“Hide the decline” refers to the decline in the Briffa MXD temperature reconstruction in the last half of the 20th century, a decline that called into question the validity of the tree ring reconstructions. (I’m going to analyze the letters on another occasion.) In the IPCC Third and Fourth Assessment Reports, IPCC “hid the decline” by simply deleting the post-1960 values of the troublesome Briffa reconstruction – an artifice that Gavin Schmidt characterizes as an “a good way to deal with a problem” and tells us that there is “nothing problematic” about such an artifice (see here.
Not only were the post-1960 values of the Briffa reconstruction not shown in the IPCC 2001 report – an artifice that Gavin describes as being “hidden in plain sight”, they were deleted from the archived version of the reconstruction at NOAA here (note: the earlier Briffa 2000 data here does contain a related series through to 1994.)
Interestingly, the values from 1402-1994 were sent from Tim Osborn to Michael Mann on a couple of occasions, including here.
Here’s a comparison showing the portion that was deleted from the NOAA archive in red.