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This Is the Way the World Ends
Dave Knott writes "The CBC's weekly science radio show Quirks and Quarks this week features a countdown of the top ten planetary doomsday scenarios. Nine science professors and one science fiction author are asked to give (mostly) realistic hypotheses of the ways in which the planet Earth and its inhabitants can be destroyed. These possibilities for mankind's extinction include super-volcanoes, massive gamma ray bursts, and everybody's favorite, the killer asteroid. Perhaps the most terrifying prediction is the reversal of the Earth's magnetic field (combined with untimely solar activity), a periodic event which is currently 1/4 million years overdue."
Also from the most recent material I have read the threat of a "nuclear winter" was a gross beat up. We have had multiple volcanic events that discharged more particles into the atmosphere than would happen with optimal usage of warheads to cause a "nuclear winter"
In a serious nuclear war you can get a lot more material into the air than that. Here is some very recent analysis on the subject, using one of the latest climate models. (Try this paper and this one.) This research group also does work on volcanic events, which the model's response has been tested on. They find that even a regional Indian-Pakistan exchange, each country using 50 Hiroshima-sized bombs, can have pretty significant global climate impacts (almost 1.5 C cooling). They do assume an "optimal" scenario, where the bombs are aimed at the highest population centers, causing maximum burning and thus particulate emission. The "winter" only hangs around for a decade or two, but it's worse for a "full scale" MAD scenario.
For the full "global thermonuclear war" scenario, they see cooling of up to 30 C (~ 60 F) over some regions! The global temperature drops by 8 C, which is colder than an ice age. It doesn't last long enough to form continental ice sheets, of course. But sticking around for a decade or two is Very Bad for plant life and the animals which depend on it. (And this is just the temperature effect, not counting the reduced sunlight for photosynthesis, any burned vegetation outside cities, effects of fallout, etc.)
A full nuclear exchange during the Cold War would have involved up to 10 gigatons of explosives. Even very large volcanic eruptions like Thera were only 0.5-1 gigatons (and I suspect that burning cities would emit more particulate matter). World War III wouldn't have been a Dinosaur Killer, and it wouldn't have sterilized the planet, but it would have had damn large effects on the biosphere.
Thus far, there has been no good proof that there's any sort of reality in it.
Pretty much all the studies in your link conclude that there is reality to "nuclear winter", if by that you mean "significant cooling as a result of a large nuclear exchange". What's contested is mostly how much smoke there would actually be. Compared to that, the climatic effects of particulate matter in the atmosphere are relatively well understood. A few people criticized the early models which assumed that the atmospheric doesn't respond dynamically (note your link was published 20 years ago). Modern models which have dynamical circulation bear out the same results (e.g., here). The weak link remains assumptions about what gets injected into the air, not the models themselves. You can get very large variations in particulate emissions if you tweak your assumptions about how the war plays out.
Don't confuse scientists speculating on things with real empiricism. There's lots of interesting ideas and theories, something with mathematical or computer models to back them up. That doesn't mean any of it has a thing to do with reality.
Large climatic effects from particulate emissions are pretty much undeniable. You don't need a fancy theory or model to know that. Particles of that size reflect sunlight. And lo, we see it happen from volcanoes. We even know how much particulate matter the volcanoes emit. The models reproduce the observed volcanic climate effects.
The main uncertainty, as I said, is in how much burning will take place.
String theory would be a good example.
Sigh.
String theory is not a good analogy. While there may be uncertainty about nuclear winter, there is still vastly more experimental evidence underlying our understanding of particulate emissions and atmospheric circulation models than there is about string theory. Comparing the former to the most theoretical of all theoretical physics is grossly exaggerating for effect. The two levels of uncertainty are not comparable.
It is, in fact, not a theory. It makes no testable prediction.
Both those statements are false.
People always try to compare string theory to a model of particle physics like the Standard Model. That's not the right comparison. String theory is a theoretical framework. The correct comparison is to quantum field theory in general.
"Quantum field theory" makes very few testable predictions, because it makes no assumptions about what particles exist or how they interact. To make predictions, you have to construct a specific model within QFT, such as the Standard Model. That is, you have to say that quarks and leptons exist, there are three forces whose interactions take a particular form, etc.
String theory is a theory in the same sense quantum field theory is: they are both frameworks in which you can write down predictive models. String theory by itself doesn't say much other than particles are made of strings. To make predictions, you have to write down a specific model. And you can write down something like the Standard Model (or one of its GUT generalizations) in string theory. It will make the same predictions as the SM in low energy regimes.
The problem with string theory is not that it doesn't make testable predictions. It's just as predictive as QFT is; in fact, QFT is just a limiting case of string theory, so any prediction you make in low energy QFT, you can make in string theory. And its predictions are certainly testable, because you can write down string models that are demonstrably false (the same is true of QFT models, such as all models before the Standard Model). It's hard to think of an experiment that could falsify all possible string models, but the same is true of one that could falsify all possible quantum field theories.
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