In the linked article I believe that Zeke Hausfather has done a good job of explaining consensus climate scientists position that BAU emissions will like fall (roughly) between the RCP 8.5 the RCP 6.0 forcing scenarios, and he even note that uncertainties in climate models with regard to carbon cycle climate feedbacks may justify leaning towards use of RCP 8.5 by recent studies.
That said, here I position out a few (of many) reason for believing the Hausfather's logic errs on the side of least drama (ESLDs):
a) The recent CMIP6 results indicate that cloud feedback is primarily leading to ECS values on the order of 5.3C (as opposed to 3C). Thus, Hausfather's focus on carbon cycle uncertainties ESLDs; and both Hausfather and CMIP6 ignore the ice-climate feedbacks characterized by Hansen et al. (2016) for abrupt ice sheet collapse.
b) The authors of RCP 8.5 defined it based on 90% percentile of the published peer-reviewed literature as the “very high baseline emission scenario” available at that time; and then Hausfather distorts this definition in to the "high-end of possible baseline emissions scenarios", which it is not. In this regard, Hausfather downplays the importance of methane emission and methane's chemical interactions in the stratosphere to increase the GWP (global warming potential) of emissions from such sources as: hydrofracturing of shale deposits; lakes, and agriculture; particularly over the 20-year timeframe when the GWP for methane emission could be as high as 105 time that of carbon dioxide.
c) The AR5 definition of pre-industrial global mean surface temperature almost certainly ESLDs (due to historical errors in measured land and sea temperatures); thus we are likely already closer to conditions that will support a cascade of feedback mechanisms than Hausfather conveys in his article.
Title: "Explainer: The high-emissions ‘RCP8.5’ global warming scenario"
https://www.carbonbrief.org/explainer-the-high-emissions-rcp8-5-global-warming-scenarioExtract: "A sizeable portion of recent studies on future climate impacts have focused on a warming scenario called “RCP8.5”. This high-emissions scenario is frequently referred to as “business as usual”, suggesting that is a likely outcome if society does not make concerted efforts to cut greenhouse gas emissions.
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According to the researchers who developed it, RCP8.5 was intended to be a “very high baseline emission scenario” representing the 90th percentile of no-policy baseline scenarios available at the time.
The creators of RCP8.5 had not intended it to represent the most likely “business as usual” outcome, emphasising that “no likelihood or preference is attached” to any of the specific scenarios. Its subsequent use as such represents something of a breakdown in communication between energy systems modellers and the climate modelling community.
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The literature around the development of RCP8.5 makes it clear that the scenario represents the high-end of possible baseline emissions scenarios rather than the most likely “business as usual” outcome. The original paper outlining the RCPs suggest that there is no reason to think that a high-emission RCP8.5 baseline would be any more likely than a lower emission RCP6.0 baseline in a no-policy world.
However, its position as the only non-mitigation scenario considered in the IPCC AR5 along with relatively poor communication between energy modelling and climate modelling communities led to a widespread misperception both in the media and in the academic literature that RCP8.5 was the expected “business as usual” outcome in a world without any future climate policy.
While worst-case outcomes are important to take into account, particularly given the uncertainties in the magnitude of carbon cycle feedbacks, it is important that they not be considered in isolation. Taking the range of possible baseline outcomes from 6.0 to 8.5 W/m2 forcing would provide a more realistic set of scenarios for studying climate impacts in a no-policy future."