Check out the thread, "Are We Living in the Most Peaceful Times in History." War has always been a major survival risk to adults. Yet since WWII, averaged over the whole planet, it no longer is. Peace has of course not increased monotonically and there's no way to be positive that we won't backslide into another world war, but the odds of that will drop considerably when Bush crawls back under his rock in Texas.
This threat remains frustratingly unproven, since the evidence is far less abundant and consistent than we'd insist on before canonizing any other scientific theory. Still even in the worst case it's unlikely to cause the literal extinction of our species. Communities with solar or nuclear power can become self-contained, raise their drawbridges, feed themselves with hydroponics, and perhaps even sequester themselves in giant arcologies with climate control.
There are entire (large) countries where racism is at least illegal, if not completely obliterated. Religious differences are a far bigger risk. Still it's been argued that what we Americans see as "Islamic terrorism" is really politically and economically motivated, and what other people see as a Christian revival here is just the religion's death throes, before it fades away as it is doing in Europe and many other places.
The second derivative of the entire global population has been negative for a couple of decades. (I.e., the rate of increase, as measured by the birth rate, is falling.) The signs are everywhere. Many Western countries are already greying and their "social security" Ponzi Schemes are dependent on immigration to avoid collapsing. At the other end of the scale, in countries where the average family once had twelve children, they now have eight. Prosperity is the most powerful contraceptive and the human race's per-capita GDP is rising steadily. I've seen predictions that the population will peak before the end of this century, and then start falling.
It would take something like an asteroid hit to wipe us out, along with most of the other large non-aquatic species. The probability of that is of course non-zero but it's so small as to qualify as "extraordinary speculation" on a science board. I assume you want to discuss serious threats that are worth worrying about, or perhaps even lobbying Congress for action.
Not to veer off into science fiction.
Again, in a science fiction story it's easy to postulate something like that, but in real life the probability is too small to qualify as a major concern. The Plague only killed off about a third of the population of Europe, and the result was that the remaining two-thirds suddenly became fifty percent wealthier. That was arguably one of the causative factors in the progress of the last half millennium.
I forget, did I already say something dismissive about science fiction?
To give it the brief consideration it deserves: Physics is a rather mature science and we know quite a lot about how the universe works. (Compare this to, say, the science of psychology and its domain, the human mind.) It's extremely likely that the lightspeed limitation on space travel is both real and insurmountable. Voyages of interstellar exploration may take thousands of years and require generation-starships that are miniature cities. It will be difficult for one civilization to even find another one, even harder to send any information about it back home, and a herculean project to send back artifacts. The concept of a galactic civilization (or an interstellar war) is preposterous.
Ultimately the human race won't survive.
That's an extraordinary assertion and you are hereby challenged to provide the extraordinary evidence that supports it, in accordance with the scientific method. Our species is about 200,000 years old. The first stirrings of tool-building, which mark the beginning of human culture as qualitatively different from other animal societies, goes back two million years. If you're talking about a timeframe with six or seven zeroes, well then sure, the species
Homo sapiens may be extinct, but I see no evidence to suspect that our descendant species will not still be recognizable as "humans," just as our ancestral species were, with even a continuity of civilization going back to 9000BCE when the first villages were built and we gave up nomadism.
But will we survive the next 100 years?
30-50 years ago, with the Cold War raging, nuclear weapons proliferating, and "duck and cover" drills being practiced in schools, that was a very topical question. But now communism is dead in most places and emasculated in the rest. There are far fewer operational nuclear weapons than there used to be. Today the biggest threat to our survival is religion, particularly the Abrahamists, who rise up in paroxysms of genocidal violence every few generations and seem to be thinking about it now. But there's no way "religious" terrorists (which they probably are not, as I noted earlier) can get their hands on enough nuclear weapons to bring about Armageddon.
