The latest critique of consensus studies is an attempt to re-analyse the data from Verheggen et al. (2014) to suggest that the consensus amongst climate scientists is only 47%, not 97%. I only have two things to say about this. Firstly, As Michael Tobis points out, you do need to consider who you include as a scientist, what question you are asking, and how you go about asking it. Secondly, these consensus studies are not to inform those who work in – or understand – this scientific area; it’s for those who do not and for those who dispute the existence of a strong consensus. If you analyse survey data that aims to address this issue and conclude that the level of consensus with respect to AGW is less than 50%, then you’re wrong. It’s clear (whether you look in the scientific literature or speak to relevant experts) that the level of consensus about anthropogenically-driven warming is very strong.

There’s a very simple reason for this. It’s because it is extremely difficult to construct a physically plausible scenario in which anthropogenic influences contributed less than 50% of the warming since 1950. Ed Hawkins has a nice back-of-the-envelope attribution calculation and I thought I might do something similar here, except the other way around. I want to see if it is possible to construct a scenario in which less than 50% of the warming since 1950 could – plausibly – be anthropogenic.

So, what do we know? We’ve warmed by about 0.6oC since 1950, and the change in anthropogenic forcing since 1950 is about 1.7Wm-2 ± 1Wm-2. In the absence of feedbacks, the equilibrium climate sensitivity (ECS) would be 1.2oC per doubling of CO 2 (3.7Wm-2). If we assume that the TCR-to-ECS ratio is 0.7, then the transient climate response (TCR) would be about 0.85oC. Let’s start by assuming that the actual change in anthropogenic forcing is the smallest plausible (0.7Wm-2) and that the TCR is indeed 0.85oC. This would imply anthropogenic warming, since 1950, of about 0.15oC, which is indeed less than 50% of that observed (0.6oC).

Here’s the first problem, though. The Planck response to 0.6oC is 1.9Wm-2. We currently have a planetary energy imbalance of about 0.5Wm-2, which means that the change in radiative forcing/feedback since 1950 is about 2.4Wm-2. However, we’ve assumed here that the change in anthropogenic forcing since 1950 is only 0.7Wm-2, and we’ve assumed that there are no feedbacks to anthropogenically-driven warming. Therefore, something else must have produced a change of 1.7Wm-2. It can’t be the Sun, or volcanoes, so it can only be the response to the internally-driven warming (0.45oC).

However, the physical processes (clouds, water vapour, …) that would produce such a response are the same as those that would produce feedbacks to anthropogenically-driven warming. How can there be such a strong response to internally-driven warming and (as assumed) no feedback response to anthropogenically-driven warming? It doesn’t seem possible. Additionally, if the response to the internally-driven warming (0.45oC) is 1.7Wm-2, then that implies a feedback response of 3.8Wm-2K-1, which is large enough that internally-driven warming could produce a runaway, which is again virtually impossible.

Let’s do one more step. Let’s assume that the change in anthropogenic forcing is still only 0.7Wm-2, but that the anthropogenic warming is 50% (0.3oC). This implies a TCR of 1.6oC and an ECS (assuming a TCR-to-ECS ratio of 0.7) of 2.3oC. This would then imply a feedback fraction of 0.5, and – consequently – a feedback response of 1.6Wm-2K-1. So, 0.3oC of anthropogenically-driven warming is associated with a change in anthropogenic forcing of 0.7Wm-2 (assumed) and a feedback response of Wm-2 – a net radiative response of 1.2Wm-2.

The remaining 1.2Wm-2 (2.4 – 1.2) is therefore associated with internally-driven warming and is still substantially greater than the feedback response to anthropogenically-driven warming. This, again, seems rather unlikely. Also, this would produce an internally-driven feedback response of 1.2/0.3 = 4Wm-2K-1 which would again imply that internally-driven warming could produce a runaway, which is – again – highly implausible.

I’ve made some fairly extreme assumptions here, and it has still turned out very difficult to construct a physically-plausible scenario in which less than 50% of the warming since 1950 was anthropogenic. As far as I can tell, if I change any of the numbers so as to make them a bit more realistic (make the change in anthropogenic forcing larger, and assume more reasonable TCR and ECS values, for example) it gets even more difficult to construct such a scenario. This is why it is extremely likely that most of the warming since 1950 has been anthropogenic; it’s very difficult to construct a scenario in which this is not the case. It really doesn’t matter what various surveys may or may not say; the strong consensus exists because those who work in this field realise that it is extremely likely that most of the warming since 1950 has been anthropogenic. Massaging numbers in surveys is not going to change this.

Of course, I’ve done this rather quickly and have rounded various numbers, so this isn’t intended to be precise. It’s also possible that I’ve missed something, in which case feel free to point it out. In fact, I’d be quite keen to see a physically-motivated argument as to how more than 50% of the warming since 1950 could have been non-anthropogenic, as I’ve never seen one and I don’t think it is actually possible to do so. Feel free to prove me wrong.