Which have never actually been observed. The whole "positive feedbacks" thing is based on computer models which have been falsified by the actual data.
Besides that, it's based on the fact that we're already seeing things like polar ice melt and emissions from permafrost.
We don't have accurate enough geological evidence to support the kinds of "positive feedback" claims that are being made. Hansen's book lays out a partisan case.
> we're already seeing things like polar ice melt and emissions from permafrost.
What were these things doing a thousand years ago? Five thousand years ago? Ten thousand years ago? We don't know. And that means we don't have the data to put what we're currently seeing in proper perspective. And nobody is claiming that we do: as I said, the claims of possible disaster based on positive feedback are based on models, not data. And the models have been falsified by the data.
It seemed pretty convincing to me, since there were multiple independent lines of evidence that all pointed to the same conclusion.
No, I've read scientific papers on the topic, his and others. I don't trust what is said in books on science for lay people; I've caught too many scientists misrepresenting the science when they write for lay people, because they know they can get away with it (and not just in climate science--I've caught physicists doing it about general relativity and basic QM, which are a lot more solidly nailed down by evidence and controlled experiments than climate science is).