AR6 Climate “Muddles”

ATTP has a new post which highlights a fundamental problem with increasing complexity of AR6 climate models. This is based on a new article in Nature by Zeke Hausfather et al . Some of the latest models in AR6 are running far too hot. The more sophisticated they try to become  so the hotter they seem to run. Modellers introduce new complexity through informed guesswork to parameterise it. The article notes however that such “hot models ‘ have problems.

“Numerous studies have found that these high-sensitivity models do a poor job of reproducing historical temperatures over time and in simulating the climates of the distant past. Specifically, they often show no warming over the twentieth century and then a sharp warming spike in the past few decades, and some simulate the last ice age as being much colder than palaeoclimate evidence indicates”

ATTP summarises the article as follows

The Hausfather et al. article was basically suggesting that researchers who use climate model output to assess the impact of climate change should aim to follow a similar practice to what was presented in the most recent IPCC report. Use GWLs, rather than simply focussing on scenarios going to 2100, weight the models if the warming trajectory is relevant, and try to consider which models may be best suited to the problem that is being considered.

So I thought it would be interesting instead to go right back to basics and use the simplest CO2 green house climate “model” possible. From this we can derive the radiative forcing due to rising levels of CO2 giving a logarithmic dependence

Following this we can simply make an estimate for the net change in surface temperature due to CO2.

S = \epsilon\sigma T^4
DS = 4\epsilon\sigma T^3 DT
DT = \frac{DS}{4\epsilon\sigma T^3}

Averaging over clouds(\epsilon = 0.5), oceans and land(\epsilon = 0.95) we then get a global averaged \epsilon = 0.65

If we assume that each increment in forcing DS is offset by the same increase in Black Body radiation due to a small surface temperature rise DT, then we can iterate through the temperature rise due to increases in CO2 concentrations. We take Pre-industrial CO2 levels as 280 ppm with an associated temperature of 288 K

Here is the result of this “model”.

Simplest possible Climate Model.

We can see that temperatures should rise by ~1C since pre-industrial times which proves to be surprisingly accurate!  A  doubling of CO2 levels would then lead to a net temperature rise of just 1.5C due to the logarithmic increase in forcing. This is not a  “climate  breakdown”.

I once worked on Magnetohydrodynamic (MHD) code for modelling Tokamak plasmas. That too was immensely complicated but recent breakthroughs in fusion have mainly been  experimental. So I suspect the same will apply to our current dependence on ever more complex climate models predicting imminent doom.


About Clive Best

PhD High Energy Physics Worked at CERN, Rutherford Lab, JET, JRC, OSVision
This entry was posted in AGW, Climate Change, climate science. Bookmark the permalink.

21 Responses to AR6 Climate “Muddles”

  1. forbin says:

    Hello Clive,

    Thanks for the post . One wonders when this simplied model will change government position . I think it will be ignored as there;s too much invested in the old models and could be embarrising,



  2. Thank you, most interesting

  3. entropicman says:

    Climate sensitivity?

    • Clive Best says:

      Climate sensitivity is ~ 1.5C

      • entropicman says:

        Your climate sensitivity of 1.5 seems low. Consider the temperature formulation of the CO2 forcing equation.

        ?T=5.35ln(C/Co) climate sensitivity/warming effect of forcing

        (IPCC quote a warming effect of forcing of 3.7W/C)

        Due to lag we don’t see the full effect of CO2 increase in real time. We can calculate transient climate response using ?T to date.

        TCR= ?T*warming effect/5.35ln(C/Co)

        1.2*3.7/5.35ln(420/280) =2.05

        TCR to date is 2.05, larger than your 1.5 estimate, so the full ECS will be larger again.

        • entropicman says:

          Your browser failed to parse deltaT. Instead it out up ?T.

        • Clive Best says:

          I was simply using Stefan Boltzmann to estimate the temperature rise needed to offset the CO2 forcing where I estimated the emissivity.

          IPCC instead talk of a sensitivity parameter lambda where Delta T = lambda DS and quote lambda = 0.8 K/(W/m2). So that indeed gives much higher values (for ECS) = 4C

          So if I take epsilon as 0.95 instead of 0.65 TCR rises to 2.2C

          • Entropic man says:

            That sounds more likely..

            The low climate sensitivities discussed by writes such as Nic Lewis always seemed too low to account for the warming already observed.

            If the low sensitivity were genuine it required that either that CO2 forcing was a lot greater than expected or that an unknown factor was doing a lot of the warming. The former disagrees with the physics and the latter would be hard to miss in this highly monitored era.

  4. Clive,
    I think your comparison between your model and observations ignores that we’re not currently in energy balance/equilibrium. If we were in energy balance at today’s atmospheric CO2 concentration, warming would probably be closer to something like 1.5 – 1.7C, than 1C.

    • Clive Best says:

      This is not true in this model. At all stages there is a balance between incoming energy from the sun (constant) and outgoing energy including CO2 molecules in the upper atmosphere. The temperature change at the surface is just calculated using the Stefan Boltzmann.

      The energy imbalance balance you refer to is the deep ocean equilibrium. Radiative balance depends only on surface temperature.

      • Clive,
        Yes, I realise it’s not true in this model. That’s essentially my point. Your model predicts ~1C of warming today, relative to pre-industrial, which you suggest is “surprisingly accurate”. However, this is essentially equilibrium warming. The warming since pre-industrial is not warming to equilibrium. Given today’s level of atmosopheric CO2, we’d probably expect something closer to 1.5 – 1.7C if the system warmed to equilibrium.

