Assuming current technology, is there any catastrophic event from space/earth that humanity are able to see/detect/calculated it coming. (Would prefer a space event but earth ones are welcomed)
This event should be able to wipe out at least 90% of living things.
OR
if we go for the less classic solutions (meteorites, comets, asteroids and so on) there isn't much we can actually predict. Stuff coming from the sun usually comes without warning and it takes very little (compared to astronomic times) to reach earth. Other things are:
1) not very visible until it's too late (Aliens? )
2) Visible enough that we can know it in advance (Comets or even incoming planets)
3) Too sudden (Solar Flares)
4) The information of the event comes with the event itself (Gamma Ray Bursts, which travel at the speed of light so you get a visual of the event at the same time of the damage being done)
An idea could be a Rogue Black Hole (yes, they are a thing): a black hole wandering around the universe and ends up in our solar system.
Since the black hole is not "eating" anything at the moment it's undetectable (It should be emitting Hawking Radiation though, but i don't think it's that much detectable, it should be very faint actually).
All of a sudden it gets close to Neptune with catastrophic results, and everyone is aware of that black hole now.
The black hole has now matter swirling around it (an accretion disk) and it would be very bright because of the heat emanated by the disk, probably it could even be seen with naked eye (it mostly depends on the disk size at this point. But we can safely assume it'd be easily visible at a certain point)
Now we have 2 scenarios, depending on the black hole size:
The black hole is very tiny and almost at the end of its life (we're talking the same order of magnitude of atoms, if not tinier. A black hole of $1*10^{-9}$ nanometers, way less than an hydrogen atom which is 0.053 nanometers, will have a 673468.0 metric tons mass and a lifespan of 813 years)
The black hole is big enough to influence other celestial bodies (a coin sized black hole will have roughly the same mass of the earth, so i think that'd be enough)
In the 1st case the black hole won't have a long life span but it can have a direct effect on the earth by evaporating next to it, emanating a lot of radiation. So our scientists might calculate that in a few years the black hole that perturbed Neptune will get close to the earth and end its life close to it exploding with terrible force, emanating gamma rays which will kill most of the life on earth
The 2nd case is more catastrophic. The black hole can't reach the earth or it will completely destroy it.
The best course of action for a not-so-catastrophic catastrophe would be if it hit mars and then the debris would hit the earth or if it passed so close to the earth to perturb its orbit causing earthquakes and possibly a new ice age, or even move the earth closer to the sun (and/or tilt its axis) and make it hotter. In any case this will do a lot of damage to the current life on earth
This video might give you an idea on how catastrophic a coin sized black hole is (and also how a very tiny black hole might "affect" life on earth on evaporation): https://www.youtube.com/watch?v=8nHBGFKLHZQ
I haven't said anything about the black hole's speed but that can be arbitrarily set to suit your needs (depending on how much "sciency" you want to go, you might need to do some calculations). Just remember that you need to make it go quite fast to not be caught by the sun's or other planets gravitational field, becoming part of the solar system and ending up in a stable orbit before unleashing its wrath on the earth
Also remember: Black holes do not suck things! they have a huge gravity pull but no bigger than the thing they formed from. If the sun would instantly turn into a black hole, planets would still have the same orbit (well, we would die because of no more heat but that's not important). The only difference would be in the area which was previously occupied by the sun, that's the point where a black hole's curvature of space time (which is its "gravity") does the weird things. This is just to say that what i wrote is kinda plausible with today's knowledge of black holes.
More on Black Holes lifespan:
https://www.quora.com/How-long-does-a-black-hole-last
Black Hole features calculator:
http://xaonon.dyndns.org/hawking/
Ruling out the classic meteorite, an interesting option can be a big and opaque cloud that cross the solar system between sun and earth blocking the sun for some decades, let say a century, and thus fast driving earth to a ice age.
While we can discover the cloud 5 years beforehand, we cannot prepare to an ice age so fast and moreover, with no light (or 80-90% less) all life on earth are basically doomed, except, perhaps, some niche where some extremophiles can survive.
The problem with global warming.
The scientists assume that Venus or Mars looked similar to Earth, and they also assume that it is the global warming that turned them into hell.
The temperature on Earth rises slowly, and nobody knows when is the critical point of no return, but it is going to come. In such scenario, no living organism would survive.
Humanity can calculate that World War III is coming in advance. If the war goes all the way to the nukes for any reason, such as a group of extremists that hid themselves within the ranks of both sides turns them on, they can wipe out most of the living things overnight and then the fallout will do the rest, killing out things for the next few decades.
We can, more or less, detect the possibility of it happening before the war even starts. If we go for the real world example, then I should say that I am already getting vibes that a new global war is coming and it might bring nukes with it, or maybe even weapons that are more horrible.
When it comes to space, we can only detect a disaster beforehand if it moves slower than the speed of light. For example, as luck would have it, one of the Voyager probes might detect an asteroid coming towards Solar system and the scientists can calculate that it is on a collision course with Earth in five years.
From the setup for Arthur C Clarke's The Songs of Distant Earth:
Scientists in the 1960s discover that the neutrino emissions from the Sun – a result of the nuclear reactions that fuel the star – are far diminished from expected levels. At a secret session of the International Astronomical Union it is confirmed that the problem is not with the scientific equipment: the Sun is calculated to become a nova around the year AD 3600.
Easily adaptable to your desired shorter timescale.
How about the solar system enters a galactic high density dust cloud. The reduction in radiation would invoke a severe ice-age but, apart from spectacular dawns and dusks, not much else. Would that work?
There's a 3% probability that the whole solar system breaks down. But for an unlucky butterfly effect, the disintegration of Phoebes (Mars' satellite) caused this probability to become a fact. Earth's orbit around the sun is being pushed further and further, causing the planet's climate to veer slowly into a perpetual Ice Age. Luckily enough, the Earth's new orbit allows for some fortunate species to survive. Including a bunch of human beings.
I'm thinking biology rather than astrophysics. The Tasmanian devil is facing extinction because of an infectious cancer-inducing virus. Could that happen to humanity?
It would have to be extremely infectious and very fast spreading so medical science does not beat it. Something like a flu pandemic. It would initially not cause any life threatening symptoms, so the CDC and similar agencies would not be very alarmed. By the time a global epidemic of highly aggressive cancer was noticed it would be too late. Almost everyone would have been infected.
Nature might spring something like this on us but prob ably only once in a billion years. But we might do it to ourselves. Research by some crazy scientists working for an equally crazy government with the aim of creating a doomsday weapon. They don't realize their latest brew is a success because their lab is destroyed by a natural disaster or insurrection the next day. Or their insane leader decides that because he is dying then everyone else must go with him.
By the way, if 90% deadly in one year is sufficient for your plot then all you need is nature, in the form of a particularly virulent pandemic flu that makes the Spanish flu look like a dress rehearsal. Or some other virus mutating across the species barrier in one unusually successful mutation. It could happen.
The human species went through a genetic bottleneck about 80,000 years ago and has less genetic diversity than most other species. We may be spectacularly vulnerable to a particular new mutation of an old virus. Like smallpox in the Americas, but global.
