonlinear climate sensitivity. Equilibrium 4.88 K (likely range, 4.29 to 5.44 K) per CO2 doubling for warm phases.