I’ve been reflecting a little on some of the recent discussions I’ve had, mostly on Twitter, with those who have more expertise in emission scenarios, and energy systems, than in physical climate science specifically. I’ve found it a somewhat frustrating experience, but it seems that there’s also frustration on the other side too, specifically with the use of RCP8.5, which many seem to think is unrealistic.
One simple issue is that I think many climate scientists simply see the representative concentration pathways (RCPs) as bracketing the range of possible concentration pathways that we could follow, without thinking specifically about how we might do so. One reason for the latter is that there isn’t an easy way to determine how we might follow a specific concentration pathway; it will depend on our future energy pathway, how the carbon sinks respond, and on whether or not some carbon cycle feedbacks start to operate.
My impression is that many emission/energy experts think that we should put more focus on considering the impact of following a pathway that is likely, rather than simply considering a small sample of concentration pathways, at least one of which is seen as very unlikely. Climate models are, however, very computationally expensive, and so there is a limit to how many scenarios can be considered. It’s also important to be able to compare different climate models, so using the same basic scenarios can be important for that reason too. It’s also often possible to extrapolate between scenarios to work out how the system might respond if we followed some scenario that hadn’t explicitly been considered by a climate model.
One might think that if the highest concentration pathway isn’t really all that plausible anymore, that there shouldn’t be as much focus on it. There is certainly some merit to this. However, there are also reasons to still consider this scenario. One is that the impact of climate change depends mostly on how much we are likely to warm, which depends on how sensitive the climate is to these radiative perturbations. If you use RCP8.5 with a climate model that has a climate sensitivity near the middle, or lower half, of the range, then this can also represent what might happen if we follow a lower concentration pathway, but climate sensitivity turns out to be higher.
This is essentially my perspective; until someone can rule out the higher levels of warming (by considering both the plausible range of concentrations pathways and the range of climate sensitivity) then I think it is still important to understand the impact of these higher levels of warming.
I also think that this whole basic debate ignores something that I happen to think is key. We’re moving the Earth’s climate into a regime that we have probably never experienced before. We have some idea of what might happen, but we can’t really know for sure. Maybe we’ll be very lucky and find that it evolves smoothly and in ways that we can manage. Maybe, however, we’ll discover that some natural processes have been masking some of the forced warming, and that we have more warming in the pipeline than we expected, or that we’ve crossed some tipping points that lead to substantial, irreversible, changes on a short timescale.
Clearly we can’t avoid some future warming, since we can’t simply stop using fossil fuels overnight. However, rather than arguing about whether or not climate scientists are using the optimal concentration pathways in their models, maybe we should just recognise that we might not want to face too much more future warming and should spend our time finding ways to limit our future emissions. That’s just my view, of course. Other people probably have a different perspective.