I’ve been thinking a bit more about the debate around high emission scenarios, which I found rather frustrating. I think it’s an important issue, but the manner in which some people choose to frame this does make it difficult to have a serious discussion about it.
However, it does seem as though we no longer live in no climate policy world and, hence, it’s now very unlikely that we will follow an emission pathway that could lead to an RCP8.5 concentration pathway. This is – in my view – a very good thing and does mean that we may already have done enough to avoid some of the more extreme outcomes. On the other hand, I do think one should be careful of how to interpret this.The figure on the right shows the relationship between cumulative (total) emissions and warming. If we’ve largely now ruled out an RCP8.5 concentration pathway, then we may well have ruled out more extreme levels of warming (> 4oC, for example). However, it seems that we may still not have ruled out something close to an RCP6-like concentration pathway. Given the range of warming associated with this pathway, this could still lead to something close to 4oC of warming, which some regard as potentially having extremely serious consequences.
The other issue, which (somewhat ironically) those who criticise the use of RCP8.5 also point out, is that achieving some of our stated targets is going to be incredibly difficult. The figure on the left shows mitigation pathways that would give us a 2/3 chance of limiting warming to 2oC. If we’d started reducing emissions in the mid-1990s, we could have done so relatively gradually. Today, we’d need to halve emissions in about 15 years and get to almost zero emissions by around 2060; an extremely challenging prospect. Plus, there would still be a roughly 1/3 chance that we could do this and still end up with warming exceeding 2oC.
So, although it seems great that we may already have done enough to avoid some of the more severe outcomes, we still aren’t doing enough to achieve our stated targets and are still heading for a level of warming that could lead to very serious consequences. Furthermore, there are still factors that are uncertain, such as climate sensitivity itself, and how the carbon sinks are likely to respond as we continue to emit CO2 and, hence, continue to warm.
So, as much as I’m pleased that it appears that we’ve already implemented enough climate policy to avoid what might be regarded as a worst case scenario, it still appears that we’re heading in a direction that could still lead to pretty severe impacts. Just because the outcome is unlikely to be as catastrophic as it could have been does not mean that there’s nothing left to do.