An interesting footnote but I suspect it won't mean much in the overall scheme of things. They're saying a 0.3 degree C decrease per millennium based on one indicator, tree rings, in the face of a 0.8 degrees C increase over the last two hundred years using all the tools.

Anthony Watts' misdirect about tree rings was addressed in one the videos I posted that you didn't watch and is the root of the cranks "hide the decline" slur from the manufactured Climategate scandal.

Tree ring data has been used with coral, ice core samples, historical records and
ocean sediments to place the temperatures before the 1880s when we started
keeping the books on weather.

The tree ring data tracked along with the other indicators until the 1960s when it deviated (whether due to drought, acid rain or other pollution is unknown) but since
we were keeping temperature records then anyway, it wasn't that important.

That's the decline Mann wanted to hide because there were other more reliable
indicia that supported the scientific assertion that the atmosphere was gaining and
retaining energy due to all the carbon mankind has been belching into the air since
the industrial revolution began.

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