Luckily we were able to discuss it further and clear up some of the confusion.

> The problem with the argument is, well, why doesn't this apply to everything? Massive changes happen all the time in the world, and anything could be a disaster if our only standard for "could" is lack of knowledge that it won't be. If someone said "we need to focus our global efforts on stone-faced office buildings, because humanity might go extinct if we keep building so many all-glass ones", we'd demand more than a lack of evidence.

I can't think of any comparible examples where human activity is known to be causing massive problems... Various forms of pollution maybe? But I would argue for action there too.