Yesterday we discovered that climate scientists think that human emissions of greenhouse gases are warming the world. Stop the presses!
Today we’ll look at what 1,868 practicing climate scientists think of sensitivity.
Verheggen and the PBL asked them, ‘What is your estimate of equilibrium (Charney) sensitivity, i.e., the temperature response (degrees C) to a doubling of atmospheric CO2? Please provide both a best estimate and a likely range (66% probability interval.
Slightly under half (49%) of the scientists provided a best estimate. (Wow. More than half the climate scientists surveyed refused to give an estimate for sensitivity?)
The average sensitivity given was about 2.7 C. (The IPCC provides a range of 1.5C to 4.5C.)
Fewer scientists were willing to provide upper and lower bounds.
852 scientists (46% of the total surveyed) provided a lower bound for sensitivity. The average was about 1.6C.
831 scientists (44% of the total) provided an upper bound for sensitivity. The average value was about 4.3C.
Those whose best estimate of sensitivity was below 2.5C were asked to indicate why their estimate was lower than the IPCC’s best estimate of 3.0C. 271 scientists had given a low best estimate, 14.5% of the total.
Respondents were free to offer more than one answer and many did.
44% said ‘natural variability has been underestimated.’
33% said ‘models overestimate current warming.’
31% said ‘cloud cover acts as a negative feedback.’
29% said ‘the effect of natural forcings is underestimated.’
22.5% said ‘positive cloud cover feedback is overestimated.’
22.5% said ‘positive water vapor feedback is overestimated.’
21% said ‘natural aerosols act as a negative feedback.’
20% said ‘energy balance calculations show climate sensitivity is small.’
A similar follow-up question was asked of those who indicated they felt the best estimate of sensitivity was higher than 3.5C. However, the data has not yet been made available for this follow up question.