How much of an effect would a global thermonuclear war *really* have to life on Earth?
Considering everything that life on Earth has already been through (what with having experienced five “big” mass extinction events these past 4,000,000,000 years that life has been around), aside from the dozens, hundreds, or even thousands of impact events and supervolcanic eruptions, it has been hypothesized that Earth has already been through:
#1). Struck by a Mars-sized object over 4,000,000,000 years ago.
https://web.archive.org/web/20170402015342/http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html
#2). Almost completely being covered in snow essentially turning the entire planet into a "Snowball Earth" over 2,000,000,000 years ago after the “Great Oxygenation Event”.
#3). Hit by a gamma ray burst originating from a hypernova over 400,000,000 years ago. This basically incinerated and irradiated the entire surface of the planet, killing off most lifeforms of the time.
And yet, it would seem that life on Earth managed to survive through all of that. Also consider this: the most powerful nuclear device ever detonated by humans was the “Tsar Bomba” exploded with the equivalent of 57 megatons of TNT. By comparison: the asteroid that struck the Earth about 65,000,000 years ago and is apparently responsible for driving dinosaurs into extinction (while allowing the distant ancestors of humans and of all mammals to survive and later even somehow thrive through all of that) exploded with the equivalent of 100,000,000 megatons of TNT. The asteroid impact was of such great intensity that it released ejecta off of Earth's surface and far out into the Earth's atmosphere leading into its exposure to infrared radiation. The reentry of the irradiated debris then rained back down as giant fireballs across the entire surface of the Earth, thereby causing global firestorms and creating a sort of nuclear winter effect (effectively killing exposed organisms, and the ancestors of today's mammals lucking out by digging themselves underground.) The global debris layer deposited by the impact contained enough soot to hint that the entire terrestrial biosphere burned, with an implication of this being that this would have caused a global soot-cloud blocking out the Sun. Widespread fires increased the carbon dioxide content of the atmosphere and caused a temporary greenhouse effect. Furthermore: rain and ocean water became acidic. Photosynthesis was later inhibited by a dust cloud that blocked sunlight, and resulted in a severe drop of global temperatures. There are currently a total of 13,840 nuclear warheads--only 3,750 of which are active. In the year 1986; there were up to 70,300 nuclear warheads. The majority of which seem to be much less than 25 MT each. Not only that--but the asteroid responsible for the Cretaceous-Tertiary Extinction Level Event (dinosaur death) wasn't even the biggest impact event. There were probably about a dozen more even bigger than that millions of years before. Some asteroids may even contain natural nuclear fission reactors with trace amounts of uranium--perhaps even plutonium.
While there is no doubt that a global thermonuclear war would have an absolutely devastating effect to human civilization in the short-term, how much of an effect could it have to the human species in the long-term (at a geologic timescale)? If the species Latimeria chalumnae was around 400,000,000 years ago and is still around today, how likely is it that the species Homo sapiens will still be around 400,000,000 years from now? The distant ancestors of Homo sapiens managed to live through the asteroid impact that drove dinosaurs to extinction about 65,000,000 years ago, why can't Homo sapiens live through something like that? There are still millions of people around the world today living exactly like how their ancestors did thousands of years ago. There's still pastoral nomads and hunter-gatherers, and it's estimated that there's about 20 uncontacted tribes in the world today (in the Amazon and in New Guinea.)
But anyway. The human species itself managed to survive through the Toba catastrophe that occurred about 70,000 years ago (which resulted in the global human population being reduced down to around 2,500 individuals.) While it may very well be that most humans of the 21st century (especially those of first world nations) would be poorly adapted to a post-nuclear world and most likely die off shortly after a nuclear war, there are still millions of humans throughout the world today who have been living exactly as their ancestors did millennia ago. So there would have to be at the very least 2,500 individuals grouped up together, perhaps with most (if not all of them) living within a government installation (maybe something like the Cheyenne Mountain Complex) to use as a fallout shelter. I'm not so sure on how many advances have been made in genetic modification to be used for hydroponics laboratories these last few years, but I think hydroponics may prove to be highly beneficial in such an event. Those last surviving individuals could help preserve whatever knowledge possible, and pass it down to future generations.
