How many nukes would cause a "Nuclear Winter"?

I was just wondering…are there any estimates on how many megatons of nukes, roughly (very roughly), it’d take to cause a “nuclear winter”?

Since the largest weapon ever detonated was 57 megatons, I’d guess that it’s somewhere above that. And according to Wikipedia, Krakatoa’s eruption was equivilant to about 200 megatons, and while it did effect the world’s climate, it obviously wasn’t a world-ender.

Naturally, of course, I’d imagine that the details of how and where the nukes detonate would make all the difference. (Which could make a comparison to the effects of volcanic eruptions problematic. I’d guess a mountain exploding with a few hundred megatons would kick up more ash and dust than, say, a nuke going off in the air over a desert.) I’d assume airbursts for optimum blast effect over urbanized areas would be what one would mostly figure on, but I could be wrong.

So, have there been any reliable studies on the subject, or is the whole thing so subjective and debated that any estimates would be useless?

Crap…nothing? Nothing at all? :frowning:

Briefly, 100 megatons over 100 cities should do it.

This interprets it a different way saying that 5000 megatons is enough.

I’m sure there are plenty of variables that come into play that are above my scope of understanding.

How much cooling do you need before it’s a nuclear winter?

There are historical cases of volcanic winters, including one caused by Krakatoa. The earlier eruption of Mount Tambora is thought to have caused mid-summer frosts in New York State and snow in June in New England. If that will do, the eruption of Tambora is estimated at about 24500 megatons.

I’ve heard that it would do more damage per megaton if the explosions were scattered over the world instead of concentrated in one place.

If you’re interested in impact winters, here are some fun sites to play with:

Earth Impact Effects Program

Solar System Collisions

Short answer: We have no idea.

Long answer: Really, we have no idea, but we’ll exploit it for all of its political, shock, and entertainment value.

Expanded answer: The term “nuclear winter” has its genesis in the “TTAPS” report by R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and most prominantly, Carl “billions and billions” Sagan. (Climatologists had previously noted the impact of suspended particles upon global climate but no one had previously addressed the impact of a large scale “catastrophic” nuclear exchange.) I can’t find the TTAPS report online but the basic idea is that dust, vaporized solids, and residuals released by burning (NO, NO[sub]2[/sub], CO[sub]2[/sub], etc.) generated by the thermal updraft of a nuclear explosion would remain in suspension in the atmosphere, choking out sunlight and causing a drop in average global temperature, which would cause further cloud accumulation, which would cause still lower temperatures, ad nausum, resulting in a massive (> 10 deg C) drop of average global temperature for years or decades, resulting in virtual elimination of all “higher” life on Earth, e.g. all large mammals, as hypothetically occured with the large dinosaurs during the K-T Extinction Event. Here’s Sagan’s own words on the topic.

The study was based upon examination of climate models based upon meteorlogical data accrued from examination of large dust storms on Mars. (This was used as the primary exemplar rather than climatological observations of Earth because Mars represented a rather uncomplicated environment and a fairly comprehensive data set.) The study is valid insofar as the internal methodology, but there are a number of highly questionable assumptions that provide a rather questionable foundation as far as its applicability to a nuclear exchange. First of all, the model used was highly simplified; it assumed a more-or-less uniform distribution of “events” over land masses, using a 2D model that assumed isonomic conditions at the same latitude. Furthermore, critics of the study claim that parameters of the study were selected to generate “worst-case scenerio” results; in other words, Sagan et al selected the particulars of their model to conform to their hypothesis rather than offereing a more stochastic answer.

