What is the lowest level event or series of events that could, potentially, destroy human civilization? I got the idea for the question from a conference I went to a few weeks ago that was discussing disasters and how fragile our infrastructure is to systemic failure, so thought I’d ask 'dopers for their thoughts. We all know a major event could take down human civilization, but what do you think is the least event that could do it world wide? This could be anything from a few nukes detonated at strategic places to, I suppose, a key or several key resources being depleted with no alternatives to, perhaps, just doing nothing about global warming. Whatever you think would be the smallest event that could take it all down and push us back to those sweet, sweet hunting and gathering days of yore…
Whoa, your last sentence seems to add a whole new constraint. Do we need to leave a certain level of habitability after our work is done?
I would assume a drive for the most insanely deadly pathogen would probably give you fair bang for your apocalyptic buck. Otherwise, you can save your pennies and hope that a 10-mile asteroid is headed to Earth pretty soon - genocide for free, pretty much.
I would also question the premise, about how fragile our infrastructure is to systemic failure. I think the major lesson of modern civilization since 1750-onwards is that globalization is a great preventer of ancient problems such as disease, famine, even sewers. The greatest catastrophes which have been inflicted upon modern civilization have been self-inflicted, and the minor catastrophes which have afflicted mankind from the beginning (famine, fire, disease) have been eradicated or ameliorated. More fragile? Not hardly!
Depending on how heavily the world economy depends on the US, a particularly cataclysmic Yellowstone eruption could take the US back to the stone age (and relatively uninhabitable), thus ending the New York stock market and significant chunks of the Internet. A subsequent nuclear winter would ruin enough crops worldwide to cause widespread starvation in other countries. If the right countries fail to survive all that, whoever’s left is going to be doing plenty of hunting and gathering.
A gamma ray burst would do a nice job of wiping all the life off the planet, but it’d take a good chunk of the biosphere with it and we’re talking about an habitable earth afterwards, so.
Note that neither of these take any effort on our part (and could happen at any moment); I figured they were noteworthy for that.
I’d say a “one-two-punch” of the right type of plagues with the right timing would have a good chance of rendering humanity extinct or close to it with minimal effort. Extinctions often seem to happen that way; some disaster kills off most of a species, and then a second kills the remainder before the population can rebound.
Both diseases are highly infectious, but slow to show symptoms; nobody knows they exist before they’ve become a global pandemic. Both are highly lethal, but one starts first and manifests symptoms many years later than the other. The faster pathogen kills off most of humanity quickly; but well before that happens the sneakier pathogen has already infected most people, and most of the non-infected probably die during the pandemic.
Then a decade or two later the second pathogen goes to work and kills off the survivors. Humanity might survive a 90%+ death toll twice in a row, but I really doubt civilization would.
One of those giant electro-killing burst thingies (hey, that’s Science Talk, right there!) that render all cell phones useless would lead to mass suicide on a scale that would cripple the face of the planet. 
I think an asteroid strike large enough to cause a “nuclear winter” of, say, 3 years would do the trick. There would be starvation on a massive scale, and the incredible stress on human civilization would probably result in wars.
Gama ray hulk is the technical term I believe. That’s a bit too big for what I’m looking for, but certainly would do the trick.
My initial thought was one piece of space junk too many would cause a Kessler syndrome Gravity thingy that would render all of our satellite comms and the like inoperable. If something else of moderate damage happened at the same time (maybe a solar storm that took out power for a few weeks or months), we’d get Der Trihs one-two punch thing and that might take down civilization.
I’m not looking to destroy all humans, just what would it take at a minimum to take down the current civilization.
I would guess that a solar storm big enough to bring down the power grid for weeks or months would be big enough to destroy / disable all the satellites, making your Kessler syndrome an unneccessary redundancy. Also, no power would suck, but would not, I think turn us back into hunter-gatherers. Well, some of us at least: many would die, but some would survive much like they do today, still raising crops and living their lives.
ETA: except for the line about “hunting and gathering days” I think six months without power would do enough damage to “take down the current civilization” in it’s recognizable form.
I think, from an ROI perspective, it would be cheaper just to destroy all people than it would be to make civilization “revert” back to some prior state. I mean, you did say you were on a budget and all… 
It would mean we could not put satellites back up…or anything else. And as I said, it’s a one-two punch…first you get hit with having all the satellites taken out and you can’t do anything about it. As you start to try and recover from that you get hit with something else. So I don’t see it as an unnecessary redundancy. The second punch could be something else, I just used a solar storm as an example.
