The previously assessed most likely (catastrophic) scenario now deemed low probability

A very good summary, as might be anticipated, from How to Understand the New IPCC Report: Part 1, Scenarios by Roger Pielke Jr..  The subtitle is "Contrary to what you've been reading, the massive new IPCC report offers grounds for optimism on climate science and policy"

Worth reading in its entirety.  A couple of points worth making before getting into the quotes.

1)  The technical details making up the bulk of the periodic IPCC Reports always tend to be much less alarmist than the summary overview.  The summary overviews tend to be much less alarmist than the mainstream media readings of the summary.

2)  Climate change is real and has always been real.  That is not the central issue.  The central issue is the extent to which human activities may be contributing to an acceleration of those changes or an exacerbation of those changes.  A secondary issue is - which changes, depending on their causal nature, should be mitigated tactically versus strategically.

The southwest US has long cycle droughts.  Does rising CO2 levels either accelerate that baseline problem or do they make the droughts worse (but on the same periodicity)?  Given that there is a natural cycle, even without the imputed CO2 contribution, there is some climate change that is natural and consequential.  If CO2 were to turn out to have no acceleration or exacerbation impact, we would still need to assess what an appropriate response to increased heat, desiccation and reduced rainfall might be.  Do we choose to adapt to the natural cycle or do we choose to change the cycle (if feasible)?  

These nuances are usually steamrolled in the effort to stir up climate hysteria.

Back to Pielke:

The current IPCC report is notable because it has stated that among the 5 “illustrative” scenarios that it emphasizes, it assigns no likelihood to any of them.  

That is important because it represents a 180-degree turn from the previous IPCC assessment, which identified one scenario as most likely, called a “reference scenario.” Because the 2021 IPCC has decline to attach a likelihood to its scenarios, that means that in order to properly interpret the new IPCC report, you and I have to assess the likelihoods of different scenarios.

It is important.  This is just plain old risk planning.  How much upside benefit is there, how much downside risk,  how much would risk mitigation cost, and which scenarios are most likely.  

Much to its credit (and seemingly at odds with its claim to assign no likelihoods to scenarios), the IPCC has concluded — just as we have in our research — that several of its scenarios are of low likelihood. This is very good news because these implausible scenarios are the report’s most extreme scenarios.

For my technical readers, the scenarios judged unlikely by the IPCC are high emission (“such as RCP8.5 or SSP5-8.5”) and the scenarios “in line” with current policies are intermediate scenarios (“RCP4.5, RCP6.0 and SSP2-4.5”).

This is huge news. Fantastic in fact. Why? The extreme scenario RCP8.5 was in the most recent IPCC report identified as our most likely future. Now IPCC has completely reversed that, and it is now considered low likelihood. There could not be a more profound change in the scenario foundation of climate science.

Instead of apocalyptic warnings about “immediate risk” a top line message of this report should be: Great News! The Extreme Scenario that IPCC Saw as Most Likely in 2013 is Now Judged Low Likelihood. I am actually floored that this incredible change in such a short time apparently hasn’t even been noticed, much less broadcast around the world. 

Indeed.  The extreme scenarios were always unlikely and yet they drove the policy debate.  Read the article for the details for why the extreme scenarios were never plausible in the first place.  

Despite acknowledging the low likelihood of the most extreme scenarios RCP8.5 and SSP5-8.5, which were the dominant focus of the 2013 IPCC report, the extreme scenarios dominate the current report as well. This is obvious from the table below which shows the number of mentions of various scenarios in the new report. 

Pielke notes that the most implausible, indeed non-viable, scenarios still take up 50% of the reports discussion.  It's like visiting a doctor having the sniffles and a sore stomach and the conversation focusing on the probability that you have Ebola.  Conceivable but to all practical purposes untrue.

Pielke, Lomborg and others are right.  Climate changes.  There are human practices which likely contribute in some ways to the natural climate rhythms but we are very clear yet on which practices and to what extent they impact outcomes.  While CO2 is a candidate, the more consequential locally are almost certainly land-use practices as well as urban density (more efficient energy consumption but also more impact on locality due to things such as the heat-island effect.)

Climate change is a valid and important point of discussion.  We just aren't really discussing it.  Instead we are still dealing with hysteria and edicts.

As an aside, and warranting a separate post.  It is interesting that the three big controversies at the moment and for the past couple of years are all model forecasting issues.  I.e. the models have demonstrable error rates in their scenarios - election outcome forecasting, climate forecasting, and Covid-19 mortality forecasting.  

75% of the noise is due to poor model performance and bad communication practices surrounding the poor model performance.  We are not focusing on solving agreed problems.  

And for an example of a bad communication practice look at that IPCC flip-flop.  The most emphasized and deemed most probable outcome has been, in the space of a few years moved from most probable to low likelihood.  It is good that they are being more transparent now but we are still left with the residual loss of trust from their exaggeration of an improbable scenario earlier to a more realistic one now.  The scenarios haven't changed that much, they are just being more honest about the probabilities.  Their communication practices have undermined the trust accorded to them.