old school…not! I disproved the crystal sphere idea ages ago. 
obviously, since I said “like asteroids and comets”, I meant astronomical causes.
“we” certainly do not use A.U. to measure something at a distance of 1000 light years. AU are used on the scale of a solar system, not interstellar space.
I’ll have to check references to figure out which are the closest possibilities, but this is not sci-fi.
my telescope is bigger than your telescope 
Yes ‘We’ do. ‘We’ (Queen Victoria and I) decided that AUs are so great we use them to measure everything. When I go to the auto parts store (or almost anywhere else) I insist that all measurements be shown in AUs.
In fact, I will thank you to convert all future references of light years to either AUs or candlesticks.
So, how many femtoparsecs are there in a furlong?
There are some completely reputable scientists who believe that the world nuclear arsenal represents a sufficient source of destruction to end the phenomenon of human life on the planet. That assessment does not generally include every single weapon being exploded. It would not sterilize the planet, but it would probably eliminate enough of the food web to cause massive extinctions among earth’s species. The folks in New Zealand surviving assumes that the weather effects are not as severe as the strategic exchange models indicate. Those models only consider a couple of thousand nukes. Tens of thousands of nukes is not a scenario that is often examined.
I think the effects would include many things not dreamed of in the models.
Tris
Perhaps I had best clarify my earlier remarks… The way that a nuclear war or asteroid strike would cause extinctions is by affecting the weather, as several posters have pointed out. In a worst-case scenario, you might end up with a 5-year winter, say. However, we humans stockpile food. It’s not uncommon for a single family to keep enough basic staples in the house to last a full year, and if a few folks raided their local grocery stores/grain silos, some folks could eat for five years or more. Further, although the ecosystem would be devastated, we could also save seeds for a few plants that we considered important, and likewise save specimens of various animals.
As to the RHIC/black holes: There is a miniscule (say, about 1 in 10[sup]40[/sup]) chance that an experiment like the one at Brookhaven could produce a black hole. Even if it did, however, that black hole would evaporate instantly, if not quicker. The net result would be the release of as much energy as was put into the experiment: Possibly enough to do some damage to the lab, but not more than that. Such a black hole could not possibly swallow anything in its brief lifespan, much less the entire Earth.
Finally, with all do respect to Phobos, a neutron star collision wouldn’t do the trick, either, although it would do a pretty good job of killing everything on half of the Earth. Remember, the duration of such events is counted in the seconds, and no matter how strong the gamma source, you’re not going to get any significant transmission through the Earth itself. In order to sterilize the whole planet from space, you’d need something with a duration of a day or more, in or very near the plane of the equator.
A nuclear exchange might kill most humans, but if as little as a few thousand live the race is still viable. We as a species are tough bastards. We can live in climates as varied as the difference between Abu Dabi and Anchorage. We can live at those places, adapt to conditions, and still successfully interbreed. Humans can live on basically refuse, as the Middle Ages proved. Look at Louis XIV’s France. Look at how badly the pesants lived. Yet they did live, and live long enough to raise children that in turn lived long enough to raise children, etc. Humans have lived for thousands of years in places like northern Siberia and Greenland. Yak-herders live in Upper Mongolia, in the unforgiving steppes, and live fairly well considering how bad-off some humans have lived. The packers in Nepal and occupied Tibet would not be much affected by an exchange. They’d live at high altitudes without oxygen, like their ancestors have done for generations. Food webs in the hit areas would be affected, and shock waves might circle for a while, but life is a robust system. It can take such a small thing as some radioactive bombs in stride. Earth, and the life thereof, survived the Yucatan Impact 65 million years ago. That must have had effects similar to that of a large exchange. And when life survives, humans survive.
I love how Straight Dopers can casually and rationally discuss anything, up to and including the annihilation of life on Earth. 
Regarding the Black Death: “A third of Europe” isn’t actually high at all; general estimates put it right about there. Some estimates for the total death toll from the various forms of plague (bubonic, pneumonic, etc.) range up to half, but then you have to decide how you define Europe, where you draw the borders and how you calculate the population. In any event, in terms of total world population, the toll isn’t that high, because you leave out the Americas. The Black Death (known at the time as the “blue sickness,” FWIW) swept through Asia prior to its arrival in Europe, but records outside of China are pretty spotty.
The Great Influenza Pandemic around World War I is estimated to have killed between 17 and 22 million people, worldwide. Given the population of the time, that’s still the winner for deadly disease outbreaks in known history. AIDS is approaching that total death toll, particularly in Africa, but the higher total population worldwide means it constitutes a lower percentage.
Possible disease threats in the future? There’s always the possibility another strain of influenza could turn up, and chew a wide swath. Also, tuberculosis could be a big problem; lots of multiply-resistent varieties have been turning up, including some that respond to no, repeat, no antibiotics. In other words, if your body doesn’t fight it off on its own, it kills you. Not good, obviously, and TB isn’t the only infectious agent for which multiple resistance is rapidly increasing.
You also can’t discount the possible re-release of smallpox. After its eradication in 1977 (IIRC), samples were retained only in two secure locations in the U.S. and Russia, ostensibly for research purposes; but the Russian military is widely known to be experimenting with smallpox as a bioweapon. (Read Biohazard by Ken Alibek if you want nightmares.)
If a disease takes us out, it won’t be something horrifying like Ebola; that would burn too quickly through the infected population to be passed on to others. Rather, it’ll be something innocuous we think isn’t a problem, like TB or streptococcus.
(Can you tell my wife’s an epidemiologist?)
