CMIP6 ECS – Watts Up With That?

CMIP6 ECS – Watts Up With That?

By Rud Istvan

This post was motivated by May’s second here on paleoclimate, by comments to  Bradley Jr’s  climate models repost from AIER, and my previous posts and comments about climate models and equivalent (observational) or equilibrium (modeled) climate sensitivity (aka ECS). All are hyperlinked below. This post summarizes some of the most important climate basics at a simple high level, while pulling together the myriad underlying specifics via linked references to a lot of stuff posted at WUWT over the years. Hopefully this is helpful at both levels for the many newer WUWT commenters. It also indirectly takes scientific aim at some of the extreme skeptics who also comment here, like those who deny that the greenhouse effect exists, or that ECS cannot be significantly positive.

First, the green house effect (GHE) is real although often misunderstood. Unlike a real greenhouse (which works by inhibiting surface convective cooling), the GHE works by inhibiting radiative cooling in the troposphere. “Greenhouse” gases (GHG) absorb and omni-directionally re-emit infrared radiation (IR), inhibiting (retarding) cooling IR escape to space. Greenhouse gasses (so also their ‘backradiation’) cannot warm—incoming insolation is solely responsible for that. But as first shown experimentally by Tyndall in 1859, GHG can inhibit radiative cooling. The level in the troposphere where the GHE ceases to be a factor is called the effective radiating level (ERL). The ERL is also why the GHE can never saturate. More GHG just raises ERL, meaning the ERL emission temperature is cooler so less efficient thanks to the troposphere temperature lapse rate. Which is why Guy Callendar realized in his 1938 paper that increasing CO2 has a decreasing logarithmic effect. For what follows, that also means that whatever the ECS per CO2 doubling might be, it remains about constant over a much broader concentration range.

Second, the notion that there must be some positive ECS to doubled CO2 is logically valid. Given steady insolation heating but reduced offsetting radiative cooling, Earth’s surface temperature must rise until the resulting increase in IR offsets the GHE so that TOA radiation ‘equilibrium’ is re-established. ECS can be established two basic ways. First is by problematic climate models that predicted about 3C in AR4 (CMIP3), about 3.4C in AR5 (CMIP5), and now about 3.7-4.5C in upcoming AR6 (CMIP6, specifics follow). Second is via various observational methods all suggesting about 1.7C. This 2x discrepancy was so great that AR5 explicitly declined to provide a central ECS estimate because of it.

Third, the climate models ARE problematic for an inescapable reason. Thanks to computational intractability of important climate phenomena size scales (e.g. Willis Eschenbach’s many TStorm posts here), they are forced to parameterize. Parameterization drags in the attribution problem, both on long time scales as Andy May just showed, and on short time scales of just the past century. Attribution very simply is how much of the observed past temperature change was natural, and how much was CO2 ‘forcing’. IPCC assumes (by charter!) all forcing, ignoring natural variability. MBH 1999 attempted to remove millennial time scale natural variability (erroneously) via its infamous hockey stick handle—erasing the historically well documented but only natural MWP.

Now we have coming soon for AR6 the newest and ‘best’ CMIP6 climate models. While their results are not yet finalized, it is already apparent that the climatariat and reality continue to diverge. The interim CMIIP6 ECS results are as follows.

The high ECS from the 40 reported models is 5.6C, the low is 1.8C (INM CM5, the Russian model close to observational methods). The mean is now 4.5 (yellow/blue), very far from observational methods. The median has risen to 3.7C. So, CMIP6 has significantly INCREASED ECS uncertainty (the opposite of what ongoing science is supposed to do), while further raising its modeled central tendencies ( now at greater variance from the observations that caused AR5 heartburn). As the text around the linked graphic above shows, the warmunists there find this both plausible, and cause for mild celebration!?!

It is apparent that those who believe in anthropogenic global warming do not understand the intractable fundamental problem with their climate model approach. They are digging themselves a deeper hole. The Army’s first rule of holes is, when in one and wanting out, first stop digging.

Or to paraphrase a very famous Feynman lecture summation:

‘It does not matter how beautiful your theory is. If it disagrees with observations (experiments based on the theory), IT IS WRONG.’


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