Would it be possible for tornados to increase in frequency and/ or intensity to a level where human life as we know it would not be sustainable?
I’m unclear on what makes you think this is even plausible enough to ask about. Tornadoes are basically unheard-of in most of the world, and even in the places where they’re most common, they still have only a very minor impact on human life. What would it even mean for them to make human life unsustainable? Every acre of land on the planet being hit by a tornado every year?
Oh, you’re no fun!
Maybe the tornadoes couldn’t tear up every acre, but the associated rainfall could wash it all away …
People would simply move underground or underwater. Problem solved.
That’s quite the username/appropriate post combo…
Is this a scenario for “Sharknado 6”?
OP should read Cat’s Cradle by Kurt Vonnegut, which ends with life on earth being (mostly) wiped out (with a few scattered survivors here and there). One of the meteorological phenomena he posits for the post-apocalypse earth is ubiquitous and incessant tornadoes. (Of course, it doesn’t help that he also posits that most of the liquid water on earth is eliminated. That makes life difficult too.)
No. As Chronos notes, tornadoes are a pretty minor weather phenomenon. Like any storm, they are produced by temperature differences in the atmosphere. By the time global weather patterns had become so screwed up to produce tornadoes over most of the Earth’s land surface, humans would already be extinct due to other more important causes (extreme heat, torrential rains, hurricanes, etc.)
Tornadoes are no-hopers for making life impossible on the surface.
But here’s something a little more plausible for the OP to worry about: Hypercane - Wikipedia. If we ever got the ocean temps up that high these things would happen. And as we see with Harvey, they’d make quite a mess at landfall.
They’d still be limited to the tropics. Although with those kinds of ocean temps in the tropics, temps we now associate with the tropics may well be found a lot further north. So we’d have hypercanes where we now have hurricanes and hurricanes where we now don’t have them.
Such fun.
Here’s another kind of weird weather it’d be nice for us to not have: Raindrop 
My favorite title was “Sharknado 3: Hell No”.
PS. I can’t believe I just added Sharknado to my spell checker dictionary.
Dennis
Great minds think alike!
Great minds think alike!
In stereo even!
Unlike duck’s quacks, 2Bits’ posts echo. 
How pray tell, might one encounter a tornado with no water?
Where is the mechanism by which the tornado is existing now coming from?
Right … the energy source for tornadoes comes from the condensation of water vapor into it’s liquid state, releasing 2.2 KJ/kg of water vapor … in the “Ice-9” scenario there would be very very little water vapor available due to a (presumably) very very low vapor pressure of the “Ice-9” … but then again in a fictional story I suppose we can have our “Ice-9” sublimating at extremely high rates … as believable as the death penalty for not wearing shoes …
I think again we’d see near continuous rainfall across the tropics before sea surface temperatures reach 50ºC … as much as I hawk the frictional limits in the troposphere, we can add temperature inversion in the stratosphere as a major inhibitor for hypercane growth … I don’t know why Wikipedia states this as “hypothesis” without specifying the experiment that can be performed, looks more like speculation to me …
Agreed it’s speculative. But fun speculative.
Tornados:
Back in the 90s I moved to tornado country. Which led me to do a bunch of research on them. This was the early days of the public www and there were lots of homebrew knowledge sites about topic X set up by folks with an interest in X. And sometimes even actual factual expertise to go with their interest. 
Nowadays wiki has sorta cornered the market on well-sourced crowd-sourced wisdom. Not back then. Rather the opposite.
It’s gone now, and not even the Wayback Machine can find it, but there was a truly awesome website run by some crank who insisted tornados were actually electromagnetic. His “proof” was something like “The narrow powerful end of a tornado is at the ground where the flux lines emerge from the Earth; If they were actually an air vortex, the narrow powerful end would be at the top where the storm is.” He claimed to be an actual degreed engineer, so of course you could trust that he knew whereof he spoke. He had actual math formulas and everything.
It was less nutty than Flat Earthers, not paranoid CTish at all, and far more coherent than timecube. That it was abject bunk from end to end didn’t mean it wasn’t entertaining and superficially plausible.
It’s a shame he & his site are gone. They’re the sort of monument to a bygone time that we should have worked harder to save.
The relevance to your post being that since tornados are actually electric, they’ll still work fine even after the ice-9 has sequestered most of the liquid water. They might even work better given stronger static electric fields in the absence of all that conductive water vapor.
One could also explain the Ice-9 tornado scenario by noting that the whole story is, uh, y’know, fiction. My take: Vonnegut understood that the oceans have a very stabilizing effect on weather, so he posited that without oceans the weather would be very unstable, which he envisioned as incessant tornadoes.
Interesting side-fact: Kurt Vonnegut’s older brother, Bernard Vonnegut, was an atmospheric scientist, who invented silver-iodide cloud seeding to produce rain. One might wonder how he would have written it.