I'm personally more inclined to think we are less vulnerable than ever before.
I doubt any virus would wipe out an entire population. An infectious agent spreads better among a numerous population, but as it kills its hosts, the population becomes smaller, which slows the infection rate. So there is negative feedback here, and I doubt that can lead to a total extinction of the host species. Not to mention that we have modern medicine now, and that we understand how viruses work.
Climate change is an issue, but I have hard times imagining how it could wipe out all human beings. Worse case scenario, I guess, would be a sharp decline in agricultural production. So hunger riots and stuff. But even that would at worse reduce the population until a new equilibrium is reached. And I don't see that equilibrium to be at 0.
Nuclear war is a serious threat. But it's not like it would destroy the surface of the Earth or anything. It would provoke a nuclear winter though. I suppose a nuclear winter is about as bad as a volcanic winter is, or worse. Again, the main difficulty would be a sharp decline in food production. And that also would require a new population equilibrium to be reached.
IMHO human beings, nowadays, can deal with an awful lot of contingencies. We can predict them, we can store resources, we can plan ahead...
There really is a case to make about the idea that humans will survive a mass extinction. Annalee Newitz wrote a book about it
Yet all our ancestors have survived. I don't see why we, who are considerably more intelligent, who can predict celestial events, who can store food and water for years, who can build nuclear-bomb-proof bunkers, can generate energy from urianium dust, would do worse a job at surviving than tiny mouse-like animals 65 million years ago.
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