        Additionally, the change in forcing we’ve experienced since pre-industrial is not simply CO2. There are other GHGs and there are aerosols. So, there is quite a large uncertainty in the change in forcing, which is one reason why there is an uncertainty in climate sensitivity.

        • Clive Best says:

          The climate never really is in equilibrium. Seasons affect the northern and southern hemispheres differently. Volcanic eruptions change aerosols as much if not more than humans. There are oceanic oscillations like El Nino and AMO. Farming and cities change the Albedo etc.

          I have my doubts that ECS really has any meaning in this context. Otherwise you could argue that the warming today includes the latent warming playing out today from the early industrial revolution. The only measure we have a handle on is the measured global temperature at one time.

          • No use arguing over ECS as there is simply not enough discriminatory power to claim one model over another. It’s just a trend with the limitations inherent in that, and with the climate variations due to ENSO creating havoc every few years (cf. the AGW “pause”).

            OTOH, geophysics and geophysical fluid dynamics is providing all the variation in which we can discriminate the models against much more complex time series. If the climate scientists would get their heads out of the sand and acknowledge what some of the obviously disenchanted former NASA JPL scientists are suggesting regarding the impact of torque on the Earth’s dynamics we can start filling in the missing pieces. See research by James H. Shirley, Claire Perigaud, Steven L. Marcus. I can also fill in the gaps but have even less pull.

  5. Charles k May says:

    I don’t think there is any valid climate model without including natural variability. I know Dr. Curry and others are proponents of this. I think the current models are close to worthless and are unworthy to be used for policy decisions.

    To add further from what you have posted, I added what you see below. This comes from Prof. Scafetta;

    I take this opportunity to share my latest article on “Geophysical Research Letters”:

    Advanced Testing of Low, Medium, and High ECS CMIP6 GCM Simulations Versus ERA5?T2m – Scafetta – 2022 – Geophysical Research Letters – Wiley Online Library

    I test all the CMIP6 GCMs used now by the IPCC (the CMIP6s) from 1980 to today and show that the great majority of these models are incompatible with the observed warming trends. Essentially the models show excessive warming which is not observed.

    Those that seem to better agree with the data are those with low equilibrium climate sensitivity, even though these too have conspicuous deficiencies.

    In any case, only the low ECS models could at most be used for policy, but these models only predict moderate warming for the next few decades even in the worst-case emission scenario. Thus, climate adaptation policies should be sufficient to address future climate changes.

    Plain Language Summary

    The last-generation Coupled Model Intercomparison Projects (CMIP6) global circulation models (GCMs) are used by scientists and policymakers to interpret past and future climatic changes and to determine appropriate (adaptation or mitigation) policies to optimally address scenario-related climate-change hazards. However, these models are affected by large uncertainties. For example, their equilibrium climate sensitivity (ECS) varies from 1.83°C to 5.67°C, which makes their 21st-century predicted warming levels very uncertain. This issue is here addressed by testing the GCMs’ global and local performance in predicting the 1980–2021 warming rates against the ERA5-T2m records and by grouping them into three equilibrium climate sensitivity (ECS) classes (low-ECS, 1.80–3.00°C; medium-ECS, 3.01–4.50°C; high-ECS, 4.51–6.00°C). We found that: (a) all models with ECS > 3.0°C overestimate the observed global surface warming; (b) Student t-tests show model failure over 60% (low-ECS) to 81% (high-ECS) of the Earth’s surface. Thus, the high and medium-ECS GCMs do not appear to be consistent with the observations and should not be used for implementing policies based on their scenario forecasts. The low-ECS GCMs perform better, although not optimally; however, they are also found unalarming because for the next decades they predict moderate warming: ?Tpreindustrial?2050 ? 2°C.

  6. Scott Jones says:

    Or we could just use the raw data and average the global temperature over a 12 month rolling average and find that the current global average temperature is 13.66C or 287K.
    Not much of a trend or a threat. refer Temperature.Global website (no .com or .org)

  7. Javier says:

    “The more sophisticated they try to become so the hotter they seem to run.”

    A clear indication of a troubled paradigm.

    Models and ECS studies assume all the warming is coming from changes in GHGs, since natural factors and internal variability have been ruled out without evidence. They are all worthless despite costing a huge amount of money.

    “We can see that temperatures should rise by ~1C since pre-industrial times which proves to be surprisingly accurate!”

    Don’t get your hopes too high. Your model, as everyone else’s, assumes that the location where the radiation exchange at the ToA happens has no effect, and that the GHG effect (in your case just CO2) is global and doesn’t change over time except through their concentration changes. The truth is that GHG concentration is VERY DIFFERENT at different parts of the globe (that pesky H2Ov) and experiments not well constrained changes over time. As an example, OLR over the winter North Pole depends exclusively on the amount of heat transported there, that changes over time. And in the polar atmosphere conditions an increase in CO2 results in a higher emissivity, because the atmospheric CO2 is warmer that the surface (as per van Wijngaarden & Happer 2021). The amount of energy transported to the North Pole has a huge effect on the Earth’s energy budget and its changes are neglected in every GC model.

    Have you checked monthly changes in ToA OLR for the 70-90ºN region? Very ilustrative.

    All those assumptions are wrong. Climate science has become the equivalent to the discussion about how many angels can dance on a pinhead. Reality is being ignored while climatology more and more resides inside computers.

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