I messed up something on Twitter so, of course, I now have to write a blog post. There seems to be this narrative developing that we’re either tracking well below RCP4.5/SSP2-4.5, or that it’s now a plausible high-end pathway, and I was trying to point out that we might want to be a little a cautious about making such claims. I managed to mess up the cumulative emissions from this pathway, but I think my general point still mostly stands.
In case people don’t know, the RCPs are Representative Concentration Pathways that present a set of possible future atmospheric greenhouse gas concentration pathways, ranging from very high (RCP8.5) to very low (RCP2.6). These have now be largely been replaced by a set of scenarios that combine Shared Socioeconomic Pathways (SSPs) with Representative Concentration Pathways, SSP2-4.5 being a middle of the road socio-economic pathway in which social, economic and technologicl trends do not differ much from historical patterns, combined with an RCP4.5 concentration pathway. You can find more detailed descriptions in this Carbon Brief article.
Thanks to a number of people on Twitter (Paul Skeoch, Chris Parker and Zeke Hausfather) I now know the correct cumulative emissions for these different pathways. As shown in the Figure on the right, SSP2-4.5 has cumulative emissions between 2020 and 2100 of about 2700 GtCO2. For context, this is very similar to what has been been emitted to date (~2500 GtCO2), so SSP2-4.5 essentially implies doubling emissions by 2100.
Similarly, emissions today are just over 40 GtCO2 per year, so SSP2-4.5 has average emissions for the rest of the century that are about 80% of current emissions. Something to bear in mind is that there is some uncertainty in associating emissions with concentrations, and earth system feedbacks could act as an additional source. Hence, we could emit less and still follow an SSP2-4.5-like pathway.
Of course, we’d like to do much better than something like SSP2-4.5. There are policies that have been, and will continue to be, implemented that will reduce emissions, at least relative to what they might have been. We are developing and implementing alternative energy sources. People are also changing their behaviours so as to reduce their personal carbon footprints. However, to claim that we’re doing better than something like SSP2-4.5, when emissions have yet to peak, seems a bit premature. Similarly, implying that it’s now a plausible high-end, or worst case, scenario when continuing with current emissions would exceed it, also seems somewhat over-optimistic.
To me, this is a situation where there are multiple positions that can be held at the same time. There can be what we’d like to happen, what will probably happen given where we are today, what could happen if we do more in the future, but also what might happen if something goes wrong and things don’t work as expected.
In a sense, this is quite a tricky situation. In my view, it is important to be optimistic and to both hope, and try, to do better than we might think is currently possible. At the same time, it’s also important to be aware of what could go wrong and the potential impact of that scenario. It would be nice to think that something like SSP2-4.5 is now a plausible high-end scenario, but it just seems slightly over-optimistic. On the other hand, if emissions do indeed peak in 2023, then this may become a more reasonable inference.