I’m not a climatologist or meteorologist, and even if I were I daresay that climate models are not yet sufficiently detailed and reliable to offer a reliable answer to the question, but there are a number of other issues that challenge the hypothesis on a qualitative level, to whit:[ul]
[li]Most weapons in a large-scale exchange, except for the low- to medium-yield bombs directed at hardened targets (missile complexes) are likely to be air burst in order to maximize the damage radius and personnel lethality.[/li][li]Distribution of targets is not uniform over the Earth’s surface, or even land masses, but rather would be concentrated in North America, Western and Central Europe, Russia, the Ukraine, Kazakhstan, and Central/Eastern China, i.e. all targets in the Northern Hemisphere. There is little in the way of strategic targets in the Southern Hemisphere save (perhaps) for some Pacific territories of the US and France and possibly the eastern coast of Australia. [/li][li]Given that the bulk of the dust cover would be in the Northern Hemisphere, it is unlikely that more than a minor amount would migrate across the equator to the South. Even if the most dire predictions about sun blockage are genuine, South America, Africa, and Pacific Oceania should be relatively unscathed.[/li][li]Claims that dust would remain suspended for a decade or more without any means of replenishment are laughable. Heavy particulates (cast-out dirt and vaporized rock) would nucleate precipitation and would be returned to ground within the span of a few years or less. Persistance of suspended matter it unlikely to cause long-term climatological change, though it most likely would causes short-term changes in temperature and opacity in local and regional areas where atmospheric conditions promote suspension, i.e. the Los Angeles Basin.[/li][li]Gaseous residuals (NO, NO[sub]2[/sub], CO[sub]2[/sub], other hydrocarbon compounds) could remain suspended for longer but their effect is unclear; indeed, they could promote a counterbalancing or even overbalancing “greenhouse effect”…or they might become supersaturated and precipitate out. We really don’t understand the long-term effects of these “pollutants”, claims of industrially-induced global warming to the contrary.[/li][li]Comparisons to the K-T Extinction Event (65M years ago) are scarcely the basis for sound scientific theory. While it is generally accepted that the Yucutan Impact Event was the initiator or catalyst for K-T, the actual mechanism that lead to mass extinction is hotly debated among paleontologists and evolutionary biologists. Supporters of Punctuated Equilibrium theory, for instance, generally argue that the change in conditions (whatever they may be) didn’t kill off the larger dinosaurs directly but gave other species, particularly small mammals, an advantage that permitted them to oututilize the larger established species. [/ul]A couple of other points of note:[ul][/li][li]As mentioned, air bursts don’t produce a significant amount of fallout (as compared to ground bursts); not only is the vacuum from the thermal pulse less concentrated, hence drawing up less material to altitude, but material doesn’t tend to be as pulverized, meaning it will stay suspended for less time. [/li][li]Furthermore, higher-yield ground bursts do not increase the amount of fallout linearly; bombs become less and less efficient with yield, as much of the energy is reflected back into the air. As a bomb becomes larger, the crater depth in porportion to yield decreases and ultimately, the crater becomes flattened, ejecting more material to the sides (and back onto ground) than sucked up through the vacuum channel.[/li][li]On the other hand, ground penetrators, such as the low-yield “bunker busters” currently being promoted by the W. Bush Administration, generate significantly more fallout than comperable ground-level burst (and without achieving sufficient penetration to damage plausibly deep vaults). The thermal pulse, however, tends to be absorbed and contained by the surrounding ground material rather than the air, and so the airborne-persistance of the fallout is low.[/ul][/li]
In retrospect, examining Sagan’s statements and positions subsequent to the study, it is obvious that there is a political agenda at work; the study is “tuned” to support the notion of a catastrophic result to a large nuclear exchange, and is thus an instrument for the argument against proliferation and for disarmorment. The motive is sufficiently noble, but the means descend to the kind of manipulative bad science that Sagan was known to otherwise proselytize against (see his The Demon-Haunted World).

The truth of the matter is that neither our meteorlogical models nor our accounting of the extent of a nuclear exchange simply aren’t reliable enough to offer an accurate estimate of the climatological effects of the alleged “nuclear winter”. It it extremely doubtful that it would be as dire as predicted by TTAPS and promoted by the End-Of-Ages Brigade; on the other hand, there’s little question that throwing significant amounts of particulate matter into the air will have some effect, at least in the short term, as evidenced by regional and even slight global effects of large volcanic eruptions (Krakatoa). There are plenty of reasons to argue against the horrors and potential for out-of-control escalation of nuclear war–one need only look at pictures of Nagasaki and Hiroshima, or read up on the Cuban Missile Crisis–without resorting to scientific flimflammary.


Stranger, you should have taken the time to track down the original paper and read it before you responded. That second line is a cheap shot born of misunderstanding of the tools used, the time in which the study was done, and some plain old inaccurate information.

