Thread by @RogerPielkeJr

🧵Some might be curious why the IPCC focuses on the scenarios that it does

After all these scenarios are the foundation of the entire report's look to the future & assessment of possible impacts and the worth of different policy approaches . . .

The short answer is that the highest priority scenarios were selected for scientific purposes first & considerations of plausibility absent

Here is how the CMIP6 exercise justified its baseline (BAU/reference) scenarios

➡️science & unmitigated baseline

gmd.copernicus.org/articles/9/346…

Decisions on what scenarios to prioritize were made in 2015/16 but build on earlier decisions of CMIP5, IPCC 2007 & SRES 2000 and even earlier

The IPCC AR6 report in 2021 is really an assessment based on scenarios that were determined to be most relevant as much as 20 years ago

We detail much of this history and its consequences for climate science in this epic paper:
doi.org/10.1016/j.erss…

It is more than a little silly to have a breathlessly-awaited, embargoed report presenting analysis of out-of-date scenarios, but here we are!

If you want a more readable, shorter version of the story you can read this:
issues.org/climate-change…

The IPCC could have decided to re-evaluate scenarios for the AR6 based on plausibility

That would have meant discarding or deemphasizing work once labelled highest priority - hard to do give sunk resources, published papers, egos

It might have looked like this . . .

Reimagined CMIP6 scenario use

Where we are headed on current trends:
SSP2-4.5

What the world might look like under more aggressive emissions reduction policies:
SSP2-2.6, SSP1-2.6

Exploratory (implausible what ifs):
SSP3-7.0, SSP1-1.9

Fanciful (to advance science):
SSP5-8.5

This CMIP6 figure illustrates clearly why holding firmly on to 8.5 as a reference scenario vs 4.5 is appealing for purposes of advocacy/messaging esd.copernicus.org/articles/12/25…