Trying to figure out a post-apocalyptic scenario and looking to stay somewhat within the realm of possibility. Given how tech levels MIGHT be in a 50 years, what would you say would be the most likely cause of a catastrophe that wipes out maybe 90% of Earth's population?

I know a giant asteroid is out of the question - IIRC we've pretty much mapped out every asteroid that has a chance of coming close to the Earth and would have a lot of advanced warning. A comet, maybe? They tend to be more unpredictable.

Nuclear war also seems unlikely as it would essentially mean mutually-assured destruction and I can't see why major nations would want themselves to be annihilated as well.

How likely is a bio-weapon? Would a super-contagious viral plague work too fast and be quarantined too easily?

Or would the catastrophe be slower and more insidious, brought on by climate change and dwindling resources, leading to reduced territory and infighting/civil wars around the world?

The Four Horsemen

Global warming is making crop failures look likely. Famine can kill large numbers of people and historically has done so often. Tens of millions died in the midcentury famines in China and the USSR. If global warming makes the tropics unsuitable for agriculture that would lead to famine in Africa and India for sure and possibly China. Starving people do not drop in their tracks - these people will migrate and destabilize the regions where they show up.

Migration of desperate people leads to conflict. This is happening now. https://www.nationalgeographic.com/news/2015/3/150302-syria-war-climate-change-drought/

If you add war (and its friend pestilence) to this context you might reach your 90%. Major nations with a view for the long term do not want to be annihilated but in the context of famine and desperate chaos, people who come into the possession of nations might not have the same long term view as people do now.

The most likely suspects

1. Molecular Nanotechnology (either weaponized or accidental). ~31%


2. Artificial Super Intelligence (terminator scenario). ~27%


3. Large scale wars, including nuclear and terrorism. ~27%


4. Engineered or natural pandemic. ~15%

I've summarized the Source and redistributed the probability under the assumption that there was such an event. Note that the source references extinction by 2100 not 90% by 2119, however, I think that you would need an extinction level threat to kill 90% of people, it just needs to be slightly milder, and none of the above scenarios are likely to have an exclusive 100% kill condition.

Notes on other answers:

Climate change is likely to have an impact on the world moving forward, but it's extreme to say that it could kill 90% of people in the next 100 years. Millions possibly, billions probably not, over 6 billion is extremely unlikely.

Famine as a global risk is unlikely due to how diverse food sources are, a long term problem might get you as high as 50% but as people die the demand for food decreases, and people move to the places where they can get food (or else die) so the problem quickly remedies itself.

Alternatively there's always the rapture

I also believe climate change is the most likely source of catastrophe over the next century. Not only starvation and migration-induced wars, but weather pattern changes that could lead to much more rapid changes in habitability of populated regions. For instance, a five year drought in Brazil would probably depopulate half of South America, and have knock-on effects I can't readily foresee.

There is still the risk of nuclear war, as well. Major nations may not start it (as you say, MAD makes this MADness), but a small, impoverished nation with a hereditary dictatorship and a new nuclear arsenal might well start a fight they think their enemy hasn't the will to pursue -- and the counterstrike might trigger a secret treaty (like the one that put the US into WWII in Europe as well as the Pacific), leading to a less restrained exchange.

I still wouldn't rule out an asteroid, either -- at least as the trigger. If a rock like the one that exploded over Chelyabinsk came in on a more vertical trajectory over a major city, it might be days or longer before the victim city's national government knew it wasn't a nuclear attack -- and by then it would be days too late for whoever they thought was likely to strike that way, that week. And again, counterstrikes would escalate.

At least a nuclear war would offset a lot of global warming...

I would argue famine is a good contender in general.

Over the last 100 years, we've come to rely on refrigeration and a global network of food shipment to supply local needs across the globe. As a planet, it's almost as if we've collectively decide to put all the production of any particular farm production into one ecosystem, and just ship the products everywhere it's needed.

This effectively means that we've concentrated the population of every species of domesticated crop and animal into regions that could easily be swept with disease and exterminated entirely.

I read recently that the African Swine Flu recently destroyed 1/3 of the world's pork production with very little fanfare. Apparently with this much concentration it doesn't take much to cause big problems in our food supply.

This route is also a good one to pursue if you want to avoid modern politics bleeding into your world. No one country and no one political party has given this particular issue a stigma, which lowers the risk of your audience getting offended by perceiving your world as a soap-box for your own ideologies.

another Carrington event

A solar based EMP would essentially throw us back 150 years in technology.

No major city would survive the first winter. Only hardened electronics, or those stored in nice faraday cages would survive. An estimate of 1/2 to 2/3rds of the worlds population would be dead within a year.

The biggest problem with coming up with a disaster scenario here is the need to kill 90% of the population. That's going to be a very difficult thing to do. Most of the disaster scenarios mentioned in other answers (climate change, a Carrington event, a limited nuclear war) would not come close to that.

The IPCC says climate change could cause a loss of global GDP of .5-2% by 2100. That's bad, but it's a far cry from killing 90% of the population. Even the most extreme scenarios for climate change would not kill a large percentage of the population.

A strong Carrington-level event would certainly kill a lot of people. Even a short-term (months) disruption of power grids and communication networks and electronics in general would result in large losses of population as food distribution and production failed, but 90% is not realistic.

A nuclear war - even a large-scale one - would not kill 90% of the population. Even at the height of the cold war, with potentially thousands or tens of thousands of warheads detonated in a conflict, no one was predicting anything close to 90% loss of life. In 1979 the Office of Technology Assessment issued a report to Congress in which they estimated loss of life in a full scale exchange between the Soviet Union and the U.S.A as being 35-77% of people in America, and 20-40% in the Soviet Union.

The Effects of Nuclear War

To be fair, they admit that there are a lot of unknowns and some of their estimates are just educated guesses. Still, even if you accept the worst case, it's hard to get to 90% because there are a lot of countries that are very spread out and likely not to be targeted. Australia, Indonesia, Africa, South America... Lots of population would likely survive, and many of them would be in the Southern Hemisphere where even fallout patterns can't get to them.

There are two disasters that could easily kill 90%, or even wipe out humanity completely. Those are a giant impactor (asteroid or comet), or a biological weapon. We discover new comets and asteroids all the time, and while we have no doubt found all the close ones that could threaten the Earth with an extinction-level event, you can always have one appear from outside the solar system. We'd know it was coming for a long time, since anything big enough to destroy mankind would be spotted long before it got here, but we'd be utterly unable to do anything about it.

A biological weapon could wipe out humanity if it was designed to do so. Imagine a virus that has no symptoms other than to make the person who contracts it be sterile. Or a virus that is contagious for a long time before it begins to show symptoms, and then you die shortly thereafter. There would be no way to find the carriers, or even to know the disease existed before it had spread through enough populations that it could never be stopped. Maybe 10% of the people have an immunity.

If you had said that 1/3 or even 1/2 of the people die then I think you could plausibly use any of the disaster scenarios mentioned. But 90% is a really hard number to get to. I'd go with either a major impact or a biological weapon.