Ok that made me cry 
Words do indeed have meanings, as I am well aware. They often have multiple and subtly nuanced meanings. Merriam-Webster defines one of the meanings of “existential” as “grounded in existence or the experience of existence”. Hence the philosophy of existentialism: “a philosophy that emphasizes individual existence, freedom and choice. It is the view that humans define their own meaning in life, and try to make rational decisions despite existing in an irrational universe.”
Sure, “existential threat” could be interpreted in a narrow sense to mean a threat to our very existence, and hence a discussion about the imminent extinction of homo sapiens. But as I said, this is not a very useful or interesting discussion. A much more productive discussion grounded in reality would be about which threats pose the greatest risks to the quality of our experience of life on earth as we know it. I think it’s increasingly clear that climate change is by far the greatest such threat.
This is beyond cool. You are sending my mind to places it’s not familiar with. Thank You!
Extinction is highly unlikely, so let’s go each of these in turn.
Pandemics: While this is the concern of the hour, it’s interesting to note that very common diseases tend not to be very fatal; very fatal diseases tend to burn though populations. Even a nightmare scenario like Steven King’s The Stand Virus stands a good chance of simply missing something like the Sentielese or very long voyage ships. That said, Covid-19 is absolutely a wake-up call on this threat and I’d have to point out that we’re going to be considerably harder and more robust against pandemics after building much more infrastructure against them.
Climate Change. Humanity will not survive Earth turning into Venus, but it’s also clear that human actions are fueling climate change, so terminating these acts will stop and gradually reverse the consequences. A worst case scenario may mean mass famine and a forced rebuilding of industrial processes, but extinction isn’t really possible.
Drinking Water Contamination. Drinkable Water is a serious but stupid problem; we can desalinize water if the costs became too high, but largely this is people crapping where they sleep. The consequences can be stark–Mexico City could completely collapse over a lack of water–but this would be a national health crisis, not an extinction crisis. Water is a problem, and resources need to go to fixing it locally, but that’s the extent of it.
AI Takeover. Of all the options on this list, this one is hardest to gauge. My hunch on this is that this wins, but the mechanics here are dodgy. In practice, Skynet could not remotely fire the US nuclear arsenal, 1960s Retrotech will serve as a important barrier to such violence. There are fundamental questions of ‘if’ in this; the human brain is trinary, genetics have four different options, computers are built on switches. So it’s not immediately clear to me that an AI could takeover in the first place.
Beyond that, humanity is likely to run into problems long before AIs are so capable of this. Humanity will probably be scared of this threat…and this might not be enough. An Asimov ‘I Robot’ scenario, where Humanity has handed the keys over to Robots and gradually disappears through demographics would require acceptance of extinction, which is…bizarre.
A scenario where it’s 3000 AD, Humanity has gone extinct because everyone decided to marry supermodel AI Gynoids and Androids and all pockets of Humanity have finished this journey qualifies, and ironically leaves those AI servitors as direct heirs of human civilization. The mechanics of an AI takeover may very well be something that humanity simply doesn’t reject. But even this kind of scenario is a stretch–it wouldn’t be very difficult to keep a steady stream of new humans being born and this kind of very accommodating AI would probably have its own important need to keep humans around.
Earth Impactor. Even a Dino-Killer probably doesn’t take out people in submarines, underground mines or in space. The last of these is a somewhat narrow window–how much longer until a human population on Luna, Mars, or further could rebuild humanity? It probably is not much of a leap to go from 1,000 people on Mars to 1,000,000 - once the economics and the means to build from local resources are in hand, that seems inevitable. Basically a super doomsday impactor needs to head to Earth and needs to do it within the next 100 years–it has been 65 MILLION years since this last happened.
Super Volcano. The Tambora explosion in the early 1800s created a ‘year without a Summer’ and is the largest volcano in human history. While a similar eruption from Yellowstone would probably break the United States as a nation, it would probably rate around the same level of badness - regional problem, not an existential problem.
I am now imagining the Sentinelese very gradually coming to the realization that everybody else is Gone.
It would probably take them long enough to venture out that the illness would have disappeared – at least, if we presume that it only attacks humans, and is infallibly deadly. After all those years of increasingly shrunken and encroached on space, a culture that had been trapped on a single small island has the entire planet to expand into –
hmmmm. There’s a book in there somewhere.
Be a hell of a plot twist if the climate change the rest of the 7 billion of us created by burning fossil carbon caused their island to sink beneath the ever-rising waves, inexorably drowning the last of them 150 years after the rest of humanity was gone.
