Hey Benjamin! Thank you so much for the very detailed response to what I now, upon reflection, realize was a pretty offhand comment on a topic that I’m definitely not an expert in. I’ve looked more into the IPCC report and the paper from Sherwood et al (which were really interesting) and this has been on the back of my mind for a while.
I definitely better understand what you are getting at in the sentence I quoted. But I will say that I’m still not convinced the wording is quite right. [1] I’ll explain my reasoning below, but I also expect that I could be overlooking or misunderstanding key ideas.
As you explain, the IPCC report draws upon multiple lines of evidence when estimating climate sensitivity (process understanding, instrumental record, paleoclimates, and emergent constraints) and also makes a combined assessment drawing on each of these lines of evidence.
Since the climate system is so complex and our knowledge about it is limited, there are limits to how informative any individual line of evidence is. This gives us reason to add uncertainty to our estimates drawn from any individual line of evidence. Indeed, the authors address this when considering individual lines of evidence.
However, they decide that it is not necessary to add uncertainty to their combined assessment of equilibrium climate sensitivity (drawing from all of the lines of evidence together) since “it is neither probable that all lines of evidence assessed are collectively biased nor is the assessment sensitive to single lines of evidence.”
This is a pretty narrow claim. They are basically saying that they feel the combined assessment of ECS in the Sixth IPCC report is robust enough (drawing from multiple separate lines of evidence that are unlikely to be collectively biased) that they don’t need to account for unknown unknowns in framing it. [2][3]
The combined assessment of ECS is only part of the full report. I worry it’s an overstatement to make a general claim that “The IPCC’s Sixth Assessment Report attempts to account for structural uncertainty and unknown unknowns” (and in doing so vindicates low probability estimates of existential catastrophe from climate change), when in reality, the report only says that accounting for structural uncertainty isn’t needed when framing a particular estimate (the combined assessment of ECS), which itself is only one component supporting the broader conclusions of the report, and the broader threat models from climate change.
What does accounting for unknown unknowns actually imply about whether “anthropogenic warming could heat the earth enough to cause complete civilisational collapse”? My take here is that it should actually decrease our credence in what I take to be otherwise strong evidence suggesting that such a catastrophe looks extremely unlikely.
Toby Ord makes a similar argument in The Precipice, actually. I quote it below:
“When we combine the uncertainties about our direct emissions, the climate sensitivity[4] and the possibility of extreme feedbacks, we end up being able to say very little to constrain the amount of warming.”
And:
“The runaway and moist greenhouse effects remain the only known mechanisms through which climate change could directly cause our extinction or irrevocable collapse. This doesn’t rule out unknown mechanisms. We are considering large changes to the Earth that may even be unprecedented in size or speed. It wouldn’t be astonishing if that directly led to our permanent ruin.”
I tend to agree that an existential catastrophe directly resulting from anthropogenic climate change is extremely unlikely, but I think accounting for unknown unknowns should make us less sure of that — and I don’t think we can say that “even when we try to account for unknown unknowns, nothing in the IPCC’s report suggests that civilization will be destroyed” based only on the IPCC report claiming that their combined assessment of climate sensitivity is robust to unknown unknowns.
I’m thinking in particular of “But even when we try to account for unknown unknowns, nothing in the IPCC’s report suggests that civilization will be destroyed” and “The IPCC’s Sixth Assessment Report, building on Sherwood et al.’s assessment of the Earth’s climate sensitivity attempts to account for structural uncertainty and unknown unknowns. Roughly, they find it’s unlikely that all the various lines of evidence are biased in just one direction — for every consideration that could increase warming, there are also considerations that could decrease it.”
This is all my interpretation of the passage from the IPCC AR6 section 7.5, cited in the 80k article: “In the climate sciences, there are often good reasons to consider representing deep uncertainty, or what are sometimes referred to as ‘unknown unknowns’. This is natural in a field that considers a system that is both complex and at the same time challenging to observe. For instance, since emergent constraints represent a relatively new line of evidence, important feedback mechanisms may be biased in process-level understanding; pattern effects and aerosol cooling may be large; and paleo evidence inherently builds on indirect and incomplete evidence of past climate states, there certainly can be valid reasons to add uncertainty to the ranges assessed on individual lines of evidence. This has indeed been addressed throughout Sections 7.5.1–7.5.4. Since it is neither probable that all lines of evidence assessed here are collectively biased nor is the assessment sensitive to single lines of evidence, deep uncertainty is not considered as necessary to frame the combined assessment of ECS.”
Also, I’m not sure if saying that they “account for unknown unknowns” is precisely what’s going on here — rather they feel their combined assessment of ECS is so robust that they don’t need to account for them. Maybe that is “accounting for them” in a very meta way.
