To state the obvious, the findings of this research implies that ECS is currently about 5.5C, ...
There is nothing obvious about those
unbelievably high ECS values of the current generation of GCM's, the CMIP6 models.
The following highly critical paper evaluates the differences between CMIP5 and CMIP6 model generations.
From the abstract: "...Here we show that the closely related effective climate sensitivity has increased substantially in Coupled Model Intercomparison Project phase 6 (CMIP6), with values spanning 1.8–5.6 K across 27 GCMs and exceeding 4.5 K in 10 of them. This (statistically insignificant) increase is
primarily due to stronger positive cloud feedbacks from
decreasing extratropical low cloud coverage and albedo. Both of these are tied to the physical representation of clouds which in CMIP6 models lead to weaker responses of extratropical low cloud cover and water content to unforced variations in surface temperature. "
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL085782Attached figure from the same paper shows that almost all differences between the two model generations are attributable to the way that the models are handling clouds and water vapor. Low cloud water content and coverage decrease more strongly with global warming, causing enhanced planetary absorption of sunlight—an amplifying feedback that ultimately results in more warming.
The factual development is on the other hand that global cloud cover has been slowly
increasing over the last decades, not decreasing.
We can't even say for sure that clouds are a positive feedback. (Most of the 27 evaluated models have a +ve feedbakc , but 2 have a -ve feedback). The basic physics says that clouds are a negative feedback. NOAA says: "The cloud radiative effect (CRE) on the Earth’s present-day radiation budget can be inferred from satellite data by comparing upwelling radiation in cloudy and non-cloudy regions. The figure at right shows that cloud conditions exert a global and annual SW CRE of approximately -50 W/m2 and a mean LW CRE of approximate 30 W/m2. The net global mean CRE is approximately -20 W/m2 implying
a strong net cooling effect of clouds on the current climate. Given the large magnitude of SW and LW CRE, clouds have the potential to cause significant climate feedback."
The issue of cloud feedback is much more complex than that, but the starting point is actually a -ve feedback.
From the papers' Conclusions: "While some high ECS models closely match the observed record (e.g., Gettelman et al., 2019), others do not (e.g., Golaz et al., 2019). Do the former models achieve their results via unreasonably large negative aerosol forcings and/or substantial pattern effects (Kiehl, 2007; Stevens et al., 2016)? It is worth noting that cloud feedbacks are enhanced in CMIP6
primarily over the Southern Ocean, a region of efficient ocean heat uptake (Armour et al., 2016). This implies that the enhanced surface SW heating is less likely to manifest as surface warming during transient climate change than if the heating were focused elsewhere (Frey et al., 2017). This cloud feedback pattern could make it easier for high ECS models to simulate the observed surface temperature record without requiring a large negative aerosol radiative forcing or large historical era pattern effects."
This conclusion raises the issue that the unbelievably high ECS models are manipulated in a false way as a means to achieve a goal, namely the high ECS value.
What's
obvious, is that these latest "bottom up" models still are unable to properly handle the feedback of clouds and water vapor.
https://www.gfdl.noaa.gov/cloud-radiative-effect/Figure byline: "Figure S7. Contributions of forcing and feedbacks to ECS in each model and for the multi-
model means. Contributions from the tropical and extratropical portion of the feedback are shown
in light and dark shading, respectively. Black dots indicate the ECS in each model, while upward
and downward pointing triangles indicate contributions from non-cloud and cloud feedbacks,
respectively. Numbers printed next to the multi-model mean bars indicate the cumulative sum
of each plotted component. Numerical values are not printed next to residual, extratropical
forcing, and tropical albedo terms for clarity. Models within each collection are ordered by ECS."