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Since this century is all about how the world is doomed and that the end of mankind is near, I think it'd be fun to come up with doomsday scenarios
The rules:
When a doomsday scenario appears to be thoroughly discussed (more than 1 person should respond to it), then anyone could post their doomsday scenario afterwards and the cycle continues. Be serious with your responses and doomsday scenarios. I don't want to see "Molten cheese enveloping everyone on the western hemisphere" or anything of the sort.

My doomsday scenario is a black hole coming at the Earth at the speed of light. What exactly would happen? Would we die before we know it or would we feel the effects before dying?

I think that we would die instantly and faster than any other object coming at us since time slows down near a black hole. If time stood still and an object came at us at the speed of light, then that would theoretically be a faster death than when something like an asteroid came at us in real time.

The problem is, we don't truely know what hapens when an object enters a black hole, so do we know that everyone would die?

That's false in the sense of what you mean. If you enter a black hole you will, without doubt, get instantly ripped into a countless amount of pieces. It's what happens afterwards which is questionable.

I'm thinking the nuclear fallout would kill everything, so no society. Cockroaches would rule the world if they could find anything to eat.

I think you should rephrase that question...

Nuclear warfare would kill millions, but there would be survivors if they managed to go to an underground bunker in time.
Would a nuclear bomb destroy the ozone layer? If it could, then the outside would be uninhabitable for a long time until nature replenishes the ozone layer naturally (since everything would be unprotected from the sun's radiation).

That and the fact that the entire world would be riddled with deadly radiation for thousands of years.

The first part, "If no one did survive" means that everyone died
The second part, "how long do you think it would take for everyone to die?" asks that if there were survivors (which is explained in the first part) then what would be the estimated amount of time it would take for them to die.

Next doomsday scenario:
Global warming results in the melting of greenland and, in response, the flooding of the world. Would we live on mountains or hills and make cities on them? Would it seperate the U.S. into different countries because there would be large bodies of water inbetween large bodies of water? How could we stop the flooding and maintain a balanced ecosystem? You should take some time on asnwering this because it may come in handy very soon...

Are we allowed to respond to previous ones?

That depends entirely on the mass of the black hole. If you specify a mass I could probably give you a rough answer.

If all the nukes in the world were set off, in ways to try to make humans extinct, it would be debatable whether or not anyone would survive. However, in a standard nuclear war probably fewer nukes would be set off and they would not be set off in the right ways to make people completely extinct, so I'd say at least a few hundred thousand people in various places around the world would survive the effects of the explosions (this counts fires, radiation, etc, as well as the actual blast). Afterwards it would take them a long time to rebuild, but if they were smart enough I think they could keep from sliding into a primitive state of technology long enough to build up civilization again.

Greenland is mostly made of rocks and dirt. I assume you mean that only the ice melts. Also, that other polar ice would also melt (it would be hard to melt only Greenland's ice and leave all the other ice intact, and essentially impossible the way we're doing it).

I think we may try to reverse it as soon as possible. For example, if we genetically engineered some robust bacteria that liked to live near the surface of the ocean and reflected lots of light, we may be able to increase the Earth's albedo and cool things down again. If we can't do that, then at least we could start painting stuff white, or put up giant shades in space to reflect the light away from the Earth.

However, if that didn't work, then I suspect people would survive by living on mountains above the water level. The big question here is in ecological terms, whether the damage to the environment would be enough to, say, reduce the ozone in the atmosphere and let us all get fried by ultraviolet, or reduce the oxygen so we all suffocate.

A black hole doesn't have any more mass than whatever goes into it, i.e. the star, the surrounding planets, etc., so it's essentially just the mass of the star. If our sun was instantly transformed into a black hole, our orbit around it would remain the same, but we would freeze to death within... a rather quick amount of time. I'm sure current science could keep a few settlements of us alive for rather lengthy periods of time, but our biggest source of energy would be gone. If a black hole somehow managed to travel at the speed of light towards Earth, it would most likely miss us by countless billions of miles (because our solar system is moving very, very quickly, along with the rest of the galaxy and all galaxies in fact). If somehow it hit our solar system at the speed of light (one chance in a horribly large number), it is still likely that its speed (although we're moving quickly, we're nowhere near the speed of light) would simply rip our solar system apart. Heck, at the speed of light, perhaps light could escape it (going in the opposite direction), or rather "leak" out the back. Still, if a black hole passed through our solar system at the speed of light, there is little chance that it would actually hit anything, and we would simply be thrown off course. I'm not even sure about "ripped apart," but we would certainly feel substantial ripples. If it hit the sun, we'd definitely be goners. Obviously the same would apply if it hit Earth.

So then global warming. A UN panel is expected to publish a report that human influence has raised global warming incredibly, and that by 2100, catastrophic events will occur because of our influence if we persist along this path. I know 93 years seems like a long time, but it's hardly a drop in the bucket. The world's societies need to combat this together. We definitely need to reduce and eliminate air pollution from factories (in fifteen years, I don't expect any pollution from vehicles). Remember that story about the elevator to space a while back? Perhaps we could use that method to transport ozone effectively and efficiently to the upper atmosphere where it belongs. I don't know how to solve this problem, but it undoubtedly does need solving.

I don't foresee nuclear warfare as a real possibility anymore. Humans are too scared to do that, and the only use of nuclear weapons, if any, will be by rogue states and terrorist organizations. It will cause hefty devastation if used of course, but I can't possibly see that used on a scale that was considered during say the Cold War.

