Mini advent calendar of Xrisks: nuclear war

Fair enough. But it does mean we can't take his estimate at face value.

I'd think not, if it's just Europe. It's a good question how long it would take to re-create present technological level

• from the remaining shielded tech
• just from books

Actually, when I did my calculations my appreciation of Szilard increased. He was playing a very clever game.

Basically, in order to make a cobalt bomb you need 50 tons of neutrons absorbed into cobalt. The only way of doing that requires a humongous hydrogen bomb. Note when Szilard did his talk: before the official announcement of the hydrogen bomb. The people who could point out the problem with the design would be revealing quite sensitive nuclear secrets if they said anything - the neutron yield of hydrogen bombs was very closely guarded, and was only eventually reverse-engineered by studies of fallout isotopes (to the great annoyance of the US, apparently).

Szilard knew that 1) he was not revealing anything secret, 2) criticising his idea required revealing secrets, and 3) the bomb was impractical, so even if somebody tried they would not get a superweapon thanks to his speech.

I think cobalt bombs can be done; but you need an Orion drive to launch them into the stratosphere. The fallout will not be even, leaving significant gaps. And due to rapid gamma absorption in sea water the oceans will be semi-safe. Just wash your fishing boat so fallout does not build up, and you have a good chance of survival.

Basically, if you want to cause an xrisk by poisoning the biosphere, you need to focus on breaking a key link rather than generic poisoning. Nukes for deliberate nuclear winter or weapons that poison the oxygen-production of the oceans are likely more effective than any fallout-bomb.

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Well, the textbooks need to be not just destroyed but accessible, and many advanced physics textbooks depend on other math texts and the like. But most of those are wide spread, so this seems like a reasonable assumption. So it may be possible to avoid spending as much in experimentation. But until the 20th century, experimentation was for physics not a massive set of costs. A more serious issue is going to be engineering, but again, an assumption that basic textbooks will be accessible seems reasonable. The serious issue is more how much one can take advantage of economies of scale and comparative advantage in order to get to the point where one has enough free resources to do things like build solar panels and the like.

Moreover, there's a large amount of what may be things like institutional knowledge or simply isn't commonly written down in textbooks. For example, China for decades now has had trouble making their own high-performance jet engines (1), and during the cold war the USSR had a lot of trouble making high-perfomance microchips. In both cases there likely were other problems at play in addition to technical know-how. but this suggests that there may be more serious issues for many technologies than just basic textbooks.

"We're not sure if we could get back to our current tech level afterwards" isn't an xrisk.

Yes it is. Right now, we can't deal with a variety of basic x-risk that require large technologies. Big asteroids hit every hundred million years or so and many other disasters can easily wipe out a technological non-advanced species. If our tech level is reduced to even late 19th century and is static then civilization is simply dead and doesn't know it until something comes along to finish it off.

The world still has huge deposits of coal, oil, natural gas, oil sands and shale oil, plus large reserves of half a dozen more obscure forms of fossil fuel that have never been commercially developed because they aren't cost-competitive.

The problem is exactly that: They aren't as cost competitive, and have much lower EROEI. That makes them much less useful and not even clear if they can be used to actually move to our current tech level. For example, to even get >1 EROEI on oil shale requires a fair bit of advanced technology. Similarly, most of the remaining coal is in much deeper locations than classical coal (we've consumed most of the coal that was easy to get to).

Plus there's wind, geothermal, hydroelectric, solar and nuclear. We're a long, long way away from the "all non-renewables are exhausted" scenario.

All of these require high tech levels to start with or have other problems. Geothermal only works for limited locations. Solar requires extremely high tech levels to even have positive energy return. Nuclear power requires similar issues along with massive processing procedures for enough economies of scale to kick in. Both solar and have terrible trouble with providing consistent power which is important for many uses such as manufacturing. Efficient batteries are one answer to that but they require also advance tech. It may help to keep in mind that even with the advantages we had the first time around, the vast majority of early electric companies simply failed. There's an excellent book which discusses many of these issues - Maggie Koerth-Baker's "Before the Lights Go Out." It focuses more on the current American electric grid, but in that context discusses many of these issues.

There are particulate events in the climate record that a model of nuclear winter could be calibrated against -- any major volcanic eruption, for example. Some have even approached the level of severity predicted for a mild nuclear winter: the "Year Without A Summer" following the 1815 Tambora eruption is the first one I can think of.

This isn't perfect: volcanoes mainly release fine rock ash instead of the wood and hydrocarbon soot that we'd expect from burning cities, which behaves differently in the atmosphere, and while we can get some idea of the difference from looking at events like large-scale forest fires there are limits on how far we can extrapolate. But we should have enough to at least put some bounds on what we could expect.

Different components in the model can be tested separately. How stratospheric gases disperse can be tested. How black soot rises in the atmosphere, in a variety of heat conditions, can be tested. How black soot affects absorption of the solar radiation can be simulated in laboratory, and tested in indirect ways (as Nornagest mentioned, by comparing with volcanic eruptions).

The point non-programmers tend to miss here is that lack of testing doesn't just mean the model is a a little off. It means the model has no connection at all to reality, and either outputs garbage or echoes whatever result the programmer told it to give. Any programmer who claims such a model means something is committing fraud, plain and simple.

This really is a pretty un-bayesian way of thinking - the idea that we should totally ignore incomplete evidence. And by extension that we should chose to believe an alternative hypothesis (''no nuclear winter') with even less evidence merely because it is assumed for unstated reasons to be the 'default belief'.