I thought I might briefly reflect, again, on the whole RCP8.5 discussion. In case anyone missed it, there has been a lengthy online discussion about RCP8.5, which is a concentration/forcing pathway that leads to a change in forcing of 8.5 W m-2 by 2100 and is associated with high emission pathways. The criticism is that this pathway is seen as unrealistic by energy modellers. For example, here is a paper that suggests that a vast expansion in 21st century coal use is implausible. Here is another paper that suggests that re-carbonisation is unlikely.
So, why do we continue to see the use of RCP8.5 in climate modelling and climate impact studies? One reason, as this Carbon Brief explainer highlights, may be that the concentration pathways used as inputs to climate models were finalised before the associated socio-economic pathways had been fully developed. There may also have been a communication breakdown between those who develop these socio-economic models and those who run climate models and do impact studies.
Ultimately, it’s great that energy modellers seem to be in agreement that the socio-economic pathways underpinning the high emission pathways are unrealistic. I think this is a positive outcome. However, I do think that one should be careful of then concluding that the more extreme climate outcomes are no longer possible. Ultimately, there are a lot of steps between what we do as a society and the resulting climate impacts.
Even if it seems unlikely that we will suddenly start to re-carbonise, can we actually rule out that someone won’t develop a clever way of extracting fossil fuels that we had thought were not recoverable? Can we be sure that some future event won’t lead us to decide to increase our fossil fuels use? In addition to that, there isn’t a simple relationship between emissions and atmospheric concentrations. The latter is really what determines how much our climate is going to change. Given uncertainties in how the natural sinks will respond, and the possibility of some sinks becoming carbon sources, we can’t rule out that even if we follow a lower emission pathway, we won’t end up on a higher concentration pathway.
Finally, what we’d really like to understand is the impact of climate change. If climate sensitivity turns out to be on the high side, then the impacts could be severe even if do follow a lower emission pathway. Similarly, some of the impacts could be more severe than we expect even if climate sensitivity doesn’t turn out to be on the high side; Great Barrier Reef, Arctic sea ice, Greenland, West Antarctic ice shelf. So, even if the emission pathways associated with RCP8.5 are very unlikely, we still can’t rule out that we won’t experience some of the impacts typically associated with this high concentration pathway.
I guess my point is that even though energy modellers seem to think the energy pathways associated with RCP8.5 are unrealistic doesn’t mean that we should then conclude that the more severe impacts of climate are also unrealistic. There are many uncertainties in the chain that takes us from what we do as a society to the resulting climate impacts. In some respects this doesn’t substantially change what we would should probably do – reduce emissions. We should, in my view, simply be careful of becoming complacent because energy modellers regards the socio-economic pathways associated with RCP8.5 as being unrealistic.