To clarify what I mean by unknown unknowns, here’s a climate-related example: We’re uncertain about the strength of various feedback loops, like how much warming could be produced by cloud feedbacks. We’d then classify “cloud feedbacks” as a known unknown. But we’re also uncertain about whether there are feedback loops we haven’t identified. Since we don’t know what these might be, these loops are unknown unknowns. As you say, the known feedback loops don’t seem likely to warm earth enough to cause a complete destruction of civilisation, which means that if climate change were to lead to civilisational collapse, that would probably be because of something we failed to consider.
But here’s the thing: generally we do know something about unknown unknowns.[1] In the case of these unknown feedback loops, we can place some constraints on them. For example:
They couldn’t cool the Earth past absolute zero, because that’s pretty much impossible.[2]
They almost certainly couldn’t make the earth hotter than the Sun (because at some point the Earth would start forming a fusing ball of plasma, and the Earth isn’t heavy enough to be hotter than the sun if it turned into a star).
In fact, we can gather a broad variety of evidence about these unknown unknowns, using various different lines of evidence. These lines of evidence include:
The physics constraining possible feedback processes
The historical climate record (since 1800)
The paleoclimate record (millions of years into the past)
Accounting for these multiple lines of evidence is exactly what the 6th Assessment Report attempts to do when calculating climate sensitivity (how much Earth’s surface will cool or warm after a specified factor causes a change in its climate system):[3]
In AR6 [the 6th Assessment report], the assessments of ECS [equilibrium climate sensitivity] and TCR [transient climate response] are made based on multiple lines of evidence, with ESMs [earth system models] representing only one of several sources of information. The constraints on these climate metrics are based on radiative forcing and climate feedbacks assessed from process understanding (Section 7.5.1), climate change and variability seen within the instrumental record (Section 7.5.2), paleoclimate evidence (Section 7.5.3), emergent constraints (Section 7.5.4), and a synthesis of all lines of evidence (Section 7.5.5). In AR5 [the 5th assessment report], these lines of evidence were not explicitly combined in the assessment of climate sensitivity, but as demonstrated by Sherwood et al. (2020) their combination narrows the uncertainty ranges of ECS compared to that assessed in AR5.
That is, as I mentioned in the main post “the IPCC’s Sixth Assessment Report… 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.”
As a result, even when accounting for unknown unknowns, it looks extremely unlikely that anthropogenic warming could heat the earth enough to cause complete civilisational collapse (for a discussion of how hot that would need to be, see the first section of the main post!).
It’s of course true that there are some kinds of unknown unknowns that are impossible to account for — that is, things about which we have no information. But these are rarely particularly important unknown unknowns, in part because of that lack of information: in order to have no information about something, we necessarily can’t have any evidence for its existence, so from the perspective of Occam’s razor, they’re inherently unlikely.
At least, in macroscopic systems. You can have negative absolute temperatures in systems with a population inversion (like a laser while it’s lasing), although these systems are generally considered thermodynamically hotter than positive-temperature systems (because heat flows from the negative temperature system to the positive temperature system).
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).
Thanks for this comment Tyler!
To clarify what I mean by unknown unknowns, here’s a climate-related example: We’re uncertain about the strength of various feedback loops, like how much warming could be produced by cloud feedbacks. We’d then classify “cloud feedbacks” as a known unknown. But we’re also uncertain about whether there are feedback loops we haven’t identified. Since we don’t know what these might be, these loops are unknown unknowns. As you say, the known feedback loops don’t seem likely to warm earth enough to cause a complete destruction of civilisation, which means that if climate change were to lead to civilisational collapse, that would probably be because of something we failed to consider.
But here’s the thing: generally we do know something about unknown unknowns.[1] In the case of these unknown feedback loops, we can place some constraints on them. For example:
They couldn’t cool the Earth past absolute zero, because that’s pretty much impossible.[2]
They almost certainly couldn’t make the earth hotter than the Sun (because at some point the Earth would start forming a fusing ball of plasma, and the Earth isn’t heavy enough to be hotter than the sun if it turned into a star).
In fact, we can gather a broad variety of evidence about these unknown unknowns, using various different lines of evidence. These lines of evidence include:
The physics constraining possible feedback processes
The historical climate record (since 1800)
The paleoclimate record (millions of years into the past)
Accounting for these multiple lines of evidence is exactly what the 6th Assessment Report attempts to do when calculating climate sensitivity (how much Earth’s surface will cool or warm after a specified factor causes a change in its climate system):[3]
That is, as I mentioned in the main post “the IPCC’s Sixth Assessment Report… 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.”
As a result, even when accounting for unknown unknowns, it looks extremely unlikely that anthropogenic warming could heat the earth enough to cause complete civilisational collapse (for a discussion of how hot that would need to be, see the first section of the main post!).
If you’re interested in diving into this further, I’d suggest taking a look at the original paper “An Assessment of Earth’s Climate Sensitivity Using Multiple Lines of Evidence” by Sherwood et al., or Why low-end ‘climate sensitivity’ can now be ruled out, a popular summary by the paper’s authors.
It’s of course true that there are some kinds of unknown unknowns that are impossible to account for — that is, things about which we have no information. But these are rarely particularly important unknown unknowns, in part because of that lack of information: in order to have no information about something, we necessarily can’t have any evidence for its existence, so from the perspective of Occam’s razor, they’re inherently unlikely.
At least, in macroscopic systems. You can have negative absolute temperatures in systems with a population inversion (like a laser while it’s lasing), although these systems are generally considered thermodynamically hotter than positive-temperature systems (because heat flows from the negative temperature system to the positive temperature system).
From the introduction to section 7.5 of the Working Group I contribution to the Sixth Assessment Report (p.993).
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).