I’ve noticed a fair amount of dicussion about the Representative Concentration Pathways (RCPs) varying from they’re scientific fraud, they’re overly optimistic, to the details are being hidden so the IPCC can’t be trusted. I haven’t actually paid to much attention to the details, since these RCPs just seem like possible scenarios, ranging from rapid emission reduction through to continuing to increase our emissions. Just because some are unlikely to actually materialise, doesn’t mean that they shouldn’t be presented.
Furthermore, it’s not really clear to me why the details matter all that much. From a physical climatology perspective, what matters is the emission pathway. How the system responds then depends on physics and chemistry, not on the underlying details (population growth, GDP growth, ….). These details may have some importance, but – as the figure on the right seems to show – the warming we will experience really just depends on which emission pathway we end up following.Admittedly, we can’t model all possible emission pathways. However, as the figure on the left shows, the amount of warming depends primarily on total (cumulative) emissions, not really on the specific pathway. Hence, if you have considered a large range of potential pathways, then you can use that – at any time in the future – to extrapolate what sort of pathway would need to be followed to achieve some goal (not that we should necessarily have a goal, but it could be done if need be). This is essentially what motivates the carbon budget argument – to have a certain chance of limiting ourselves to a specific level of warming means that there is maximum amount of CO2 that can be emitted to achieve that goal. If we know how much has already been emitted, then we can estimate how much is left, without needing to know much about the specifics of the emission pathway. If you do want to know more about the details, Skeptical Science has a lengthy post with lots of information about the RCPs. There is, however, one thing about which I think there is some confusion that I’ll highight here. The term emissions doesn’t simply mean burning fossil fuels, it actually refers specifically to emission into the atmosphere. If you consider the figure on the right, the energy usage in 2100 is greater than today for all pathways (not surprising). Furthermore, the total amount of energy from fossil fuel sources also increases for all pathways.
The reason the emissions go down along certain pathways is because there is an assumption that we will develop carbon capture and storage (CCS) technologies, that will allow us to continue using fossil fuels without emitting all the CO2 into the atmosphere. So, as I understand, even if we follow an RCP2.6 energy pathway, if we don’t develop such technologies, emissions will continue to rise. In fact, I think this is where some of the criticism is coming from. The argument being that such pathways present an overly optimistic scenario, allowing us to justify continuing to increase our emissions on the basis that we will develop some future technology that will save us.
Anyway, I’ve written this all rather quickly and have probably illustrated my own ignorance, more than anything else. As I mentioned in an earlier post, some of the criticisms of the RCPs just seem like the latest tactic to dismiss the scientific evidence associated with global warming. Some of it, however, does seem more informed, so I may well be misunderstanding some aspects of this. Hence, I thought this might be an opportunity to open up a bit of a discussion about the RCPs. I might learn something.