Why do you not endorse this for countervalue targeting?

Because that kind of countervalue targeting isn't a thing.  I intend to write on this more, but there tends to be a lot of equivocation here between countervalue as "nuclear weapons fired at targets which are not strictly military" and countervalue as "nuclear weapons fired to kill as many civilians as possible".  The first kind absolutely exists, although I find the countervalue framing unhelpful.  The second doesn't in a large-scale exchange, because frankly there's no world in which you aren't better off aiming those same weapons at industrial targets.  You get a greater effect on the enemy's ability to make war, and because industrial targets tend to be in cities and have a lot of people around them, you will undoubtedly kill enough civilians to accomplish whatever can be accomplished by killing civilians, and the other side knows it.  

The partial exception to this is if you're North Korea or equivalent, and don't have enough weapons to make a plausible dent in your opponent's industry.  In that case, deterrence through "we will kill a lot of your civilians" makes sense, but note that the US was pretty safely deterred by 6 weapons, which is way less than discussed here.  

Is this really a fair description of IR Realism?

It's not a fair description of all IR realism, which I think is a useful theory for illuminating certain interests of state actors.  But some proponents (and the article Stephen linked confirms that Mearsheimer is one of them) seem to elevate it to a universal theory, which I don't think it is.  Frankly, Realism is the intellectual home of Russia-apologism, and while I'll admit that Mearshimer seems to come by this honestly (in that he honestly thinks Russia is likely to respond with nukes and isn't just a Putin fanboy) I take a rather different view of how things are likely to turn out if we keep backing Ukraine.

OK, remember that we're dealing with nuclear weapons, which inspire governments to levels of paranoia you maybe see when dealing with crypto.  Dropping a dozen nukes down a mine somewhere is not going to happen without a lot of paperwork and armed guards and a bunch of security systems.  And those costs don't really scale with number of warheads.  Sure, if you were trying to disperse the stockpile during a period of rising tension, you could take a few infantry companies and say "hang the paperwork".  But that requires thinking about the weapons in a very different way from how they actually do, and frankly they wouldn't be all that useful even if you did do that, because of the other problems with this plan.

Are you saying that missile warheads could not be quickly configured to be used as a gravity bomb?

Yes, I am.  The first commandment of nuclear weapons design since 1960 or so has been "it must not go off by accident".  So a modern missile warhead has an accelerometer which will not arm it unless it is pretty sure it has been fired by the relevant type of missile.  And trying to bypass it is probably a no-go.  The design standard is that one of the national labs couldn't set a US warhead off without the codes, so I doubt you can easily bypass that.

it would only take a few surviving bombers if the warheads could be used as gravity bombs

A modern bomber is a very complex machine, and the US hasn't set ours up to keep working out of what could survive a nuclear exchange.  (This is possible, but would require mobile servicing facilities and drills, which we do not have.)  Not to mention that they can't make a round-trip unrefueled from CONUS to any plausible enemy, and the odds of having forward tankers left are slim to none.

My understanding is that the warning systems are generally designed such that the ICBMs could launch before the attacking warheads reach the silos. I do have significant probability on counterforce scenarios, but I can't rule out counter value scenarios, so I think it's an important question to estimate what would happen in these counter value scenarios.

Even leaving aside the ICBMs, "countervalue" was one of McNamara's weird theories, and definitely wouldn't be implemented as a pure thing.  If nothing else, a lot of those warheads are going after military targets, not cities.

Other people have probably done more rigorous analyses now, but my rough estimate in 2015 was that 1500 nukes to the US would destroy nearly all the suburb area of 100,000+ population metro areas.

Maybe if they were targeted specifically with that goal in mind, but again, that seems unlikely, particularly with modern guidance systems.  You'll do better for yourself shooting at specific things rather than asking "how many civilians can we kill"?  A lot of those will be far away from cities, or will have overlap with something else nearby that is reasonably hard and also needs to die.

In reality, there is significant dilution, but another fire model generally found the smoke going to the upper troposphere (the Lawrence Livermore National Lab study).

I might be misreading it, but that paper seems to bury a lot of the same assumptions that I'm objecting to.  They assume a firestorm will form as part of the basis of how the fire is modeled, and then explicitly take the 5 Tg of stratospheric soot per 100 fires number and use that as the basis for further modeling.  For other fuel loadings, the amount of soot in the stratosphere is linear with fuel loading, which is really hard to take seriously in the face of the "wildfires are different" assertion.  Sure, they accurately note that there are a lot of assumptions in the usual Turco/Toon/Robock model and talk a good game about trying to deal with all four parts of the problem, then go and smuggle in the same assumptions.  Points for halving the smoke duration, I guess.

Edit:

I would think the non-deployed warheads could just be stored deep underground so they would nearly all survive.

Deep bunkers like that are expensive and rare, and even if the bunker itself survived, ground bursts are messy and would likely leave it inaccessible.  Also, there's the problem of delivering the warheads to the target in an environment where a lot of infrastructure is gone.  Missile warheads are only of use as a source of raw materials, and while you might be able to get gravity bombs to bombers, you wouldn't get many, and probably couldn't fly all that many sorties anyway.  It's a rounding error, and I'm probably being generous in using that to cancel out the loss of deployed warheads.  (Why do we keep them, then, you ask?  Good question.  Some of it is in case we need to up deployed warheads quickly.  A lot is that various politicians don't want to be seen as soft on defense.)

Furthermore, the mostly steel/concrete city centers are generally not very large, so even with a nuclear weapon targeted at the city center, it would burn a significant amount of suburbs. So with 1500 nuclear weapons countervalue even spread across NATO, a lot of the area hit would be suburbs.

