When analyzing complex systems with multiple interacting variables it is useful to note the advice of Enrico Fermi who reportedly said “never make something more accurate than absolutely necessary”. My recent paper presented a simple heuristic approach to climate science which plausibly proposed that a Millennial Turning Point (MTP) and peak in solar activity was reached in 1991.
The empirical temperature data is clear. The previous
millennial cycle temperature peak was at about 990. ( see Fig 3 in the link below) The recent
temperature Millennial Turning Point was about 2003/4 ( Fig 4 in link below ) which
correlates with the solar millennial activity peak at 1991+/.
The cycle is asymmetric with a 650 year +/- down-leg and a 350
+/- year up-leg. The suns magnetic field strength as reflected in its TSI will generally
decline (modulated by other shorter term super-imposed solar
activity cycles) until about 2650.
The temperature increase since about 1650 is clearly chiefly due to the up- leg in the natural solar activity millennial cycle as shown by Lean 2018 "Estimating Solar Irradiance Since 850 AD" Fig 5
Lean 2018 Fig 5.
This Lean figure shows an increase in TSI of about 2 W/m2 from the Maunder minimum to the 1991 activity peak . This TSI and solar magnetic field variation modulates the earths albedo via the GR flux and cloud cover. From the difference between the upper and lower quintiles of Fig 4 (in link below) a handy rule of thumb a la Fermi would conveniently equate this to a Northern Hemisphere temperature millennial cycle amplitude of about 2 degrees C with that amount of cooling probable by 2,650+/-.
The MTP in cloud cover was at about 2000.
The decline in solar activity (increase in neutron count ) since the 1991 solar activity MTP is seen in the Oulu neutron count.
Because of the thermal inertia of the oceans there is a varying
lag between the solar activity MTP and the varying climate
metrics. The temperature peak is about 2003/4 – lag is about 12
years. The arctic sea ice volume minimum was in 2012 +/- lag = 21
years. . For the details see data, discussion, and forecasts in Figs
3,4,5,10,11,and 12 in the links below.