I am sure the better informed reader can add a few more of these positive feedback loops, but in my humble opinion, these are the stronger ones, and make the process of global warming unstoppable.
You're so right! We did it! Humanity has achieved what supervolcanoes with all their CO2-emissions failed to achieve - massive positive feedback loops! Unstoppable!
For 4.5 billion years, earth might have maintained its fragile balance between supercold and Venus' twin brother engulved in heat and acid rain. It's completely beyond me how that could have lastet so long - no, it must be our special state of being the first intelligent living things, the first walking non-animals (clearly an abomination that has to be punished by nature) which tipped the scales. Now we're heading the Venus-way.
Of course this is bullshit.
Earth is currently relatively cool. It used to be a lot warmer most of the time. Whatever we do, the best we can hope to achieve is to heat it up to relatively warm and to do so quickly enough to cause mass extincion of plants and animals, opening massive ecological niches for new species to conquer.
We're possibly endangering human civilization in the process with it being undescided if the species will survive this or not. But the odds are, that we're positioned pretty well. We can eat both plants and meat and we've proven to be formidabely adoptable to various climates. Anyhow, even a complete breakdown of civilization will be unlikely to push us further back than rural africa, native americans in the rainforests or natives to australia in the outback.
So no, we're not destroying the planet. That's something completely impossible for us and quite unimaginable even for the future (if you define earth as a roughly ball-shaped lump of mostly rock floating around the sun).
No, we're not destroying life on earth. Life has endured worse than us and most of the biomass of the planet is presumably living underground anyway (as in bacteria with very slow metabolism capable to "feed" on the energies released by natural radioactivity found in very deep deep mines in south-africa, to name just one example.)
No, we're propably not destroying humanity either. Would be quite hard to achieve, given how far spread it is and how well adjusted to how many climates. And how it was so before the modern age.
But yes, we might endanger gouvernments, civilization as we know it and so forth. And Venice, of course, unless the dutch take over Italy. But why would they?