After the whole RCP8.5 fiasco, that some may remember, I got interested in thinking more formally about modelling scenarios. I even contacted a philosophy colleague to discuss the possibility of working together on philosophy of science aspects of this issue. A lack of direct relevant expertise, and a lack of time, meant I didn’t really take this any further. However, it is still a topic that I find interesting.
Any modelling will require some assumptions about the system being studied. If you want to understand how our climate might change, you need to consider possible future emission pathways. If you want to understand how a virus might spread, you need to consider what we might, or might not, do in response. This is essentially what we mean by scenarios in this context.
The choice of scenario will depend on the motivation behind the work. It might be a purely scientific question: how will the system evolve under these conditions? It could be purely modelling: how does a new model compare to an older model when using the same scenario? It could be to inform policymakers: what should we do to avoid this outcome, or what will probably happen if we follow this pathway?
One of the criticisms of RCP8.5 in climate modelling is that this pathway is implausible, but has been presented as a business-as-usual pathway. Although a reasonable criticism, much of the rhetoric around this was, in my opinion, too extreme and ignored the many valid reasons for considering this kind of scenario. I don’t want to rehash my arguments again, but I did write a number of posts that you can probably find if you’re interested.
The climate science community is now in the process of developing the scenarios for the next IPCC Assesement cycle and, unsurprisingly, the usual suspects are claiming that climate science is about to make a huge mistake because, despite advice to the contrary, they’ve chosen – again – to assign the most extreme scenario its highest priority.
Given the seriousness of this issue, and that the criticism seems to be getting quite a lot of traction, I thought it important to read the original paper that presents a perspective on the next generation of Earth system model scenarios. As far as I can see, it is not assigning the highest priority to the most extreme scenarios. It also seems to have considered most of the criticisms of scenarios, discusses the different uses for scenarios, including high-end pathways, and highlights that it would be beneficial to separate high forcing pathways for scientific purposes from the more policy-oriented framing pathway categories.
So, once again, it seems that the criticism is – at best – wildly exaggerated, or – at worst – completely misrepresents what’s being done. Of course, the latter would probably not surprise some people. It’s always been pretty clear that much of the criticism of RCP8.5 was motivated more by a desire to find something to criticise, than by any desire to be constructive. The same seems to be the case here. To be fair, if you’re a bad faith actor motivated by a desire to simply find something to criticise, it must be tricky to know how to respond if those you’re criticising actually take your initial criticisms seriously.