Guest Post by Willis Eschenbach
(Image above shows the Cray Ecoplex NOAA GAEA supercomputer, which can generate improbable future climate scenarios far faster than we simple humans …)
Someone on the web was touting the abilities of the early climate models by referencing the study Evaluating the Performance of Past Climate Model Projections by Zeke Hausfather, Henri F. Drake, Tristan Abbott, and Gavin A. Schmidt.
And yes, per their study, the climate models are just wonderful in their ability to hindcast the past and forecast the future. They put up a variety of tables and graphs to show that.
But I was interested in something different. I wanted to see what the transient climate response (TCR) of each of the models was. The Intergovernmental Panel on Climate Change (IPCC) defines the transient climate response (TCR) as the global temperature change at the time of a doubling of carbon dioxide (CO2) in a 1% per year increase experiment.
In other words, it’s the change in temperature corresponding to a 1 W/m2 increase in forcing, times the 3.7 W/m2 increase in forcing that the IPCC says will occur from a doubling of CO2.
So I looked at the model data, which the authors very responsibly posted online as an Excel spreadsheet at GitHub, and calculated the TCR for each model that they analyzed.
Then in addition to the TCR I wanted to calculate the ECS, the Equilibrium Climate Sensitivity. To do that I looked at the TCR and the ECS of 23 models from this paper. Here’s that data, along with a LOWESS smooth of the data.
Figure 1. Scatterplot, equilibrium climate sensitivity (ECS) versus transient climate response (TCR), 23 CMIP models.
So I converted the TCR values from the model performance study into ECS values, using the LOWESS line to do the conversion. Details in the endnotes.
Here are those results for the various models.
Figure 2. TCR and ECS for the 12 models investigated in the model performance paper.
Now, the interesting part of this is that the climate sensitivity (ECS) for the models covers a very wide range, from two to four degrees for a doubling of CO2 … and yet, they all do a whiz-bang job of hindcasting the temperature history of the planet.
And IF they are all “physics based” as is always claimed, I’m sorry but that’s not possible.
I call this “Dr. Kiehl’s Paradox” because back in 2007, in a paper published in GRL entitled “Twentieth century climate model response and climate sensitivity” Jeffrey Kiehl first noticed this oddity. He said:
The question is: if climate models differ by a factor of 2 to 3 in their climate sensitivity, how can they all simulate the global temperature record with a reasonable degree of accuracy?
A good question indeed. In response to that paper, I wrote a post I entitled “Dr. Kiehl’s Paradox“. In that post I discussed Dr. Kiehl’s answer to that question, as well as my own answer to that question. I followed that up with a couple more analyses entitled “Zero Point Three Times The Forcing” and “Life Is Like A Black Box Of Chocolates“.
In those posts, I showed that despite their immense complexity, the global temperature output of the climate models can be emulated very exactly by a one-line equation that simply lags and scales the forcings used as the input to the models.
Oh, yeah, a final oddity. In the Hansen 1981 model, the TCR and thus the ECS are not constant. From 1981 to 2024 the ECS goes from about 1.6°C/2xCO2 to about 2.5°C/2xCO2. The ECS then continues to increase to 2040, decreases slightly to 2075, and by 2100 it’s all the way up to 2.8°C/2xCO2. That’s almost twice its starting value.
Say what?
Figure 3. TCR and ECS for the Hansen 1981 model. Each year’s TCS is calculated using the change from the 1950 forcing and temperature to the forcing and temperature for that year.
So I have to say that I’m not impressed by the apparent agreement of the models with reality. They are not physics-based. Instead, they are simply tuned to match the past … and in the case of Hansen 1981, tuned to exaggerate the future warming.
And as my bonafide genius older brother used to say, “It’s easy to predict the future … as long as it’s just like the past”.
Unfortunately, as I showed in my last post entitled Now You Sea Ice, Now You Don’t, the climate is occasionally and quite unpredictably very unlike the past …
Regards to all,
w.
Yeah, you’ve heard it before: When you comment, please quote the exact words that you are discussing. It avoids endless problems.
For the math folks: The function I used in Excel to convert the TCR into the ECS is based on an approximation of the LOWESS curve in Figure 1. It is:
= IF(TCR < 1.6, 1.39 + 0.8 * TCR, IF(TCR < 2, -1.04 + 2.32 * TCR, 1.4 + 1.1 * TCR))
This defines three approximately straight-line sections of the LOWESS smooth, and uses their formulas to calculate the ECS depending on the value of the TCR
Related