Meanwhile they and the other nearby island tribes had long since worn out whatever small amount of modern tech they had, burned the last gallon of imported diesel & gasoline, and could never master building a primitive sailing canoe and successfully navigating the open ocean to find any other land beyond that small archipelago.
Talk about raging against the dying of the light.
They have no modern tech at all – I’m presuming that only the Sentinelese themselves (and maybe an almost equally isolated group or two far away in the Amazon) survive. All the neighboring islands have some contact and their human population would, under the conditions presumed for the story, catch the plague and die. I don’t know if the Sentinelese have ocean-going vessels, or any remaining tradition of making them; but they did get there somehow, long before fossil fuel engines existed. And they’ve scavenged wrecked ships, and do apparently build some sort of boats.
That would however be quite an ending to the story, if they weren’t able to make any boat capable of getting them to higher land; which might be quite a distance, in a planetary flood that large. – the last round of earthquakes actually apparently raised the place; most of it’s more than 66’ above sea level and some as much as 400’, and it’s protected by coral reefs, so it would take quite a sea rise to actually drown them. I don’t know how much it would take to contaminate their fresh water sources, though.
Yeah. The North Sentinelese specifically have resisted all modernity. The “tribes” on other nearby islands are somewhat more integrated with modernity. As in the occasional outboard motor, metal boat, etc. Still not fully paid-up members of the 21s Century.
At the same time, the North Sentinel island has some actual elevation. 120m or so, but that’s millennia of sea level rise, not 150 years worth. As well, the Sentinel Islands are ~50km from the mainland of Asia, not 500km or 4000km.
So my version of the future takes some poetic, novelistic license. As you said, there’s a book in here somewhere.
If there really is some more isolated nil-elevation island or atoll somewhere with hard-core primitives, then if we posit they are the last post-plague population of humans, well … the story almost writes itself.
That last part isn’t necessarily true, and in fact is rather dangerously naive thinking. The ongoing major loss of diversity in what has been called the sixth mass extinction is irreversible, and so are potentially other effects of high GHG concentrations. Furthermore, at some point runaway climate change due to ongoing feedbacks can become a reality. Significant losses of polar ice cover, methane emissions from a thawing Arctic and potentially massive releases from undersea methane hydrate clathrates may all create unstoppable climate tipping points. Hans Oeschger was among the first to recognize this possibility and identify such events in the paleoclimate record (sudden climate swings called Dansgaard-Oeschger events are named after him) and this knowledge drove his deep concern about climate change for the rest of his life.
As well, even if we avoid any runaway geophysical feedbacks, the “gradually” part of @Blue_Max’s “gradually reverse” comment will be measured in thousands, if not tens of thousands, of years.
From the perspective of not only all of us alive today, but of our entire civilization, the changes we have already baked in to definitely occur over the upcoming decades to couple centuries will be haunting future humanity practically “forever” albeit not literally forever.
Those two flavors of “forever” are a distinction without a difference.
It would not be the sea level rise, but the temperature rise that would be their biggest threat. Between being inhospitable to them, their agriculture, as well as the warming and acidifying ocean, climate change could wipe them out even if it doesn’t drown them.
The changes could be reversed more quickly if we didn’t lose our civilization. If we continue to advance, we could put solar shades at the L1 point, we could use nuclear or fusion or other non-carbon emitting power to suck that CO[sub]2[/sub] back out of the atmosphere and ocean.
We may not do these things, but we certainly won’t if civilization collapses.
ETA: I thought subscripts worked on this board…
They do: CO2. But you need CO<sub>2</sub> not CO[sub]2[/sub].
Ah, many thanks.
NH3 I just wanted to try it…
E = mc2 I wanted to try this too… How do you do this?
E=mc2. “Sup” instead of “sub”.
NH4+ Thanks.
This is not good news… Especially for the younger guys.
In the category of “existential threat”, though, this doesn’t seem to be a plausible contender unless you take the very dubious step suggested by the same author in another interview:
Even the author is acknowledging that just extrapolating from a recent trend to future estimates is making a huge and often unjustified assumption. Sperm counts and other factors in male fertility may indeed have fallen a lot in the past fifty years, but what evidence is there to think this decline rate is indefinitely extendable, or even extendable over the next few decades? How much of it is related to, for example, the unquestioned demographic aging of western populations, since sperm count generally declines with age?
Possibly the definitive take on the general subject of naive trend extrapolation was provided in Mark Twain’s 1883 Life on the Mississippi: