What was done
The authors assembled and analyzed a vast amount of data pertaining to phenomena that have been reliably linked to variations in solar activity, including frequencies of sunspot and aurora sightings, the abundance of carbon-14 in the rings of long-lived trees, and the amount of beryllium-10 in the annual ice layers of polar ice cores.  In the case of sunspot sightings, the authors used a catalogue of 235 Chinese, Korean and Japanese records compiled by Yau (1988), a catalogue of 270 Chinese records compiled by Zhuang and Wang (1988), and a time chart of 139 records developed by Clark and Stephenson (1979), as well as a number of later catalogues that made the overall record more complete.

What was learned
Over the past 1800 years, the authors identified "some nine cycles of solar brightness change," which include the well-known Oort, Wolf, Sporer, Maunder and Dalton Minima.  With respect to the Maunder Minimum - which occurred between 1645 and 1715 and is widely acknowledged to have been responsible for some of the coldest weather of the Little Ice Age - they report that the temperatures of that period "were about one-half of a degree Celsius lower than the mean for the 1970s, consistent with the decrease in the decadal average solar irradiance."  Then, from 1795 to 1825, came the Dalton Minimum, along with another dip in Northern Hemispheric temperatures.  Since that time, however, the authors say "the sun has gradually brightened" and "we are now in the Modern Maximum," which we and many others feel is largely responsible for the warmth of the Modern Warm Period.

What it means
The authors say that although the long-term variations in solar brightness they identified "account for less than 1% of the total irradiance, there is clear evidence that they affect the earth's climate."  And so they do.  Pang and Yau's dual plot of total solar irradiance and Northern Hemispheric temperature from 1620 to the present (their Fig. 1c), indicates that the former parameter (when appropriately scaled, but without reference to any specific climate-change mechanism) can account for essentially all of the net change experienced by the latter parameter up to about 1980.  After that time, however, the IPCC surface air temperature record rises dramatically, although radiosonde and satellite temperature histories largely match what would be predicted from the solar irradiance record.

Could these facts be interpreted as new evidence for the corruptness of the post-1980 global or Northern Hemispheric surface air temperature record?  With many other reasons to doubt the IPCC temperature history, they well could be.

References
Clark, D.H. and Stephenson, F.R.  1979.  A new revolution in solar physics.  Astronomy 7(2): 50-54.

Yau, K.K.C.  1988.  A revised catalogue of Far Eastern observations of sunspots (165 B.C. to A.D. 1918).  Quarterly Journal of the Royal Astronomical Society 29: 175-197.

Zhuang, W.F. and Wang, L.Z.  1988.  Union Compilation of Ancient Chinese Records of Celestial Phenomena.  Jiangsu Science and Technology Press, Jiangsu Province, China.