Background
The authors write that "the proxy record of solar variability from cosmogenic nuclides and telescopic observations of sunspots explains a substantial fraction of reconstructed Northern Hemisphere temperature variability during the pre-Industrial portion of the last millennium, with a simulated range of up to 0.4°C for plausible irradiance scaling and climate sensitivity," citing Crowley (2000) and Ammann et al. (2007); but they add that "at both the intra- and supra-decadal timescales there appear to be regional responses to solar forcing that are significantly larger than the global or hemisphere-scale response," citing Shindell et al. (2001), Woods and Lean (2007) and Tung and Camp (2008).

What was done
In further exploring this intriguing subject, Sejrup et al. worked with two sediment cores they extracted from the seabed of the eastern Norwegian Sea (~64°N, 3°E), developing a 1000-year proxy temperature record "based on measurements of δ¹⁸O in Neogloboquadrina pachyderma (dextral form), a planktonic foraminifer that calcifies at relatively shallow depths within the Atlantic waters of the eastern Norwegian Sea during late summer," which they compared with the temporal histories of various proxies of concomitant solar activity.

What was learned
This work revealed, as the seven scientists describe it, that "the lowest isotope values (highest temperatures) of the last millennium are seen ~1100-1300 A.D., during the Medieval Climate Anomaly, and again after ~1950 A.D." In between these two warm intervals, of course, were the colder temperatures of the Little Ice Age, when oscillatory thermal minima occurred at the times of the Dalton, Maunder, Sporer and Wolf solar minima, such that the δ¹⁸O proxy record of near-surface water temperature was found to be "robustly and near-synchronously correlated with various proxies of solar variability spanning the last millennium," with decade- to century-scale temperature variability of 1 to 2°C magnitude.

What it means
It would appear that the sun has "outshined" nearly all other forcings of climate change on earth over the past millennium.

References
Ammann, C.M., Joos, F., Schimel, D.S., Otto-Bliesner, B.L. and Tomas, R.A. 2007. Solar influence on climate during the past millennium: Results from transient simulations with the NCAR Climate System Model. Proceedings of the National Academy of Sciences USA 104: 3713-3718.

Crowley, T.J. 2000. Causes of climate change over the past 1000 years. Science 289: 270-277.

Shindell, D.T., Schmidt, G.A., Mann, M.E., Rind, D. and Waple, A. 2001. Solar forcing of regional climate change during the maunder minimum. Science 294: 2149-2152.

Tung, K.K. and Camp, C.D. 2008. Solar cycle warming at the Earth's surface in NCEP and ERA-40 data: A linear discriminant analysis. Journal of Geophysical Research 113: 10.1029/2007JD009164.

Woods, T.N. and Lean, J. 2007. Anticipating the next decade of sun-earth system variations. EOS, Transactions, American Geophysical Union 88: 457-458.