The IPCC estimates of the first three reports (FAR, SAR, TAR) did not give a formal definition of uncertainty, whereas the fourth (AR4) and the fifth (AR5) report denoted a 66% likelihood range for values between 2-4.5 degrees and 1.5 degrees respectively.
The first IPCC report states that for a doubling of carbon dioxide: "near the Earth's  surface,  the global  average  wanning  lies  between  +1.5°C  and  +4.5°C, with  a  "best guess" of  2.5°C". In terms of certainty they assigned three stars to this statement, which one meaning low confidence and five meaning virtually certain.[1]
The second IPCC report stated that: "No strong  reasons  have  emerged  to change these estimates of  the  climate  sensitivity" [2]
The third IPCC report states that: "Climate sensitivity is likely to be in the range of 1.5 to 4.5°C. This estimate is unchanged from the first IPCC AssessmentReport in 1990 and the SAR" [3]. This report was the first to give an estimate of the transient climate reponse (TCR).
From AR4: "It  is  likely  to  be  in  the  range  2°C to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher  than  4.5°C  cannot  be  excluded,  but  agreement  of  models  with  observations  is  not  as  good  for  those  values." In this report likely is formally defined as being more than 66% likely. Very unlikely &lt;10%.[4]
From AR5: "The equilibrium climate sensitivity  (ECS) is likely in  the  range  1.5°C  to  4.5°C,  extremely  unlikely  less  than  1°C,  and  very  unlikely  greater  than  6°C." Extremely unlikely is defined as less than 5% probable.[5]
From AR6: "Based on multiple lines of evidence21, the very likely range of equilibrium climate sensitivity is between 2°C (high confidence) and 5°C (medium confidence). The AR6 assessed best estimate is 3°C with a likely range of 2.5°C to 4°C (high confidence), compared to 1.5°C to 4.5°C in AR5, which did not provide a best estimate."[6]
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