Note that it’s in estimating only climate sensitivity that (as far as I can tell) the IPCC’s Sixth Assessment Report to make the claim that “unknowns mostly to cancel out, and we should be surprised if they point in one direction or the other” (quoted from the 80k article).
Hey Benjamin! Thank you so much for the very detailed response to what I now, upon reflection, realize was a pretty offhand comment on a topic that I’m definitely not an expert in. I’ve looked more into the IPCC report and the paper from Sherwood et al (which were really interesting) and this has been on the back of my mind for a while.
I definitely better understand what you are getting at in the sentence I quoted. But I will say that I’m still not convinced the wording is quite right. [1] I’ll explain my reasoning below, but I also expect that I could be overlooking or misunderstanding key ideas.
As you explain, the IPCC report draws upon multiple lines of evidence when estimating climate sensitivity (process understanding, instrumental record, paleoclimates, and emergent constraints) and also makes a combined assessment drawing on each of these lines of evidence.
Since the climate system is so complex and our knowledge about it is limited, there are limits to how informative any individual line of evidence is. This gives us reason to add uncertainty to our estimates drawn from any individual line of evidence. Indeed, the authors address this when considering individual lines of evidence.
However, they decide that it is not necessary to add uncertainty to their combined assessment of equilibrium climate sensitivity (drawing from all of the lines of evidence together) since “it is neither probable that all lines of evidence assessed are collectively biased nor is the assessment sensitive to single lines of evidence.”
This is a pretty narrow claim. They are basically saying that they feel the combined assessment of ECS in the Sixth IPCC report is robust enough (drawing from multiple separate lines of evidence that are unlikely to be collectively biased) that they don’t need to account for unknown unknowns in framing it. [2] [3]
The combined assessment of ECS is only part of the full report. I worry it’s an overstatement to make a general claim that “The IPCC’s Sixth Assessment Report attempts to account for structural uncertainty and unknown unknowns” (and in doing so vindicates low probability estimates of existential catastrophe from climate change), when in reality, the report only says that accounting for structural uncertainty isn’t needed when framing a particular estimate (the combined assessment of ECS), which itself is only one component supporting the broader conclusions of the report, and the broader threat models from climate change.
What does accounting for unknown unknowns actually imply about whether “anthropogenic warming could heat the earth enough to cause complete civilisational collapse”? My take here is that it should actually decrease our credence in what I take to be otherwise strong evidence suggesting that such a catastrophe looks extremely unlikely.
Toby Ord makes a similar argument in The Precipice, actually. I quote it below:
And:
I tend to agree that an existential catastrophe directly resulting from anthropogenic climate change is extremely unlikely, but I think accounting for unknown unknowns should make us less sure of that — and I don’t think we can say that “even when we try to account for unknown unknowns, nothing in the IPCC’s report suggests that civilization will be destroyed” based only on the IPCC report claiming that their combined assessment of climate sensitivity is robust to unknown unknowns.
I’m thinking in particular of “But even when we try to account for unknown unknowns, nothing in the IPCC’s report suggests that civilization will be destroyed” and “The IPCC’s Sixth Assessment Report, building on Sherwood et al.’s assessment of the Earth’s climate sensitivity attempts to account for structural uncertainty and unknown unknowns. Roughly, they find it’s unlikely that all the various lines of evidence are biased in just one direction — for every consideration that could increase warming, there are also considerations that could decrease it.”
This is all my interpretation of the passage from the IPCC AR6 section 7.5, cited in the 80k article: “In the climate sciences, there are often good reasons to consider representing deep uncertainty, or what are sometimes referred to as ‘unknown unknowns’. This is natural in a field that considers a system that is both complex and at the same time challenging to observe. For instance, since emergent constraints represent a relatively new line of evidence, important feedback mechanisms may be biased in process-level understanding; pattern effects and aerosol cooling may be large; and paleo evidence inherently builds on indirect and incomplete evidence of past climate states, there certainly can be valid reasons to add uncertainty to the ranges assessed on individual lines of evidence. This has indeed been addressed throughout Sections 7.5.1–7.5.4. Since it is neither probable that all lines of evidence assessed here are collectively biased nor is the assessment sensitive to single lines of evidence, deep uncertainty is not considered as necessary to frame the combined assessment of ECS.”
Also, I’m not sure if saying that they “account for unknown unknowns” is precisely what’s going on here — rather they feel their combined assessment of ECS is so robust that they don’t need to account for them. Maybe that is “accounting for them” in a very meta way.
Note that it’s in estimating only climate sensitivity that (as far as I can tell) the IPCC’s Sixth Assessment Report to make the claim that “unknowns mostly to cancel out, and we should be surprised if they point in one direction or the other” (quoted from the 80k article).