The full citation of the TTAPS report, for anyone who wants to look it up, is: R.P. Turco, O.P. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan, 1983, “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions,” Science, vol. 222, p. 1283-1292. (I have a PDF of the paper; drop me a line if you’d like a copy.)

This is an incorrect summation of the paper. The TTAPS report does discuss four principal environmental/climatological effects of nuclear war - obscuring smoke in the troposphere, obscuring dust in the stratosphere, the fallout of radioactive debris, and the partial destruction of the ozone layer - but the only ones actually included in the climate model are the dust and smoke. This constraint was imposed by the type of model used and the state of model development at the time (more on this below). Furthermore, to be precise, TTAPS doesn’t address extinction events; the suggestion of a possible major extinction is made by Ehrlich et al. in a companion paper (Science, vol. 222, p. 1293-1300). I would agree that the Ehrlich et al. paper reaches a bit in its conclusions, but that’s not uncommon with conceptual (rather than quantitative) models.

This paragraph… has problems.
[ul][li]First of all, while Sagan and colleagues may have been inspired to explore the concept of nuclear winter after viewing Martian dust storms, the numerical model used was actually originally designed to look at the effects of explosive volcanism on Earth. (See the first sentence of the abstract.)[/li][li]The model used was not 2D; there were 3 1D models used in the study, of which the model used to examine climatic impacts was a 1D radiation model. Yes, it is a very simple model. Such models are frequently used, even today, by researchers who are mainly interested in testing the impact of climate forcings (such as dust or aerosol particles in the atmosphere) that impact the radiation balance at the Earth’s surface. This is because the codes used to simulate radiative transfer are hideously complex and can be computationally expensive, even today, when combined with additional forcings and feedbacks - never mind back in the early 1980s. (To give you a taste of how much things have changed in the last 20-odd years: I can run a 1983-vintage 3D global climate model on a Mac G5 and run through 100 simulated years in about a day. A friend of mine used a similar version of that model for his thesis work in the late 1980s, and it took him about a month of supercomputing time to produce the same results.) It is also worth pointing out that fully 3D general circulation models (GCMs) of global climate were still in their infancy, and not well capable of modeling the effects of dust and aerosols in the atmosphere. Turco et al. used the best tool available to them at them time, so complaints from later years that a more sophisticated model should have been used are just ignorant of the reality.[/li][li]When you don’t have gobs of supercomputing time (or lifetime, for that matter) to run a suite of simulations using randomly selected starting conditions, of course you are going to first test the largest realistic impact you can determine. Besides, trying extreme parameterizations also helps in further model development - if your results based upon “reasonable” inputs are totally outlandish, then it’s a clue you have to correct something in your model.[/li][li]Furthermore, when you don’t have hard data in hand (which Turco et al. did not, seeing as we have not yet had a global nuclear war), you have to start by making some assumptions. There is nothing wrong with that, as long as your assumptions are spelled out, and Turco et al. have done just that in their paper. That way, anyone who does not agree with the assumptions is free to run their own simulations with a different set of assumptions. That happens ALL THE TIME in science; it’s part of the investigative process when you’re starting from unknowns. [/ul][/li]

You daresay incorrectly. Modern GCMs, especially those high-resolution models designed to look at regional climate change, are much better capable (though still not perfect) of modeling the effects of localized injections of dust/aerosols into the atmosphere. Since the Cold War is over, however, the focus of these models is usually the simulation of large forest fires or meteorite impacts. Most of these models show significant regional cooling on a short-term basis.

From the TTAPS report, p. 1284: “a 100-KT airburst can level and burn an area of approx. 50 km[sup]2[/sup], and a 1-MT airburst approx. 5 times that area.” The paper goes on to detail specifics of fire extent and damage in the aftermath of airbursts, and is extensively referenced, including a reference to a DoD report published in 1977, called “The Effects of Nuclear Weapons.” So it appears TTAPS used information available to them at the time.

The TTAPS report clearly states that the ID model used is only capable of modeling “horizontally, diurnally, and seasonally averaged conditions,” it’s silly to complain about the distribution of targets. Owing to the model used, TTAPS are forced to consider what conditions would be like after dust particles and the like are spread through the atmosphere, presumably 1 to 2 weeks after the detonations. Furthermore, they clearly state that, “The model predictions discussed here generally represent effect averaged over the Northern Hemisphere” (p. 1285). So what are you taking issue with, here?

