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 <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]