David Appell has written a rather strange article, purportedly about climate sensitivity, but actually about individual components of the climate system. It's only at the end that the climate sensitivity question is addressed:
“There’s really nothing in [the recent temperature record] that changes our estimates of climate sensitivity.” Calculation of that all-important number from the 20th century record is not possible, because the aerosol forcing is not well known, nor are the data for ocean warming up to the task.
“Any estimate of sensitivity requires all of the record and not just the last 20 years of it,” Pierrehumbert says. “The smaller the piece of it you take, the less certainty you have in your result.”
Nonetheless, he agrees that earlier warming may have been deceiving.
“I think it’s true that some rather sloppy discussion of the rapid warming from the 20th century has given people unrealistic expectations about the future course of warming.”
This is rather odd, because the IPCC publishes estimates of aerosol forcing. They may be uncertain, but they are hardly unknown. It's also rather odd that flat temperatures don't, in Pierrehumbert's opinion, change estimates of climate sensitivity. I'm sure someone (Ed Hawkins perhaps, or was it James Annan?) said that flat temperatures could do nothing except reduce the value.
Moreover, I also recall that Forster and Gregory reported that aerosol forcing affected the uncertainty of their estimate of climate sensitivity but not the value. I think I am also right in saying that their method is unaffected by ocean heat uptake uncertainty.
That said, the unrealistic expectations of the future engendered by sloppy discussion of 20th century warming need to be more widely recognised, so it's good to see the case being made by an RC insider.