Anything with mass traveling the speed of light would have enough energy to vaporize our entire solar system practically instantly simply by touching it, and due to the fact that it is traveling the speed of light, we would never see it until it hit us. The fact that its a black hole and not an intergalactic shoe flying at us means almost nothing. (As things with mass traveling the speed of light require infinite energy, the change in mass would only differ from a constant, whereas speed would be quadratic.)

Nuclear warfare, if not directed at every nation in the world (which seems unlikely) would be devastating in the short term, but studies show that it might be beneficial in the long run.) Animals that have grown up in the aftermath of Chernobyl, namely deer, have shown signs of increased longevity and better overall fitness. Not that we wouldn't first have problems, but the places that did survive would be pretty well off.

Global flooding would create a mass of freshwater life in saltwater areas, which would lead to mass extinctions of saltwater life and promotion of strange algae in the waters. This extinction pattern would follow up the food chain, and we would have to adapt to eat things that probably taste rather different from what were used to. Weather patterns would change as more water would be able to break into the atmosphere and would cause more violent storms, as well as possibly enhance toxins that we spray into the sky.

But all of that infinite energy would be locked up in the black hole. Perhaps that would be a white hole then? Anyway, if it were a black hole, it wouldn't destroy our entire solar system unless it managed to hit every planet. Life on Earth would be incredibly traumatic for a while, and we might be ripped out of orbit around the sun. If it were a white hole traveling at the speed of light, we wouldn't see anything. If something is traveling right at you at the speed of light, you can't see it.

You're saying this, knowing that a black hole sucks in even light? How can that be true, if even relatively small black holes have enough gravitation to pull us in? (obviously, if they also suck in light)

For the matter of that, anything with rest mass (and black holes do) cannot travel at the speed of light at all.

On the contrary, it means a lot. A shoe flying at relativistic speeds does damage by having its mass collide with the mass of something which is stopped or moving fairly slowly (such as the Earth), and convert an extremely high energy concentration in a small amount of matter into a relatively low energy concentration in a large amount of matter. However, a black hole with the same mass as the shoe would have difficulty with the colliding part; it would just pass through the Earth, and while it would cause a small (probably microscopic) explosion due to a small amount of matter getting sucked close to it and heated up, it would be nowhere near the kind of destruction caused by a shoe moving at the same speed.

You're committing the all too common 'black holes literally suck' fallacy. The gravity coming from a black hole is no different from the gravity coming from any other object, the thing with the black hole is that the place it's coming from is extremely small. A black hole with the same mass as our Sun would indeed be able to suck in light, but only light that was passing very close around it (within approximately 2.95343236 kilometers, assuming the black hole is infinitely small). And if this black hole replaced the Sun, the gravitational effect on the Earth would be very small, and the Earth would go on orbiting at its normal distance.

Why? Well, gravity diminishes by the square of the distance. That is to say, if there is, for example, a watermelon one meter away from you and another watermelon three meters away from you, the more distant watermelon will be exerting not the same gravity, not one third the gravity, but one ninth the gravity as the close watermelon (because three squared is nine). Similarly, while we say that the entire Earth is pulling on us to keep us down, the dirt that is right under our feet is exerting far more gravity per kilogram of dirt than the dirt at the other side of the Earth. The distance to the other side of the Earth is 12756.3 kilometers, or 12756300 meters, and that squared is 162723189690000, so a kilogram of dirt one meter beneath your feet is exerting one hundred and sixty trillion times as much gravity on your feet as one kilogram of dirt on the other side of the Earth.

With something as large as the Earth, the total gravity acting on you even if you, say, jump in the air, or take an elevator up to the top floor of a building, doesn't change very much, because the distance to most of the dirt that is pulling on you hasn't changed by a very large factor. However, black holes are extremely small compared to their mass. So if instead of the Earth underneath your feet there was a black hole one meter beneath your feet with enough mass to provide the same amount of gravity as the Earth, then just by jumping one more meter away from it, you would experience one quarter the gravity. This is because the mass causing the gravity is very concentrated, making it relatively easy to increase your average distance from it.

So, we have the Sun, and a black hole with the same mass as the Sun. We're assuming the black hole has a diameter of, say, 1 meter over which its mass is evenly distributed (a true black hole is infinitely small, but true black holes cannot exist before infinity time has passed, and 1 meter is close enough for our purposes) and the Sun has a diameter of 1392000 kilometers over which its mass is evenly distributed. If we go to the center of either object, then the average distance to the Sun's mass is 492146.32 kilometers and the average distance to the black hole's mass is about 0.35 meters (35 centimeters). This means that, despite their masses being the same, the black hole exerts something like 1977208160000 times as much gravitational force (my calculations may be off somewhere but at any rate the factor is very large). However, now we move out to the Earth's orbital radius. Because the Sun is so far away, the distance between its close area and its far area is almost negligible, so the result is that the Sun's mass and the black hole's mass are about the same distance away. And same distance * same mass = same gravity. The Earth is not being subjected to almost any more gravity at that distance from the black hole as it is from the Sun.

I may not have explained all this very well, but trust me, it works. Actually, it isn't necessary to trust me. Ask any physicist and they'll tell you the same thing.

This is pretty stupid, but whatever, I'll go along.

First of all about the blackhole. Sure, none of us know what will happen if we do go into one but I think its safe to assume that'd we die. I don't see why anything after that would happen. Give it a couple million/billion years and it'd blow up, thats about it.

I think there are a couple ways for us to die. Nuclear winter is the obvious one. Second is that some douche (probably the us) creates a super virus type thingy and we all die from that. Third well, iono, how about sharks attach freaking laser beams to their backs and start shooting everything?