Willard has a contrarian matrix that is intended to illustrate the evolution of contrarian arguments. It certainly appears that it evolves with time, varying from it’s not happening, it’s happening but it’s slow, it’s stopped, it’ll be good, there are higher priorities, etc. More recently I’ve noticed a number of people criticising the IPCC’s Representative Concentration Pathways (RCPs), in particular RCP8.5; a pathway in which we reach a change in anthropogenic forcing of 8.5Wm-2 by 2100.

The kind of criticisms I’ve seen have varied from claims that it’s scientific fraud, suggestions that it illustrates poor assumptions by the IPCC, and arguments that it’s not possible to following such a pathway so it should be ignored. The problem is, in my view, that these RCPs simply present possible future emissions pathways, going from one – RCP2.6 – in which we have rapid emissions reduction and finally negative emissions (which has also been criticised) through to one in which we continue to increase our emissions – RCP8.5. The different pathways simply provide information about what might happen were we to follow such a pathway. Even if a pathway is unlikely, doesn’t mean that we shouldn’t consider it. In fact, going from one extreme (rapid emission reductions) to another (increasing emissions) is a perfectly standard way in which to consider future projections. Reality is most likely to fall somewhere between those two extremes.



However, I think there is something that those who criticise these extreme pathways may not realise, or don’t want to acknowledge. Consider the figure on the right. It shows the various pathways, the range of warming for each pathway, and where we are today. We currently appear to be following the RCP8.5 emission pathway. Of course it is a long time until 2100, so we may not continue to follow it until then. Now I get the impression that some people think that to end up within some range of warming, we simply need to ultimately reach the same level of emissions as the pathway that would likely produce warming within that range. In other words, if we want to have a reasonable chance of staying below 3oC we need to ultimately have emissions similar to that of RCP6 (i.e., just over 40GtCO 2 /yr).

That, however, is not correct. What determines how much warming we will experience is our cumulative emissions, not our annual emissions. If we want to have a reasonable chance of keeping warming below 3o, then we need our cumulative emissions – not our annual emissions – to be the similar to that for RCP6. The longer we continue to follow an RCP8.5 emission pathway, the more rapidly we’ll ultimately need to reduce our emissions in order to achieve that.

If we want to have a reasonable chance of staying below 2oC, it’s even more severe. The figure on the left (from here) illustrates this very nicely. The longer we wait before starting emission reductions, the more rapidly we’ll need to do so. If, for example, we continue along an RCP8.5 pathway for another 5 years, a 66% chance of staying below 2oC would require reducing our emissions by 50% within a decade.

So, it may well be true that we’re unlikely to follow something close to an RCP8.5 emission pathway until 2100, but I can’t see anything wrong with presenting information as to what might happen if we did. Also, even if we can’t follow such a pathway until 2100, that we might follow it for another decade, or longer, does have consequences that we shouldn’t – IMO – be ignoring. Maybe I’m missing something – in which case, feel free to point it out – but it seems to me that those who criticise these extreme emission pathways either don’t realise this, or this is simply the latest tactic in the ever changing contrarian argument against actively doing anything to reduce the risks associated with climate change.