The never-ending RCP8.5 debate

This is a re-post from ATTP

I think my New Year’s resolution is going to be to not talk about RCP8.5. However, I think I will briefly summarise the state of the never-ending debate. I thought we’d reached a bit of a breakthough when Zeke Hausfather and Justin Ritchie posted an article suggesting that [a] 3C world is now “Business as Usual”, but it seems to have degenerated once more. David Wallace-Wells also has a follow up article pointing out that our climate future doesn’t look as bad as it once looked. If you’re interested, Pietro Monticone has a summary of the state of the debate.

The basic idea is that current emission projections (from the International Energy Agency) suggest that emissions won’t rise much between now and 2040. If you then make some reasonable assumptions and project these to 2100, you find that we will probably follow something close to an RCP6 pathway which will probably lead to warming of between 1.9oC and 4.4oC, with a best estimate of around 3oC. In other words, current policy suggests that business-as-usual is closer to RCP6, than to RCP8.5 (which has often been regarded as a business-as-usual pathway).

So, the basic message seems to be that we’re heading towards a world where the climate impacts might not be as apocalyptic as they could have been. Good news, in some sense. However, there are a great many uncertainties associated with these projections. Even though we may be heading towards a 3oC world, we can’t rule out that we’ll still end up in a >4oC world.

This is where I have some problems. Some are interpreting this as suggesting that we’ve essentially limited warming to ~3oC, which completely ignores all the uncertainties associated with these projections. Similarly, some are arguing that we should pay no attention to studies that use RCP8.5, which completely ignores that we still can’t rule out levels of warming typically associated with RCP8.5 (>4oC, for example). From a climate modelling perspective, RCP8.5 is simply a concentration/forcing pathway. Even if it’s now higher than is likely along business-as-usual pathway, it is still a useful pathway for investigating the higher levels of warming that are still possible.

I think it is good that we may have ruled out some of the worst case impacts, but we’re still potentially heading for a world that has warmed by more than 3oC (potentially even 4o – 5oC). Also, despite the confidence of some energy analysts, we have still yet to peak global emissions. A number of quite high-profile climate scientists (Richard Betts and Ken Caldeira) are still pointing out that given the uncertainties associated with socio-economic projections, and potential carbon cycle feedbacks, we really can’t yet completely rule out an RCP8.5 concentration pathway.

I do think it’s become very unlikely that we will follow such a high concentration pathway, but I also think that this current narrative has been poorly framed, and that some will use it to argue that we don’t really need to do much more about climate change. I do worry that in 10 years time we’ll be having a similar discussion – “yes, emissions may have continued rising through the 2020s, but – trust us – they’re just about to peak”. I hope I’m wrong.

A 3C world is now “Business as Usual” – Breakthrough Institute article by Zeke Hausfather and Justin Ritchie.
We’re Getting a Clearer Picture of the Climate Future — and It’s Not as Bad as It Once Looked – Article by David Wallace Wells.
RCP8.5 issues & comments – summary by Pietro Monticone.

Posted by on Friday, 27 December, 2019

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