My understanding regarding the threat at Brookhaven was the possibility, although an extremely remote one, that ‘strangelets’ could be produced during the experiment and that these ‘strangelets’ would, through an unquenchable though not instantaneous chain reaction, eventually convert the world to ‘strange’ matter. One person discussing that possibility was quoted as comparing it to an “‘ice-nine’ scenario”. I love when scientists refer to great works of fiction.
Homie don’t play that shit. Femto is five more decimal places than my free solar-powered calulator will handle. That’s why we insist on AUs.
A slight hijack, in response to the early posts on the Cuban missile crisis.
I spent part of my childhood in Istanbul. In order for the Soviets to access the Med, they had to send their ships through the Bosphorus. NATO and Turkey were OK with this, as long as the Sovs asked permission. A couple of times they didn’t, resulting in gunboats halting the ship. NATO forces were immediately put on alert. These situations were quickly resolved, and IIRC were simply beauracratic (sp?) bungles. (Note: I was 10 at the time. Please no detailed questions) However, judging from my Dad’s reactions, these were tense, if brief situations.
Although Cuba was obviously the mother of all crises, U.S. forces were frequently on alert throughout the cold war. I’m still amazed we managed to survive the damn stupid thing.
No disease, even half a dozen different diseases at once, will wipe out humanity.
Consider that the native Americans had no immunity to smallpox, measles, and a bunch of other Old World diseases. They were decimated by these diseases, but still some managed to survive with no help from vaccines, antibiotics or other medicines. It’d be the same for humanity in general.
I think the answer lies in what you consider “the human race”. We may have all almost died in some pre-historic incident, or remember that time when our tribe of apes first came down out of the trees, and we almost got eaten by those carnivores? whew! that was a close call! or that time when we were all a mixture of primordial ooze and the DNA almost didn’t come together because the pH was borderline acidic…
We became closest to extinction as a human race on July 1, 2000. Consider the following:
I’m getting married at 3:00 today, and plan to enjoy it and my honeymoon.
Sorry to piss on everyone elses picnic.
HAPPY 4TH OF JULY EVERYBODY!!!
ha… you are close to being right, in a weird way. The closest we have come to extinction is NOW. I mean right NOW. Since the eventual fate of mankind is either to persist perpetually, or to become extinct, we are NOW, this very moment, inching towards extinction.
I tend to view this more like the Tralfamadorians from the book Slaughterhouse Five, who can see past, present and future all at once. When one of the characters asks the Tralfamadorians how the universe ends, he explains it was an accident, one of their rocket pilots pulls the wrong lever on the quantum engine, and the whole universe is vaporized in a moment. Why not stop him, if you know how to prevent it? They reply something to the effect of “The pilot will always pull the lever, he has always pulled the lever, and he will always have pulled the lever.”
I think that’s what I’m getting at. Its quite likely that we are already extinct and just haven’t climbed into our graves yet. Perhaps we already caused too much global warming, and its irreversible and will extinct us. Or perhaps there’s some mutant ebola strain that we haven’t discovered, but its out there in the ecosphere where it will eventually emerge and destroy all mankind. I don’t know, but I suspect that if there IS some sort of extinction threat (other than the obvious military ones) we’ve probably already encountered it and don’t realize it yet.
For the philosophers out there, I recommend a satyric essay by the sci-fi writer Stanislav Lem, called “The World as Cataclysm” (something like that). He posits that Earth and man’s evolution is a series of cataclysms, destructive events starting with the formation of the stars, and that we exist only as the product of a process of elimination. Star matter went adrift into space instead of forming stars, nearby stars were destroyed or went nova, planets formed but never accumulated water, life formed on planets but was extincted by volcanoes asteroids, etc etc… Man exists as the ultimate lottery-winner, everything else was extincted but us. We are nothing special except in terms of our tremendous luck to have survived while everything else died off.
I saw a program on the Discovery Channel the other night on the subject of super-volcanoes. Apparently there’s one under Yellowstone Nat’l Park that is at least somewhat active.
Anyways, they said that the timing of last super-volcano eruption about 74,000 years ago coincides roughly with mitochondrial DNA mutation evidence that somewhere in that vicinity the entire human population was reduced to a few thousand individuals.
Taking ren’s idea to the extreme, I’d say when the entire human race was a single fertilized egg we were the most vulnerable. This would have been when the first non-human ancestors conceived the first human. At that point, many small events in the womb of one pre-human could have wiped out humanity.
Arjuna34
I can’t quote the article, but some researchers said that based on analysis of human DNA, they think that approximately 200,000 - 300,000 years ago the entire human gene pool narrowed to perhaps a few hundred individuals. There might have been a time when one single tribe was IT for the entire human race.
Sam Stone is correct. I believe the greatest threat is RIGHT NOW. In the reign of homo sapiens with respect to actual threats all past nuclear, disease based etc. etc. pale in comparison to the potential for a true species ending occurrence/event in modernity as a result of the power only now being unleased by the unraveling of the secrets of the human genome (and it’s vulnerabilities).
It may seem sci-fi but IMO Before another century (perhaps only a generation) passes someone, somewhere will be able to contruct a deadly, virulent self replicating biological agent that can be instructed to mutate (while remaining deadly) in response to counter measures (think AIDS x 10 that could be communicated via airborne vectors) If this thing is made and if it gets out of control God help us all.
I’m voting for the last ice age, lots of mammals went extinct.
Well… yeah… lot’s of animals did go extinct but man wasn’t close to being one of them. According to anthropological evidence “man” (homo sapiens) had a grand time in the ice age.