Of these, the only ones I see plausibly extincting ALL humans are an AI going full grey goo/von neumann machine and just consuming all matter on earth to replicate itself, and a sufficiently large rock breaking the world.
Is a Venus-like runaway greenhouse effect happening on earth possible? Because I think that would definitely kill everyone eventually.
Maybe, but if things got to that point I think we would get technology to start sucking carbon out of the atmosphere at a furious rate. Unless somehow the greenhouse runaway effect came so rapidly that even putting in all those carbon scrubbers couldn’t do the trick in time.
I also think we could stop it if we got serious.
Venus was also considered a potential planet to populate. In some ways it’s easier than Mars because it has an atmosphere. All habitations on Venus would need to be constantly airborn but it is possible. Just think how much easier it would be to create climate controlled floating habitats on Earth vs another planet.
The billionaires are going to survive climate change even if no one else does.
And be able to survive 180+ MPH winds…
Venus’s atmosphere is 96.5% CO2. It’s not possible for CO2 levels to reach that on Earth because Earth has mechanisms to capture CO2 (e.g. oceans, plants) that don’t exist on Venus, mostly because Earth is much farther away from the sun than Venus is. That being said, human activity could really mess up our climate, and kill a lot of people.
Runaway self-replicating nanotech? Extremely difficult to make happen – we aren’t even close to the tech level yet – but if it does happen, it absolutely guarantees human extinction. And animal extinction. And plant extinction. We’ll be down to algae and bacteria, and even they will not be having a good day in Stalingrad.
(For a fun fictional take, I recommend “The Reproductive System” by John Sladek. New Agey, and dated, but worth it for the devouring of Las Vegas.)
Or if some high-energy experiment creates a black hole of sufficient size to start gobbling… Again, not even close at this point, but if it does happen, ain’t a soul gonna survive.
Don’t underestimate drinking water contamination just yet:
“Due to anthropogenic activities, freshwater systems world- wide are confronted with thousands of compounds. In the European Union, for example, there are more than 100000 registered chemicals (EINECS), of which 30 000–70 000 are in daily use. About 300 million tons of synthetic compounds annually used in industrial and consumer products, partially find their way to natural waters (Schwarzenbach et al., 2006). A major contribution to chemical contamination originates from wastewater discharges that impact surface water quality”
Some of these synthetic compounds, especially from consumer products and prescription drugs, can affect humans (reproduction, immune systems, fetal development) at parts per billion, especially the endocrine disruptors. Current municipal wastewater treatment systems are not designed for, nor capable of removing these compounds unless there are technological innovations, and an increased awareness that it’s a existential problem. See link:
https://www.nature.com/articles/s41545-018-0029-3
ETA: Now that’s funny. I had no idea that @EastUmpqua was working on the above post when I started. I only saw their post after I posted the content below.
100% of humanity alive today will be dead in 150 years. So the calendar is a pretty existential threat to current humans.
Which also implies anything that mucks up reproductive success bad enough could/would be an extinction trigger. There’s been a lot of noise over the years about declining individual fertility. Perhaps due to environmental factors.
It’s not splashy, but it might be what does us in.
Same issue as with other examples above: statistical outliers. The effect isn’t sure to get all potential parents, and a sustaining population might arise from those who are immune. It’d be bad! But it still might not get the whole dang species. We’re mighty resilient in some ways…
I don’t think we would be able to see an astroid soon enough if it’s trajectory is directly towards Earth, especially if it doesn’t reflect much light, until it’s too late. We don’t yet have the technology to “nudge it a bit”. I think a large enough event could cause our extinction, but there might be more probable causes.
This article suggests that climate change could have very serious impacts on human’s ability to grow food, not to mention wildlife extinctions. But I agree with some of the following posts that some of us would survive this. See below:
AI might not intentionally target humans for extinction, but I could see how it might inadvertently kill us to reduce inefficiency, and preserve itself (e.g. Hal from 2001: A Space Odyssey).
But with the universe full of natural resources, what appeal would there be for that?
The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.
I don’t know what the probability is, but if it does happen the probability of extinction is 100%.
Unless the vacuum bubble nucleates in a part of the universe so distant that the region is expanding away from us faster than the speed of light – in that case we’d be quite safe.
True enough, though I’d consider those parts of the universe expanding away at faster than the speed of light to be “not the universe”. Odds are that the actual universe is vastly bigger than what we can see. It’s just sorta academic since no influence can get from there to here.
Yes, but asteroids don’t come out of nowhere; they orbit the sun, and we keep a pretty close watch on the big ones. Especially the trajectories that come close to Earth’s orbit. We certainly haven’t found them all, but we’re doing a pretty decent job of watching the skies and getting better every day.
Sure we do. The current proposal is to use repeated flyby with crafts in order to use their gravity to nudge the asteroid off course, but if needed we could also strap a rocket to it and push. An asteroid on a collision course with Earth would need just a tiny nudge to miss us by hundreds of thousands of miles.
If a high energy experiment created a microscopic black hole, it would have the gravitational force of the mass that went into it, and would most likely evaporate away rather quickly in a burst of Hawking radiation. Just because the mass is compacted enough to give it a singularity wouldn’t magically give it more gravitational attraction than that mass had in the first place; it certainly couldn’t swallow the planet.
(Hijack)
I learned just this week that there is data to suggest that the wider universe is at least 250 times bigger than the visible part. Basically, an analysis of the clumpiness of the cmb and how it correlates with the structure we see suggests that if the universe has a closed geometry it must be at least this large. If it has an open or flat geometry, it’s infinite.
To put this in context, if the non visible parts of the universe have a similar density of matter, that would mean there are some 2500 stars for every grain of sand on Earth. 
(/Hijack)
In terms of the op, Wesley_Clark nailed it way back. With billions of us on every corner of the planet, and humans being resourceful as we are, Extinction is virtually impossible at this point. You’d need an AI / aliens deliberately scouring the Earth to do it.
Or, something which completely sterilizes the Earth of all life, like a moon-sized asteroid.