AR6

CMIP6: In a Sea of Junk Models, The Met Office’s UKESM1.0 Model Stands Out as Even More Junk

There’s a post published at Watts Up With That which provides a sneak preview of some CMIP6 models runs for the upcoming release of the IPCC’s AR6 (Part 1: Physical Science Basis due in April 2021). As the author, Andy May says:

The new IPCC report, abbreviated “AR6,” is due to come out between April 2021 (the Physical Science Basis) and June of 2022 (the Synthesis Report). I’ve purchased some very strong hip waders to prepare for the events. For those who don’t already know, sturdy hip waders are required when wading into sewage. 

Andy has looked at some of CMIP6 climate model runs posted on KNMI Explorer and this is what he found:

The base period is 1981-2010 and the emissions pathway is ssp245, which is similar to the old RCP4.5 concentration pathway. Most as you can see project global warming in 2100 to be somewhere between just over 1.0C and 2.5C, which in itself is quite a spread. But then you look at UKESM1.0 (light blue) and CanESM5 (yellow – partly obscured) and they are projecting warming anywhere between about 2.5C and 3.8C. They stand out like sore thumbs in 2100, as does UKESM1.0 hindcast warming in the 1960s using historical forcings. As you can see, UKESM1.0 cools the mid 20th century cooling period by -1.5C compared to 1981-2010! That is huge and is not borne out by actual observations. I went into the reasons for this discrepancy here.

To get a clearer picture of how UKESM deviates from actual measurements, here are the graphs of Hadcrut 4 against the model runs:

Quite obviously, UKESM1.0 vastly overstates mid 20th century cooling in the northern hemisphere. Why? Because it greatly overestimates the impact of anthropogenic aerosol cooling. Here is what the Met Office say about UKESM1.0 and the physical general circulation model on which it is based:

The Earth System Model UKESM1, and the physical model (or General Circulation Model) it is based on, HadGEM3-GC3.1 are the result of years of work by scientists and software engineers from the Met Office and wider UK science community.

Analysis shows the climate sensitivity of the models is high. For both models the Transient Climate Sensitivity (TCR) is about 2.7 °C, while the Equilibrium Climate Sensitivity (ECS)  is about 5.4°C for UKESM1 and about 5.5°C for GC3.1. Future projections using the new models are in progress. When these have been analysed, we will have a better understanding of how the climate sensitivity affects future warming and associated impacts.

Very high sensitivity means that historic aerosol forcings must be correspondingly high in order for the model to align with current (presumed highly accurate) global mean surface temperature data. But the aerosol forcing is so high that it ends up unrealistically cooling the 1960s. As I pointed out:

UKESM1 massively overstates mid 20th century cooling but it has to if it is to get the rest of the historical record more or less correct with such a ridiculously high sensitivity built in. Note that it is indeed overestimated aerosol cooling which is responsible for this 20th century mismatch because it is much more pronounced in the Northern Hemisphere where most of the heavy industry was and still is.

The Met Office confirms that large anthropogenic aerosol forcings were incorporated into the development of UKESM1.0:

UKESM1 is developed on top of the coupled physical model, HadGEM3-GC3 (hereafter GC3). GC3 consists of the Unified Model (UM) atmosphere, JULES land surface scheme, NEMO ocean model and the CICE sea ice model. The UM atmosphere in GC3 is Global Atmosphere version 7 (GA7). Inclusion in GA7 of both a new cloud microphysics parameterization and the new GLOMAP aerosol scheme led to a concern the model might exhibit a strong negative historical aerosol radiative forcing (i.e. a strong aerosol-induced cooling due to increasing anthropogenic emission of aerosol and aerosol precursors over the past ~150 years) with potentially detrimental impacts on the overall historical simulation of both GC3 and UKESM1.

A protocol was therefore developed to assess the Effective Radiative Forcing (ERF) of the mainclimate forcing agents over the historical period (~1850 to 2000), namely; well mixed greenhouse gases (GHGs), aerosols and aerosol precursors, tropospheric ozone and land use change. This protocol follows that of the CMIP6 RFMIP project (Andrews 2014, Pincus et al. 2016). The aim was to assess the change in the mean top-of-atmosphere (TOA) ERF between average pre-industrial (~1850 in our experiments) and present-day (~2000) conditions. In particular to assess the aerosol ERF, with a requirement that the total (all forcing agents) historical ERF be positive. Initial tests revealed an aerosol ERF of -2.2 Wm-2, significantly stronger than the -1.4 Wm-2 simulated by HadGEM2-A (Andrews 2014) and also outside the IPCC AR5 5-95% range of -1.9 to -0.1 Wm-2. As a result of the large (negative) aerosol ERF, the total ERF diagnosed over the historical period was approximately 0 Wm-2.

They were so large initially that they had to find a method of actually reducing them:

We therefore investigated aspects of GA7 that could be causing this strong aerosol forcing and, where possible, introduced new processes and/or improved existing process descriptions to address these. The goal of this effort was to develop an atmosphere model configuration solidly based on GA7.0 that:1.Had a less negative aerosol ERF and thereby a total historical ERF of >+ 0.5 Wm-22.

The above is bad enough news for the historical authenticity of UKESM1.0 and hence its reliability in terms of future projections, but it gets worse. A paper recently published argues that anthropogenic aerosol forcings cool the climate even less than originally thought, meaning that UKESM1.0 is even more out of sync with reality than as described above:

“Our conclusion is that the cooling effect of aerosols on clouds is overestimated when we rely on ship-track data,” says Glassmeier. “Ship tracks are simply too short-lived to provide the correct estimate of cloud brightening.” The reason for this is that ship-track data don’t account for the reduced cloud thickness that occurs in widespread pollution. “To properly quantify these effects and get better climate projections, we need to improve the way clouds are represented in climate models,” Glassmeier explains further.

Oh dear, it’s not looking good for the Met Office’s ‘flagship’ CMIP6 climate model. Maybe they need to raise the white flag of surrender. It’s not much better for the Canadian model either, or in fact any of the CMIP6 13 model ensemble according to Andy May.

Historical forcings are used prior to 2014 and projected values after. The blue and orange curves are from two runs from a single Canadian model. The two runs are over 0.2°C different in 2010 and 2011, some months they are over 0.5°C different. There are multiple periods where the model runs are clearly out-of-phase for several years, examples are 2001-2003 and 2014 to 2017. The period from 2015 to 2019 is a mess.

I’m unimpressed with the CMIP6 models. The total warming since 1900 is less than one degree, but the spread of model results in Figure 1 is never less than one degree. It is often more than that, especially in the 1960s. The models are obviously not reproducing the natural climate cycles or oscillations, like the AMOPDO and ENSO. As can be seen in Figure 2 they often are completely out-of-phase for years, even when they are just two runs from the same model. I used the Canadian model as an example, but the two NCAR model runs (CESM2) are no better. In fact, in the 2010-2011 period and the 2015-2019 period they are worse as you can see in Figure 4.