First, remind me why we're looking at 1500 countervalue weapons?  Do we really expect them to just ignore the ICBM silos?  Second, note that there's a difference between "a lot of the area hit would be suburbs" and "a lot of the suburbs would be hit".  The US has a vast amount of suburbs, and the areas damaged by nuclear weapons would be surprisingly small.

"As Toon, Turco, et al. (2007) explained,

Let me repeat.  I am not interested in anything Turco, Toon et al have to say.  They butchered the stuff I can check badly.  As such, I do not think it is good reasoning to believe them on the stuff I can't.  The errors outlined in the OP are not the sort of thing you can make in good faith.  They are the sort of thing you'd do if you were trying to keep your soot number up in the face of falling arsenals.

Re firestorms more broadly, I don't see any reason to assume those would routinely form.  It's been a while since I looked into this, but those are harder to generate than you might think when that's the goal, and I don't think it's likely to be a goal of any modern targeting plan.  The only sophisticated model I've seen is the one by the Los Alamos team, which got about 70% of the soot production that Robock et al did, and only 12% of that reached the stratosphere.  That's where my money is.

>Current US + Russia arsenals are around 11,000 warheads, but current deployed arsenals are only about 3000. With Putin pulling out of New START, many nuclear weapons that are not currently deployed could become so.

Possibly the single most important goal of the deployed warheads is to stop the other side from deploying their warheads, both deployed and non-deployed.  Holding to deployed only is probably a reasonable assumption given that some of the deployed will not make it, and most of the non-deployed definitely won't.  And this was written a year before Putin pulled out of New START.  I have doubts that they'll be able to actually deploy all that many more without spending a lot of money that they frankly don't have.

>Do you have data for the 400 kT average? I think if you have multiple detonations in the vicinity, then you could have burn area outside the burn area that one would calculate for independent detonations. This could be due to combined thermal radiation from multiple fireballs, but also the thermal radiation from multiple surrounding firestorms so it creates one big firestorm.

400 kT is basically my guess looking at a list of strategic warheads, and it might well be lower.  As for scaling, that's going to be really complicated, but superlinear scaling seems unlikely.

>While it is true that most city centers have a higher percentage steel and concrete than Hiroshima, at least in the US, suburbs are still built of wood, and that is the majority of overall building mass. So I don't think the overall flammability is that much different.

Yeah, but it's clearly more fire-resistant than Japanese buildings in the 40s.  They burned really well compared to everywhere else that firebombing was tried.  US suburbs may have a lot of building mass in aggregate, but it's also really spread out and generally doesn't contain that much which is likely to draw nuclear attack.  And a lot of the outside material is reasonably flame-resistant in a way I'm pretty sure Hiroshima wasn't.

>There is a fundamental difference between a moving front fire (conflagration) like a bushfire and a firestorm where it all burns at once. 

Yeah, sorry, I've heard enough crying wolf on this (Sagan on Kuwait being the most prominent) that I don't buy it, at least not until I see good validation of the models in question on real-world events.  Which is notably lacking from all of these papers.  So I'll take the best analog, and go from there.  Also, note that your cite there is from 1990, when computers were bad and Kuwait hadn't happened yet.  Also note that the doommonger's best attempt to puzzle stratospheric soot out of atmospheric data from WWII didn't really show more than a brief gap at most.

Writer of the original article here, and I am skeptical.  Largely on the grounds that I think the odds of someone using nukes deliberately are actually fairly low, and we have a lot less nukes sitting around where accidents can happen.  I also have a complicated relationship with IR Realism, and this sounds like it's part of a theory that ends with "therefore, we should let the Russians do whatever they want in Ukraine."  Which I am very much not OK with.

I think my point should have been phrased less as "people will definitely not take you seriously" and more as "people might not take you seriously".  If I was looking for a reason to toss something, the sort of errors here would provide excellent ammo. 

More broadly, I'm glad that you guys are trying to address this.  I do think that defense is particularly tricky, for reasons I'm still trying to write up, but I also don't have the expertise to critique other areas.

Wait.  OpenPhil gave money to Toon and Robock?  Wow.  If I'd know that, I would have written a very sharp criticism of that particular decision.  

>Indeed, most of the research ever published on nuclear winter has been published in the last few years, using the latest climate modelling.

The problem isn't climate modeling.  The problem is that one of the inputs to the model is wrong by, conservatively, a factor of 50. 

>The most recent papers are getting published in Nature. I would disagree that theres a "reliance on papers that have a number of obvious flaws".

Peer review is a useful process, but not perfect, hence the existence of the replication crisis.  In this case, there's a couple of papers that keep popping up in more recent literature as the source for soot estimates that are extremely bad.  But a typical peer reviewer for nature would have no reason to critique those papers, and doesn't have the expertise to realize how bonkers some of the assumptions in them are.

I was perhaps unclear in my original comment.  I wrote up a long explanation the many, many errors those two have made in their nuclear winter models at https://www.navalgazing.net/Nuclear-Winter, which I assumed that Henry had read.  A quick glance at the paper in question turns up that they're using the very models of soot production I critique.  My expertise in agriculture is quite limited, so I can't say anything about how a given amount of soot will affect crop production.  I can say that they're relying on a model so terrible that I genuinely don't think a good-faith effort would produce anything that bad.  It's pretty hard to explain how the models get worse at exactly the rate that arsenals shrink, so the nuclear war situation stays the same otherwise.  The stuff in the 80s was probably exaggerated somewhat, but it's clear nonsense with arsenals an order of magnitude smaller today.