There are three main ways to “destroy civilization”, right?
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Destroy all the people
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Destroy all the knowledge
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Destroy all the infrastructure (while still killing vast amounts of people)
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Probably cheapest, a series of modified plagues and shit could knock us back to 700,000… or 70,000… or 7,000… survivors.
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I wouldn’t even understand how this could happen. If you wipe out all computers, we’d revert back to 1950 or so… whereupon we start a crash program to rebuild the computers we just launched.
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You’d have to come close to cracking the planet in order to have enough infrastructure problems to bring down civilization, so I don’t see that really happening. Closest is an all-out nuclear war, which is plausible enough, tbh.
XT’s last scenario is a knowledge loss, but we’d still have all the books and paper. Civilization will be fine, it just won’t have satellites, but it sure won’t put us back into “hunter-gatherer days”. It’ll take nukes and/or plagues to do that.
I once speculated about the possibility of a disease as destructive as HIV and as contagious as the common cold. I don’t know what the rate of natural immunity is, but if 99% of the population died off, the remaining 1% certainly could not sustain our civilization.
I have also read that a determined hacker could take down the electricity grid in the US. The grid is definitely not hardened either with respect to software or hardware. Remember the big power failure about 15 years ago when sagging wires on a very hot day in Ohio brought shut down the eastern half the US. Of course, they did get it going fairly quickly but it does not augur well for the grid. Neither the government nor the free market can seem to deal with the problem.
Perhaps we could calculate the 9 billion names of God?
A while back, I asked everyone’s opinion on the impact of a virus that kills off all fungus. The consensus is that it would be *extremely *bad, potentially killing off all life on the planet. Seems like a pretty good bang-for-the-buck situation.
We could just do nothing special for another 5 billion years or so…at that point, the swelling sun will swallow up the planet for free!
In 500 million years, give or take, most of the oceans will boil away, so we don’t have to wait until the sun goes all red giant hulk on us for that. Still, bit of a longish time frame…I seriously doubt human civilization would be threatened by that event if we last that long. We gots to get them now, before they escape!!
Self-replicating nanobots that can encapsulate water into gel spheres in their bodies, turning the earth into the planet Dune.
I think it all hinges on what you consider “least” to mean, and how “practical” such a thing has to be.
If it’s least active effort, just let things run without interference, and the odds of failure from some factor approaches 100% as time goes on and approaches sun-swallowing timescales. This has the added benefit of being able to capture ANY civilization-ending force rather than relying on one, and requiring no additional effort.
If it’s least in terms of tweaking of the least or smallest thing, tweak one of the fundamental forces / constants. Make the nuclear coupling constant 2% stronger, or the universal expansion a little bit stronger than gravity, and our universe itself would be drastically altered and not amenable to life as we know it.
If it’s the least change to our biology, have human molecular chirality suddenly swapped. Every human would die as essential chemical reactions fail to take place in the body. If that’s too far-fetched, the pandemics mentioned by earlier posters will work just fine. For a wild-card option, permanently eliminate all our internal bacteria - that would likely be fatal for enough of the population that civilization will collapse.
For the least change in current societal/technological structures that could end civilization, there’s so many contenders that “just leave it alone, we’re doing a fine job on our own and will have an answer shortly” looks pretty good again, but if you had to choose, anything from MAD nuclear exchanges (just a button and some brinksmanship away) to armies of rogue automated killing machines (in the works now) to a solar storm ruining most electronics and internet on half the planet (like the one in 1859 that hit us, or 2012 that missed us) and wrecking the complex and highly-interdependent global supply chains most of us rely on for food can do a good job.
On the more fanciful side, you could do something as simple as decreeing that NAND gates no longer work, and that alone would wreck most of civilization (electronics, internet, supply chains, etc), or that electrons or fungi or oil or bacteria no longer exist, etc.
One type of pathogen that could cause significant damage is a generalized form of blight that kills off a majority of the planet’s plant life. It would cause a chain reaction of mass death throughout the food chain causing mass starvation among humans leaving the survivors vulnerable to a more specific pathogen.