Ice Age Testing Reveals Challenges in Climate Model Sensitivity 

Posted: March 3, 2021 by oldbrew in atmosphere, climate, Clouds, Ice ages, modelling, predictions, research, Temperature, Uncertainty
Tags: ,

Cloud formation [image credit:NASA]

‘Challenges’ is a polite way of putting it. Is the alleged human-caused climate problem really more of a human-caused climate models problem?
– – –
Increased reflection of incoming sunlight by clouds led one current-generation climate model to predict unrealistically cold temperatures during the last ice age [Source: Geophysical Research Letters].

Key to the usefulness of climate models as tools for both scientists and policymakers is the models’ ability to connect changes in atmospheric greenhouse gas levels to corresponding shifts in temperature, says Eos.

Equilibrium climate sensitivity (ECS) is one such measure, representing the predicted warming after a doubling of atmospheric carbon dioxide (CO2) levels.

Climate models have traditionally forecast an increase of 1.5°C–4.5°C for a doubling of atmospheric CO2 from the preindustrial climate.

However, many of the latest models are finding values in excess of 5°C, which, if correct, would have significant negative implications for our ability to overcome the planet’s ongoing warming.

Zhu et al. investigated this trend by using one of the high-ECS models, the Community Earth System Model version 2 (CESM2), to simulate the climate [graphic here] during the culmination of the last ice age, called the Last Glacial Maximum (LGM).

The LGM occurred about 21,000 years ago and is frequently used to evaluate climate models. It represents markedly different conditions from the present, with much lower greenhouse gas levels, large ice sheets covering North America and Europe, and lower sea levels.

However, the LGM is sufficiently recent that there is widespread geologic evidence of both climate forcings and resulting surface temperature changes.

The authors configured CESM2 to closely mirror its use in modern climate change research, omitting only those parts (such as vegetation biogeochemistry) for which good data for the LGM are not available.

Within 500 model years after initialization, CESM2’s global mean surface temperature plunged to 11°C below the preindustrial era, roughly 5°C cooler than what the geological proxies indicate. In comparison, the model’s predecessor, CESM1, produced values several degrees warmer and within the uncertainty ranges of the proxies.

The authors attribute the discrepancy between CESM1 and CESM2 to how the latter handles clouds.

The atmospheric model in CESM2 has been updated so that the computer-simulated clouds behave more like real-world observations, which affects the shortwave cloud feedback, the ability of clouds to reflect incoming sunlight back to space under climate change.

When CESM2 was configured to use the older model’s atmospheric package—which lacks these updates—much of the excess temperature decrease disappeared.

The authors suggest that CESM2 likely overestimates the shortwave cloud feedback and therefore the ECS.

Full article here.

  1. Chaswarnertoo says:

    Models, schmodels.

  2. Phoenix44 says:

    Beware of Black Box models. They are extremely unlikely to be right.

    But hey, let’s throw away our entire economy and society based on their results.

  3. oldbrew says:

    The existence of ‘Climate Model Sensitivity’ is based on an assumption that radiative gases run the climate. Move away from that assumption and it has no useful meaning, which may explain why nobody ever manages to work out what it should be.

    The authors attribute the discrepancy between CESM1 and CESM2 to how the latter handles clouds.
    = = =
    Why ‘Social Cost Of Carbon’ Is Most Useless Number You’ve Never Heard Of
    Posted on Wed 03/03/2021

    Social cost of carbon estimates are based on questionable assumptions regarding the climate’s sensitivity to carbon dioxide emissions and projections reaching 300 years into the future.

  4. ivan says:

    When they manage to get a model that actually manages to predict, within 0.01 C the temperature one week in the future I might consider its output. Until then the first rule of computing applies – Garbage In = Garbage Out!

  5. stpaulchuck says:

    wow, still spewing on about CO2 *facepalm*

    I suppose this blog must need to print the stupid stuff as well as the solid stuff so we can see but I get so tired of the reformed ‘entrails of sheep’ Climate Catastrophe clown circus and their always failing computer models. I suspect the entrails of sheep might likely have a better track record predicting the weather/climate.

    “The climate system is a coupled non-linear chaotic system, and therefore the long-term prediction of future climate states is not possible.” – IPCC TAR WG1, Working Group I: The Scientific Basis

  6. E.M.Smith says:


    Inspecting entrails would actually have some use. (I.e. it wasn’t as daft, then, as it sounds, now). It was common for Druids to inspect the entrails of birds ahead of an advancing army. Sound daft?

    Well, both the birds and the army were living off the land. Looking at what the birds had to eat and their general health and how much body fat was in the belly, tells you rather a lot about the conditions your opponent is marching through / eating as well. Furthermore, birds and mammals share many diseases. Finding diseased birds would indicate risk to the advancing army.

    So, IMHO, inspecting the entrails of wild animals has more value and a better grounding in the real world than Climate Models.

    (BTW: I’m a computer professional who’s managed a supercomputer center where we ran models of various sorts, though not climate models, and I’ve downloaded and inspected a climate model. The code is very simplistic and, again IMHO, is mostly “Given these conclusions what assumptions can we draw? / Code in?” So I’m stating my opinion of Climate Models not from an outside bias, but an inside Tech POV.)

  7. Paul Vaughan says:

    quote: “Given these conclusions what assumptions can we draw?”

    Succinctly well$said. Homer “doh!” deuce long-run black-box sh!out putt.

  8. The first assumption of the mentioned article and the climate models is that there is an Equilibrium Climate Sensitivity (ECS) with regard to what they call greenhouse gases. However, there is no ECS with respect to CO2 or even water vapour but there may be with regard to clouds. To date no model has been able to include variables which may be able to express the effect of cloud formation, duration, density etc. In the atmosphere CO2 can only absorb radiation at a wavelength of around 14.8 micron which is not emitted from the Earths surface and occurs at a temperature around 200-220K where it is emitted to space. By the laws of thermodynamics CO2 has no part in ECS. Clouds affect the quantity of heat energy arriving at the Earth’s surface from the sun by reflection and absorption. Clouds also radiate to space.

  9. oldbrew says:

    Still pursuing their ‘climate sensitivity’ fantasy…

    MARCH 3, 2021
    High end of climate sensitivity in new climate models seen as less plausible

    Researchers at Princeton University and the University of Miami reported that newer models with a high “climate sensitivity”—meaning they predict much greater global warming from the same levels of atmospheric carbon dioxide as other models—do not provide a plausible scenario of Earth’s future climate.

    Those models overstate the global cooling effect that arises from interactions between clouds and aerosols and project that clouds will moderate greenhouse gas-induced warming—particularly in the northern hemisphere—much more than climate records show actually happens, the researchers reported in the journal Geophysical Research Letters.

    Instead, the researchers found that models with lower climate sensitivity are more consistent with observed differences in temperature between the northern and southern hemispheres, and, thus, are more accurate depictions of projected climate change than the newer models. –[bold added]
    – – –
    Suspecting that the lower they go with their so-called sensitivity, the more they approach some sort of accuracy. At least the clouds factor gets mentioned.

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