I notice that the deadline for the submission to the UK government’s IPCC inquiry has passed. I didn’t submit anything; partly because I just forgot, and partly because I’m still just an anonymous blogger and doing so would seem to be taking myself a little too seriously. I have, however, discovered that a number of other people have made submissions. It pains me to link to the blog of the most unpleasant, non-anonymous person I’ve encountered online, but it does contain links to some of the other submissions. I notice, there, that our Scottish friend has submitted evidence. A quick perusal of this would seem to confirm my suspicion that a number of people (myself included) wasted a great deal of time discussing Murry Salby’s ideas with him.
There do, however, seem to be some submissions that are rather critical of the IPCC’s statement
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period.
This statement is basically saying that it is extremely likely (95% – 100% probability) that most of the warming since 1951 is anthropogenic. As far as I understand it, this is not claiming that 95% of the warming is anthropogenic, but is an attribution statement (i.e., almost all the warming can be attributed to anthropogenic influences). Judith Curry has, however, apparently suggested that the IPCC is deliberately trying to mislead the public with this statement. My view, on the other hand, is that the reason the IPCC has said this is because it is extremely likely (virtually certain, I would say) that most of the warming since 1951 is anthropogenic.
Why? Well, we’re fairly certain that it’s not the Sun. It seems extremely unlikely that the geothermal flux has suddenly increased. The only other non-anthropogenic option is internal variability, but that can only move energy around. It can’t create energy, so can’t explain the overall warming. Additionally, as I explain here, if the rise in surface temperature was due to internal variability, we’d expect it to be accompanied by a negative energy imbalance (i.e., we should be losing more energy than we’re gaining), not a positive energy imbalance. Also, the heat content of the land and atmosphere is relatively low, so if the surface warming was simply associated with some natural cycle, the energy associated with this surface warming should have been lost relatively quickly (months or a few years).
What’s more, the known anthropogenic forcings are consistent with the overall warming. Ultimately, we have anthropogenic influences that can largely explain the overall warming and natural/internal variability processes that cannot, and hence we are virtually certain that the warming since 1951 is almost all anthropogenic. A challenge I have for those who criticise the IPCC statement is to come up with a non-anthropogenic mechanism, that’s physically motivated (i.e., not simply curve-fitting), that can largely explain the warming, and that doesn’t violate the laws of physics. If you can do that, maybe you have a point. If you can’t, you probably don’t.
I thought I’d finish this post by discussing the figure below, which is – I believe – associated with the IPCC’s attribution statement. The figure shows the observed surface warming (black), the contribution due to greenhouse gases (green), other anthropogenic influences (yellow), natural warming (purple), internal variability (no colour because it is too small to see), and the net anthropogenic contribution (orange). Each contribution has an associated uncertainty range.
This figure has been criticised by some because they claim that the overall anthropogenic contribution (orange) has much smaller error bars than would be the case if you summed (in quadrature I assume) the errors associated with all the anthropogenic contributions. This has been covered by Gavin Schmidt, but I thought I would add my thoughts here.
What those who criticise this graph fail to realise (which is quite remarkable, given who some of them are) is that the net anthropogenic contribution (orange) is not based on the anthropogenic contributions only. It’s determined by considering the anthropogenic and natural/internal variability contributions. I’ll see if I can explain this (and I’ll hope to get it roughly correct). If we consider the anthropogenic contributions only, then it is possible that they could provide much more warming than is observed. However, when the natural/internal variability contributions are also considered, it seems unlikely – if this were the case – that these could provide sufficient cooling to then match the observed warming. Similarly, considering anthropogenic contributions only, it is possible that they could have produced much less warming than is observed. It is, however, unlikely that the natural/internal variability contribution could then provide sufficient warming to explain the observed warming. Hence, by combining all the possible contributions, one can constrain the anthropogenic influence much more tightly than if one were to consider the anthropogenic contribution alone.
Assuming I’m right (and, as usual, am happy to be convinced that I’m not) I’m quite amazed that those who criticise this figure have not recognised that attribution is determined by considering all possible contributions, not just the anthropogenic ones. I must admit, that it took me a while to work out what the figure was actually saying but, having done so, it doesn’t seem all that complicated (assuming I’m roughly correct that is) and am surprised that others haven’t managed to do the same. That would, in my opinion, be true scepticism. In the spirit of scepticism, I’m happy to consider that I might be wrong and am happy to be convinced – by someone who can actually construct a coherent argument – that I am.