Why is the IPCC specializing in the eventualities it does – Watts Up With That?

Originally tweeted by Roger Pielke Jr. (@RogerPielkeJr) on Aug 7, 2021.

Some may be curious as to why the IPCC is focusing on the scenarios it is doing

Ultimately, these scenarios form the basis for looking at the future of the entire report and assessing the potential impact and value of various policy approaches. . .

The short answer is that the scenarios with the highest priority were selected for scientific purposes first and there are no plausibility considerations

This is how the CMIP6 exercise justified its basic scenarios (BAU / reference)

➡️Science & Unconstrained Baseline

Decisions about the scenarios to be prioritized were made in 2015/16, but build on previous decisions of CMIP5, IPCC 2007 & SRES 2000 and even earlier

The IPCC’s AR6 report in 2021 is actually an assessment based on scenarios that were identified as most relevant 20 years ago

We describe much of this story and its implications for climate science in this epic paper:


It’s more than a little silly to have a breathlessly awaited, embargoed report presenting an analysis of outdated scenarios, but here we are!

If you want a more readable, shorter version of the story, you can read this:

The IPCC could have decided to reassess scenarios for the AR6 for plausibility

That would have meant discarding or neglecting work that was once given top priority – hard to do, sunk resources, published papers, egos

It could have looked like this. . .

Re-imagined use of the CMIP6 scenario

Where we are on the move with current trends:

Here’s what the world could look like under more aggressive emission reduction policies:
SSP2-2.6, SSP1-2.6

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

Imaginative (to advance science):

This CMIP6 number clearly shows why sticking to 8.5 as the reference scenario versus 4.5 is attractive for advocacy / messaging purposes


Originally tweeted by Roger Pielke Jr. (@RogerPielkeJr) on Aug 7, 2021.


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