Claims that dust could remain suspended in the troposphere for decades would be wrong, but TTAPS never say that. Note also that regional cooling could happen anywhere under the appropriate conditions, not just in geographically limited areas like the Los Angeles basin (going back to the modern GCM simulations of wildfires and the like).

The radiative effects of individual “gaseous residuals” in the atmosphere are actually pretty well understood. Where it gets complicated is in understanding how the various feedbacks interact with each other to produce climatic effects. While there IS uncertainty, it is BS to take a shot at global warming here. I would suggest you do a search of GD to read the numerous threads in which global warming is discussed, so that you may edify yourself.

That K-T debate is still underway is quite true, but irrelevant to the TTAPS paper and its conclusions (which, btw, don’t mention the K-T event at all). In the link you provided, Sagan himself says: “I wish to stress, however, that our conclusions on the climatic consequences of nuclear war do not depend on this interpretation of the Cretaceous/Tertiary extinctions. The dinosaurs could have died of influenza without affecting the validity of our conclusions.” Did you not read your own link?

As mentioned above, TTAPS references government reports on the effects of nuclear war as a basis for their starting assumptions, which is more than you have done here. Now, the calculations of those effects may well have changed in the last 25+ years, and they certainly would make a difference in calculating subsequent climatic effects. I would be fine with the need to re-do simulations, in that event. But anything produced by the government after 1983 would obviously not have been available to TTAPS for their work, so it’s disingenuous of you to suggest that they were deliberately using incorrect information.

The study was designed to examine the effects of a nuclear war as described in the US government’s own reports. Whose political agenda is at work here, exactly?

As I stated above, we do in fact have models NOW that could reasonably simulate the effects of a nuclear war. Yes, the impacts don’t appear to be as extreme as outlined in TTAPS, but that is because we can now incorporate other climate forcings and feedbacks into the models that TTAPS were not able to do. It is grossly unfair, indeed intellectually dishonest, to describe a study that is merely out of date as “flimflammary” simply because the general public continues to use it as a reference long after the scientific community has moved on.

