There’s been a lengthy discussion on Twitter about RCP8.5. I think it was initiated by Roger Pielke Jr, who continues in his campaign to police the scientific community:

RCP 8.5 is by far the favorite in climate impact studies (graph).

I get why it is used so much. It offers a great way to get published (results!) & to generate news stories (climate porn!).

But every study that uses it eats away a little bit at the credibility of climate science. pic.twitter.com/pMgZsTseqF — Roger Pielke Jr. (@RogerPielkeJr) May 9, 2018

This complaint was then promoted by Matt Ridley, and the discussion ensued. It remained mostly quite pleasant, but I found it quite remarkable that people who have publicly discussed this topic for a good number of years, are still remarkably confused about something that is ultimately quite simple.

The criticism of RCP8.5 was a combination of it being completely unrealistic, and it being a scenario that climate scientists use in their studies so as to get more interesting results. In case some don’t know, RCP8.5 is one of the Representative Concentration Pathways (RCPs). These are a set of scenarios that consider different possible future atmospheric greenhouse gas (and aerosol) concentration pathways. They can also be regarded as possible future forcing pathways; pathways that produce a range of different externally-driven radiative perturbations. They go from one in which we keep the concentrations, and resulting change in forcing, quite low (RCP2.6) to one (RCP8.5) in which the concentrations keep rising, eventually reaching a level such that the change in forcing, by 2100, is 8.5 Wm-2.

These concentration (or forcing) pathways can then be used as input to climate models to try to understand how our climate will respond to a range of different possible future pathways. However, to relate these concentration pathways to our actions requires associating them with emission pathways. However, given uncertainties in the response of the natural carbon sinks, there isn’t a single emission pathway for a given concentration pathway. Even though it would now seem unlikely that we will follow an emission pathway typically associated with RCP8.5, uncertainties in carbon cycle feedbacks mean that we can’t rule out that an emission pathway typically associated with a lower RCP could lead to us following a concentration pathway close to RCP8.5. Therefore, I don’t think we can yet definitively rule out RCP8.5.

The other issue is climate scientists supposedly using RCP8.5 in order to get more interesting results (what Roger calls climate porn). It may well be that climate scientists use RCP8.5 more often than other concentration pathways. One reason for using RCP8.5 is that a pathway with a large change in forcing allows one to better distinguish between the signal (i.e., the response to the change in external forcing) and the noise (the natural variations). One can then probably still use this to estimate what would happen were the change in forcing to be lower [Edit: As Katharine Hayhoe pointed out, you can do this because a high forcing pathway will move through the levels of change associated with the lower forcing pathways]. Another reason will also be that understanding extreme outcomes can, in some cases, be somewhat more important than studying outcomes that we would probably expect to not be particularly extreme.

Of course, I do think that people should be very clear about what they’re doing. It does seem unlikely that will now follow an emission pathway that will lead to an RCP8.5-like concentration/forcing pathway. Therefore people should not present results of studies that use RCP8.5 as being somehow likely. However, this does not mean that there is no value in still considering this pathway. As already mentioned, using a pathway with a large change in forcing can help to distinguish between the forced response, and internal variability. Additionally, there is also a chance that the more we dismiss the possibility of following an RCP8.5-like pathway, the more likely it becomes that we might actually follow one.

Update:

As pointed out on Twitter (H/T Ambarish Karmalkar) many studies actually use RCP4.5 and RCP8.5. In fact, when I did a quick search, a lot of the results did use both RCP4.5 and RCP8.5. The figure on Roger’s tweet also seems to show this; the two most commonly used scenarios are RCP4.5 and RCP8.5 (with RCP8.5 appearing to be used slightly more often than RCP4.5).