JA has a nice entry on Schwartz’ sensitivity estimate pointing out where the probable errors are.
I too found the 5-y timescale rather low. Fitting in with James’s assessment that S is wandering out of his field, I was surprised by the lack of discussion of how this new result fits in with everyone else’s answers. There is a brief note that “This climate sensitivity is much lower than current estimates, e.g., the Fourth Assessment Report of the Intergovernmental Panel on Climate Change…” but that seems to be about it.
Meanwhile, Irreducible imprecision in atmospheric and oceanic simulations (thanks to JR) may be interesting, though I haven’t read it yet.