The single climate sensitivity metric used by the IPCC has a huge upper range. It is not accounted for in the IPCC projections of the 'most likely' temperature increases, that just takes the mean of the wide range of computer model results.
(The climate sensitivity applies only to atmospheric carbon dioxide. The scientists assume that the sensitivity to the atmospheric CO2 equivalent is the same as for CO2 alone.)
"8.6.3 The traditional approach in assessing model sensitivity has to consider water vapour, lapse rate, surface albedo and cloud feedbacks separately. Although this division can be regarded as somewhat artificial, because for example, water vapour, clouds and temperature interact strongly, it remains conceptually useful.
188.8.131.52.4 Despite some advances in the understanding of the physical processes that control the cloud response to climate change and in the evaluation of some components of cloud feedbacks in current models, it is not yet possible to assess which of the model estimates of cloud feedback is the most reliable.
184.108.40.206 The role of sea ice dynamics in climate sensitivity has remained uncertain for years.
8.6.4 A set of model metrics that might be used to narrow the range of plausible climate change has yet to be developed"