Yes, well that is sort of obvious, but how not good? Ukraine has brought the scenario of a nuclear war to the forefront, which raises the question, what would the outcome be? You may have heard estimates from military hawks that apart from those killed due to the blasts and those who got excessively irradiated, all would be well. Americans tend to be more hawkish because the blasts would be “over there”, although if the enemy were Russia, Russia should be able to bring it to America. There is an article in Nature (579, pp 485 – 487) that paints a worse picture. In the worst case, they estimate deaths of up to 5 billion, and none of these are due to the actual blasts or the radiation; they are additional extras. The problem lies in the food supply.
Suppose there was a war between India and Pakistan. Each fires nuclear weapons, first against military targets, then against cities. Tens of millions die in the blasts. However, there is a band of soot that rises into the air, and temperatures drop. Crop yields drop dramatically from California to China, and affect dozens of countries. Because of the limited food yields, more than a billion people will suffer from food shortages. The question then is, how valid are these sort of predictions?
Nuclear winter was first studied during the cold war. The first efforts described how such smoke would drop the planet into a deep freeze, lasting for months, even in summer. Later studies argued this effect was overdone and it would not end up with such a horrific chill, and unfortunately that has encouraged some politicians who are less mathematically inclined and cannot realize that “less than a horrific chill” can still be bad.
India and Pakistan each have around 150 nuclear warheads, so a study in the US looked into what would happen in which the countries set off 100 Hiroshima-sized bombs. The direct casualties would be about 21 million people. But if we look at how volcanic eruptions cool the planet, and how soot goes into the atmosphere following major forest fires, modelling can predict the outcome. A India-Pakistan war would put 5 million tonne of soot into the atmosphere, while a US Russian war would loft 150 million tonne. The first war would lower global temperatures by a little more than 1 degree C , but the second would lower it by 10 degrees C, temperatures not seen since the last Ice Age. One problem that may not be appreciated is that sunlight would heat the soot, and by heating the adjacent air, it causes it to rise, and therefore persist longer.
The oceans tell a different story. Global cooling would affect the oceans’ acidity, and the pH would soar upwards (making it more alkaline). The model also suggested that would make it harder to form aragonite, making life difficult for shellfish. Currently, the shellfish are in the same danger from too much acidity; depending on aragonite is a bad option! The biggest danger would come regions that are home to coral reefs. There are some places that cannot win. However, there is worse to come: possibly a “Nuclear Niño”, which is described as a “turbo-charged El Niño”. In the case of a Russia/US war, the trade winds would reverse direction and water would pool in the eastern pacific ocean. Droughts and heavy rain would plague different parts of the world for up to seven years.
One unfortunate effect is that this is modelled. Immediately, another group from Los Alamos carried out different calculations and came to less of a disastrous result. The difference depends in part on how they simulate the amount of fuel, and how that is converted to smoke. Soot comes from partial combustion, and what happens where in a nuclear blast is difficult to calculate.
The effects on food production could be dramatic. Even following the small India-Pakistan war, grain production could drop by approximately 12% and soya bean production by 17%. The worst effects would come from the mid-latitudes such as the US Midwest and Ukraine. The trade in food would dry up because each country would be struggling to feed itself. A major war would be devastating, for other reasons as well. It is all very well to say your region might survive the climate change, and somewhere like Australia might grow more grain if it gets adequate water, as at present it is the heat that is the biggest problem. But if the war also took out industrial production and oil production and distribution, now what? Tractors are not very helpful if you cannot purchase diesel. A return to the old ways of harvesting? Even if you could find one, how many people know how to use a scythe? How do you plough? Leaving the problem of knowing where to find a plough that a horse could pull, and the problem of how to set it up, where do you find the horses? It really would not be easy.
ianmillerblog 
Nuclear war not good is not the question. The question is what is better. If Putin will lose his life, as he must, he will prefer nukes.
By the way, I tend to disagree with these models. Fire bombing a city doesn’t lift soot all the way up as if it were H2SO4 from a volcano staying up there for years. I believe the soot would fall back. We have evidence from giant west coast fires… Although yes they were terrible, and chemical laden when cities burned… the smoke finally fell down.
In a nuclear bombing, the nuclear mushroom itself dissipates in a few minutes. The second mushroom is caused by a standard fire storm…
Look at the ashes in the accompanying photograph in:
I tend to disagree with models anyway. However, I think the result will be the same. If sufficient infrastructure is knocked out, it will be impossible to rebuild quickly. Our technology has got rather complicated, and you may find that very few companies can provide what you need to rebuild an oil refinery, say. Shipping would be a disaster, and look what happened simply when a number of workers got sick with a superflu? No diesel to run tractors for a month means no grain for a year. No fertilizer greatly reduces yields. It would not be pretty.
I agree with what you say 100%. This global trade world is extremely vulnerable. I had French glasses made in France by a French manufacturer of specialized mountain glasses. Except COVID and except a crucial part was made in Korea. So delays, then I was supposed to go back, but didn’t, so no (prescription!) mountain glasses… To this day. The French opticians had no idea there was just one factory in the world making that precise part. In Korea!
Unfortunately there are a lot of other specialist products made by only one or maybe two factories. Fine chemicals are notorious for that because it isn’t worth anyone competing. Sales are all done on contract, and the contract will promise a fixed relative price in return for a guaranteed sales volume. There just isn’t room for a competitor, and ther4e are so many other chemicals that could be made. This works very well, but not if the only factory is removed from the planet. Worse, nobody else will know exactly how to make it. Sue, they could learn, but after such a war there will be a lot to learn and very little infrastructure with which to learn it.