The message
from Terry Fields contains these words:

Points to note:

- only the 5-point moving average picks up a downturn circa 2004 or
so

- the 3rd order polynomial has a slightly higher correlation
coefficient than the 2nd order. The data at the 1850 end of the scale
suggests recover from the Little Ice Age....

Discuss....;-)

I think you have discovered why the Met Office have chosen a 21 point
binomial filter to smooth the curve. The more 'sophisticated' filters
ignore short term trends.

I would be interested to see how the polynomial filters would cope with
a subsequent downturn so how about inverting the 1950 - 2004 about 2005
(2006 = 2004 .... 2060 = 1950) and then doing several finish dates to
see when the downtown can first be clearly seen as well as what a mature
downturn would look like.

--
Roger Chapman