Originally tweeted by The Honest Broker by Roger Pielke Jr. (@RogerPielkeJr) on April 25, 2023.

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I talk a lot about RCP8.5, and rightly so

But there is another scenario that is just as widely misused, and that is RCP4.5

Let’s talk a bit about RCP4.5 . . .

RCP4.5 is a scenario that includes climate policy which alters the world’s trajectory from a 7.0 W/m^2 trajectory in the GCAM reference scenario

https://link.springer.com/article/10.1007/s10584-011-0151-4

RCP4.5 is often (maybe even typically) paired with RCP8.5 to demonstrate climate policy success, with RCP8.5 used as a reference scenario (or, as they say, BAU)

A good example of this practice is the 2018 US National Climate Assessment

This is flawed for at least 3 reasons…

First, as you surely know by now, RCP8.5 is not an appropriate reference scenario for anything, so using that as a baseline for any purpose is methodological malpractice

Second, you cannot pair RCP8.5 and RCP4.5 and they are from different IAMs & represent very different worlds

3rd
The big one – if RCP4.5 is indeed to be viewed as climate policy success, then we can all go home as according to the UNFCCC the world is currently undershooting RCP4.5 as you can clearly see in figure below (which illustrates SSP2-4.5 trajectory)

https://rogerpielkejr.substack.com/p/pielkes-weekly-memo-16

So if you believe climate policy stops today then you can use RCP4.5 as an upper bound scenario (not as a stress test of course, a different discussion)

Don’t just take it from me, the Biden administration’s central SCC scenario undershoots RCP4.5 to 2100

https://rogerpielkejr.substack.com/p/the-biden-administration-abandons

Don’t like Biden admin estimates?
Fine
Here is our assessment of plausible IPCC scenarios with SSP2-3.4 at the center and SSP2-4.5 as an upper bound

https://iopscience.iop.org/article/10.1088/1748-9326/ac4ebf

Why is RCP4.5 not a good proxy for climate policy success?

Well, according to IPCC AR6 it leads to a central estimate of 2.9C temperature change in 2100, which is not 2.0 or 1.5

https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-4/

So why is RCP4.5 frequently used to represent climate policy success & RCP8.5 used as a baseline, when both uses are scientifically unjustifiable?

Well, those are the scenarios that are readily available for research, so they get used even though the use is scientifically flawed

Google Scholar says there are 24,500 studies that pair RCP4.5 and RCP8.5 in research

Just look at the example titles below

To the extent that this research is supposed to say something about plausible futures, it is of little value to informing policy & may even mislead

So if you are a researcher and you think, “Hey, maybe I’ll do a comparison of XYZ in RCP8.5 and compare it to RCP4.5 to illustrate the benefits of mitigation”

Just say no

And if you are a consumer of climate science in your research, as a reporter or in policy and you come across a study that compares the fictional world of RCP8.5 and the false world of RCP4.5 as mitigation success

Move along, nothing to see here

You want to do a proper study of the benefits of mitigation based on today’s view of “current policy” trajectories?

Use RCP4.5 as an upper end and RCP3.4 (or 2.6) as your lower end to represent mitigation success

Guess how many studies exist that perform that analysis?

This thread will soon be a post at The Honest Broker, as nerdy as it is, it is a story needed to be told because the misuse of RCP4.5 – while not as flashy as RCP8.5 – is important and endemic

Comments welcome here

Post coming before long, don’t miss it:

https://rogerpielkejr.substack.com/

Originally tweeted by The Honest Broker by Roger Pielke Jr. (@RogerPielkeJr) on April 25, 2023.