On the code thread, James Smith has just posted this comment:
From the file pl_decline.pro: check what the code is doing! It's reducing the temperatures in the 1930s, and introducing a parabolic trend into the data to make the temperatures in the 1990s look more dramatic.
Could someone else do a double check on this file? Could be dynamite if correct.
This is what all the fuss is about, but the reader who sent it thinks perhaps it may be a storm in a teacup. Still, it is strange that one would want to put an adjustment like this through a temperature series.
In the comments, Steve McIntyre reckons this is probably Briffa's infamous Tornetrask bodge. This was an adjustment to deal with the divergence between tree ring widths and densities. So it's MXD versus RW, not MXD vs temperature.