Increased warming in latest generation of climate models likely caused by clouds

Posted: June 26, 2020 by oldbrew in climate, Clouds, modelling, Temperature, Uncertainty
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Are climate models getting any better, or even getting worse? Their ‘projections’ almost invariably expect more warming than is observed, often a lot more. Now the uncertainty is increasing.
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As scientists work to determine why some of the latest climate models suggest the future could be warmer than previously thought, a new study indicates the reason is likely related to challenges simulating the formation and evolution of clouds, says ScienceDaily.

The new research, published in Science Advances, gives an overview of 39 updated models that are part of a major international climate endeavor, the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The models will also be analyzed for the upcoming sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC).

Compared with older models, a subset of these updated models has shown a higher sensitivity to carbon dioxide — that is, more warming for a given concentration of the greenhouse gas — though a few showed lower sensitivity as well.

The end result is a greater range of model responses than any preceding generation of models, dating back to the early 1990s.

If the models on the high end are correct and Earth is truly more sensitive to carbon dioxide than scientists had thought, the future could also be much warmer than previously projected.

But it’s also possible that the updates made to the models between the last intercomparison project and this one are causing or exposing errors in their results.

In the new paper, the authors sought to systematically compare the CMIP6 models with previous generations and to catalog the likely reasons for the expanded range of sensitivity.

“Many research groups have already published papers analyzing possible reasons why the climate sensitivity of their models changed when they were updated,” said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research (NCAR) and lead author of the new study.

“Our goal was to look for any themes that were emerging, especially with the high-sensitivity models. The thing that came up again and again is that cloud feedbacks in general, and the interaction between clouds and tiny particles called aerosols in particular, seem to be contributing to higher sensitivity.”
. . .
Evaluating model sensitivity

Researchers have traditionally evaluated climate model sensitivity using two different metrics. The first, which has been in use since the late 1970s, is called equilibrium climate sensitivity (ECS). It measures the temperature increase after atmospheric carbon dioxide is instantaneously doubled from preindustrial levels and the model is allowed to run until the climate stabilizes.

Through the decades, the range of ECS values has stayed remarkably consistent — somewhere around 1.5 to 4.5 degrees Celsius (2.7 to 8.1 degrees Fahrenheit) — even as models have become significantly more complex. For example, the models included in the previous phase of CMIP last decade, known as CMIP5, had ECS values ranging from 2.1 to 4.7 degrees C (3.6 to 8.5 degrees F).

The CMIP6 models, however, have a range from 1.8 to 5.6 degrees C (3.2 to 10 degrees F), widening the spread from CMIP5 on both the low and high ends. The NCAR-based Community Earth System Model, version 2 (CESM2) is one of the higher-sensitivity models, with an ECS value of 5.2 degrees C.

Model developers have been busy picking their models apart during the last year to understand why ECS has changed.

Full article here.

Comments
  1. oldbrew says:

    “Cloud-aerosol interactions are on the bleeding edge of our comprehension of how the climate system works, and it’s a challenge to model what we don’t understand,” Meehl said. “These modelers are pushing the boundaries of human understanding, and I am hopeful that this uncertainty will motivate new science.”
    – – –
    How can the IPCC justify its aggressive climate assertions in light of this?

  2. Graeme No.3 says:

    Why not copy or start with the model that has been less wrong for the past 20 years?
    That would be the Russian model that ignores any effect for CO2. After all, if CO2 hasn’t had any discernable effect for 400 million years, why would it do so now?

  3. cognog2 says:

    To me all these models seem to suffer from the same error in logic; initiated in the definition of Radiative Forcing being converted to an energy flux which is then used within the models to determine an updated Climate Sensitivity.
    The definition is in units of Force (kilogram force) and the flux is in units of energy (Watts) and the only way the two can be related is through application of the sensitivity of the system involved.
    The electrical analogy here is that to convert a potential to a flux you need to know the resistance (sensitivity?) of the circuit and hence the current flow from which the flux can be calculated.
    If the scientists wish to improve the quality of their models they need to go back to this original calculation; as using Its flux value as a given in in any future model will result in potential error.
    The reason being that this flux value carries within it an automatic assumption of the original sensitivity of the system which is not likely to be appropriate.

    As an aside: In my view it seems that the original calculation failed to take into account the thermodynamic behaviour of the Hydro Cycle and its influence in the climate where large energies are moved up through the atmosphere and beyond irrespective of GHG forcings.

  4. oldbrew says:

    Most so-called GHG is water vapour anyway. Messing about with CO2 is on the margins.

  5. Ron Clutz says:

    Much revealing information is provided in an AGU publication Causes of Higher Climate Sensitivity in CMIP6 Models by Mark D. Zelinka et al. (2019). The paper goes into the key equation and notes:

    “ECS therefore depends on the magnitude of the CO2 radiative forcing and on how strongly the climate system radiatively damps planetary warming. A climate system that more effectively radiates thermal energy to space or more strongly reflects sunlight back to space as it warms (larger magnitude) will require less warming to restore planetary energy balance in response to a positive radiative forcing, and vice versa.”

    “This ECS increase is primarily attributable to an increased multimodel mean feedback parameter due to strengthened positive cloud feedbacks, as all noncloud feedbacks are essentially unchanged on average in CMIP6. However, it is the unique combination of weak overall negative feedback and moderate radiative forcing that allows several CMIP6 models to achieve high ECS values beyond the CMIP5 range.”

    Zelinka et al. is here: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL085782

    My synopsis is https://rclutz.wordpress.com/2020/01/26/climate-models-good-bad-and-ugly/

  6. ivan says:

    Most so-called GHG is water vapour anyway. Messing about with CO2 is on the margins.

    oldbrew, I am sure they know that but found it was impossible to make money from water vapour therefore that is why they chose CO2 to be the demon gas that they could charge a ‘carbon’ tax on.

    From that base reason we have all the present fuss and the models have to support that decision.

    If they want the models to get anywhere near what is seen in the real world they will have to admit that CO2 has nothing to do with climate and then they need to study and learn what does. Once they get that far those that construct the models need to learn how to model a chaotic system correctly.

  7. oldbrew says:

    ECS therefore depends on the magnitude of the CO2 radiative forcing

    Piling nonsense (ECS) on top of nonsense (CO2 radiative forcing). Where’s the data?

    They admit to ‘poor understanding’ of the far more important cloud influence. Time to bin the empty-box assertions and do some science.

  8. Gamecock says:

    Should I be able to feel this ‘Increased warming in latest generation of climate models likely caused by clouds?’

    We had the mildest June I can remember.

  9. oldbrew says:

    Virtual warming gets publicity but proves nothing.

  10. tallbloke says:

    “Our goal was to look for any themes that were emerging, especially with the high-sensitivity models. The thing that came up again and again is that cloud feedbacks in general, and the interaction between clouds and tiny particles called aerosols in particular, seem to be contributing to higher sensitivity.”

    Are the modelers still labouring under the delusion that more cloud makes the surface warmer?

  11. Gamecock says:

    “The thing that came up again and again is that cloud feedbacks in general, and the interaction between clouds and tiny particles called aerosols in particular, seem to be contributing to higher sensitivity.”

    You can’t model what you don’t understand. ‘Seem to be’ is an admission that modeling is all FAKE.

    Every improved climate model is proof that all previous models were fake.

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