Verheggen Survey and Sensitivity

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.

9 responses to “Verheggen Survey and Sensitivity

  1. The science is not settled.

    “A growing body of evidence suggests that the climate is less sensitive to increases in carbon-dioxide emissions than policy makers generally assume—and that the need for reductions in such emissions is less urgent.”

  2. Even the IPCC concedes lower sensitivity estimates – but in the science section, and not in any press releases.

    “……The bottom of the ‘likely’ range has been lowered from 2 to 1.5oC [1.5 degrees C] in the AR5, whereas the AR4 stated that ECS is very unlikely to be less than 1.5oC. It is also significant that the AR5 does not cite a best estimate, whereas the AR4 cites a best estimate of 3oC. Further the AR5 finds values of ECS exceeding 6oC to be very unlikely, whereas the AR4 did not have sufficient confidence to identify an upper bound at this confidence level. The stated reason for not citing a best estimate in the AR5 is the substantial discrepancy between observation-based estimates of ECS (lower), versus estimates from climate models (higher). Figure 1 of Box 12.2 in the AR5 WG1 report shows that 11 out of 19 observational-based studies of ECS have values below 1.5oC in the range of their ECS probability distribution.”

    • I need to work on my self-editing. And on the degree symbol using a laptop with no numeric pad.
      Yes, a sensitivity exceeding 60 degrees is unlikely. 🙂

  3. The best estimate of 3C was in AR4. AR5 had no best estimate provided. Nic Lewis calculated it as 1.64C.

  4. The problem with opinion based estimates is the rule of GIGO.

  5. Tom, today is Earth Day. Are we going to have an Earth Day post?

    45 years ago I was excited about the approaching Earth Day. I came from a coal town and the damage inflicted by the coal and steel industry was visible every day of my life.
    I volunteered to help. There were many arguments that I didn’t quite understand. Some of us wanted to invite Ernest Sternglass who wanted to come. He was vetoed without a reason given.
    On the actual day there was nothing but acrimony. There seemed to be two factions slugging it out that had little to do with the audience. A guy from the Audubon Society tried to find common ground and he was shouted down.
    A few politicians showed up with what they thought were reasonable environmental platforms. They were peppered with questions on abortion.
    It was a contest between the Malthusians and the Marxists and the Malthusians won. I wasn’t the only one who had a WTF moment.
    The featured speaker was Bob Packwood. He said that damming rivers was evil and his solution to the big environmental problems was to eliminate a tax deduction for more than 2 kids and to make “all forms” of birth control more readily available. He also put in a plug for nuclear.
    The campus organized Environmental Action to harness the students energy. It soon changed its name to Environmental Action/Zero Population Growth. It’s main goal was free distribution of birth control and legalizing abortion. I wrote an article for the campus newspaper, “How did Environmental Action Become Environmental action/Zero Population Growth Become Zero Population Growth.” They wouldn’t publish it. I doubt if there has ever been a movement more co-opted.
    Anyway I did leave Earth Day with a commitment to study human effects on climate and weather. We all know how that turned out.

  6. Pingback: Verheggen’s Consensus: Not 97%, not 47%. It’s 66%. | The Lukewarmer's Way

  7. Pingback: More on Verheggen et al: Great Survey. Pity about the report… It’s still 66%. | The Lukewarmer's Way

  8. Pingback: The Lukewarmer's Way

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