We seem to be talking at cross purposes here; I assume this is due to a lack of specificity in my original post. Let me offer some clarifications:
[li]The bulk of my comments, specifically those regarding the K-T EE, size/distribution of weapons, resultant long-term effects, et cetera were directed at application of the TTAPS conclusions rather than at the analysis and report itself. As I indicated, and you seem to agree, the model (which, you correctly point out, is a 1D model but parameterized to various latitudes, hence it is often referred to as “2D”) is highly simplified, with a number of assumptions that need to be qualified before any quantitative application to the real world can be made. The computational restrictions and lack of correlative data aren’t flaws or mistakes in the analysis, but they are limitations of its description of natural events that are commonly glossed over.[/li][li]The single actual criticism I indicated of the study was a mention that critics of the study (of which I am not qualified to issue an opinion) claim that the parameters of the study were selected to obtain worst-case results, rather than stochastically average or typical effects. It is clear that the methodological limitations and assumptions of the analysis tended toward a more severe result, though to what extent the effect and duration was overestimated is unclear.[/li][li]On that basis, the study was (and still is) a very useful tool in examining real-world events similar to a nuclear detonation, such as a volcanic eruption or large meteor impact. It should not be construed as a definitive or explicit description of the results of such an event, however.[/li][li]More advanced and descriptive climate models, using the greater computational resources and nonlinear analysis methods, have emerged and give a better understanding of the short-term effects of a detonation/eruption/impact. Long-term (>5yr) models, however, do not offer good confidence in predictions, particularly regarding highly perturbative events. [/li][li]There’s no question that suspended particulates in the air block sunlight, reduce temperature, and cause regional and even (slight, but measurable) global or at least globewise-latitudinal effects; this is not only the result of analytical models but is verified by observational data. The extent, and more significantly the duration, of the effects, however, that are critical for the veracity of claims of a persistent “nuclear winter” are not well established.[/li][li]In a brief perusal of Sagan’s and Turco’s written comments regarding the “nuclear winter” hypothesis, they do largely qualify their descriptions as being tentative; however, second-tier promoters of the hypothesis spawned apocalyptic imagery of a world turned sunless and ice-encased as the result of an exchange. I can’t see that Sagan ever made any public attempt to dispel or qualify these assertions, and indeed, his pleas for nonproliferation and disarmament were built upon sentiments of fear of a nuclear winter extinction event. [/li][li]With regard to the applicability to an actual nuclear exchange, the dispersion, type of explosion, and yield effects are very significant in terms of the amount and type of material that is suspended. The TTAPS analysis most accurately represents a large and highly dispersed number of medium yield (or larger) ground bursts, which does not reflect an actual SIOP, which would (most likely) concentrate on small-to-medium ground-burst of hardened strategic targets (missile complexes, strategic bunkers, arms depots), and large air-burst of industrial and population centers, leaving the majority of landmass untouched. (This is assuming a full strategic exchange, not a limited tactical “battlefield” exchange.)[/li][li]Your cite from (the excellent) The Effects of Nuclear Weapons (though, if we’re talking about the same publication, it was originally published in 1962 by the AEC, not 1977) indicates that the blast radius from a larger weapon is larger. (The scaling of the radius of effect, as I recall, is roughly in proportion to the square root of the yield for an air-burst at optimum altitude for that yield.) However, the depth of crater does not get proportionally deeper; hence, a larger yield doesn’t draw proportionally more material up into the air. Maximum fallout (mass and dispersal) is produced by medium-yield ground or low altitude burst. High-yield air-burst produces significantly less fallout, although a greater area is affected by the thermal pulse, resulting in more combustion products. Ground deep penetrators (not available in the early '80s) produce the most bulk fallout per yield but because the thermal pulse is limited (largely absorbed by the surrounding ground) the altitude to which material is ejected is low, and thus the range of fallout and duration of suspended material is limited. [/li][li]The indefinite suspension of large particulates is unlikely; in absence of any replenishing action or contrary atmospheric effects they will precipitate out. Cooling is more likely to promote rather than retard precipitation, though at unknown rates. Gaseous residuals can remain suspended indefinitely (provided they come to some kind of meteorological equilibrium) but as you point out the feedback from combined effects and side reactions of a large mass are an unknown. Predictive models of global warming have consistantly failed to simultaneously match data of both surface and tropospheric temperatures, indicating that there are effects that we do not understand well enough to model. Nobody (except for a few crackpots) doubts that average global temperatures are increasing, but the amount, rate of change, extent, long-term climatological effects, and root causes are subject to debate at this point.[/li][/ul]

I do have a copy of the paper, though I’ll admit that I didn’t review it immediately prior to posting and thus, any mistakes in describing the methodology of the analysis conclusions of the report are mine. (I haven’t found an on-line source for the report; if you know of one, please post so that others can review the report.) However, as I previously indicated, my comments were intended to address the wider claims of the “nuclear winter” hypothesis as discussed in popular culture and the limits of applicability of the paper to such rather than to pick nits with the TTAPS report itself. The topic itself has become so highly politicized, and indeed, at least its most prominent proponent and TTAPS author has displayed a political agenda, that a qualitative discussion has to start with a statement of unknowns and inaccurate or incomplete popular assertions. A quantitative discussion, in the (present) absence of any definitive data or model, would require a technical segue that is beyond the scope of this forum.

In any case, much incomplete and unverified analysis has been used as the basis for absolutist assertions about the effect and duration, and unnecessarily so, as there are many substantial material arguments for nonproliferation and disarmament; and it is clear that no material argument is sufficient to convince nations to disarm. Strategic planners never worried about nuclear winter because the weapons were never meant to be launched; they were just a deterrent via Mutually Assured Destruction. Kahn cheerfully illustrated the fallacy of this position (and was subsequently vilified) with his only slightly exaggerated Doomsday Machine, and Kubrick satirized this in Dr. Strangelove, with a “Doomsday Device” that would saturate the Earth with “Cobalt Thorium-G”, making the surface uninhabitable for “ninety-three years”.

To return to the question posed by the OP, “…are there any estimates on how many megatons of nukes, roughly (very roughly), it’d take to cause a ‘nuclear winter’?”, my response remains, “We have no idea,” though in retrospect this should probably be expanded to, “We don’t know, but the extent of the effect is dependent more upon the dispersal and type of blast rather than bulk yield.”