Truly, seriously, scientists can provide a distribution of possible values of climate sensitivity

The last Lewendowsky paper contained quite some statements that made my eybrows lift, just look at the last two posts. Another eyebrow moment was this statement from the seepage article:

We know from earlier work that uncertainty is no cause for inaction-on the contrary, greater scientific uncertainty should make us worry more, not less, about the potential consequences of climate change.

That seem to be a much repeated theme in his work. I saw this previously explained in Uncertainty is not your Friend and even earlier work. The idea behind this is that uncertainty means things could get worse than anticipated.

The reasoning in the “Uncertainty is not your friend”-article is explained like this (my emphasis):

Without going any further, we can already draw one conclusion from this fact: If our best guess of climate sensitivity is 3 degrees, and the uncertainty range is 2-4.5, then things could be worse than expected. We expect 3 degrees but might get 4.5-of course, we could also get as “little” as 2, but we are ignoring the vast majority of possible outcomes if we assume (or hope) that we will “only” get 2 degrees.

So clearly, uncertainty means that things could be worse than anticipated.

But the problem does not end there. There are two additional aspects of uncertainty that we need to consider.

First, we must consider the distribution of climate sensitivity estimates. We know that there is a “best” (mean) estimate, and we know that there is a range of most likely values. But it turns out that climate scientists can do better than that: they can provide a distribution of possible values of climate sensitivity which attaches a probability of occurrence to a range of possible values.

and

This final consideration concerns the effects of the magnitude of uncertainty. All other things being equal, should we be more worried by greater uncertainty or less worried? If scientists had really down-played uncertainty-as some commentators have insinuated-what would the effects be? What if uncertainty is actually greater than scientists think?

I can understand what he is saying. uncertainty does indeed mean that things could be worse than anticipated. As far as I understand the reasoning is as follows:

• It is assumed that the distribution is bell-shaped
• We can’t exclude catastrophic effects in the end of the curve → a fat tale is assumed
• Therefor, the more uncertainty, the fatter the tale and the higher the risk of having a worse effect than anticipated.

Seems logical, but does that necessarily means that action is warranted? It depends what one is focused on. If one is focused on avoiding risk, then more uncertainty would be more reason for action. If one is focused on the likelihood of a particular scenario, then the more certainty of the occurrence a catastrophic scenario, the more reason for action. So depending on your standpoint about risk, uncertainty will lead to more or less reason for action.

This brings me to the emphasized text in the quotes. Apparently he really believes that scientists could provide us with a correct distribution of possible values of climate sensitivity! I think he is heavily overstating his case here. Considering climate being a complex, coupled, chaotic system, it would be really doubtful whether scientists could be capable of doing that.

But this means that when believing that scientists are capable of doing that, then the fat tail in the distribution is something that is a certainty which one can build on and therefor it is a no brainer trying to avoid it.

There are two things I don’t think he takes into consideration. First, I don’t think there is as much certainty as he assumes. Second, that shape of the probability distribution is not something that could be empirically verified, so there is ample room for both sides to disagree on the shape as well as on the proper policy response to it.

By considering that, we enter a whole new debate, a debate that is avoided by claiming the “scientists can provide a distribution of possible values of climate sensitivity”.

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