There are multiple factors that affect both regional and global temperatures, but the most important source of long-term temperature change is solar activity. Using data from the Norwegian Sea and multiple solar proxies, the peer-reviewed research by Sejrup et al. confirms a robust and synchronous correlation between solar activity and temperatures.
“…worked with two sediment cores they extracted from the seabed of the eastern Norwegian Sea, developing a 1000-year proxy temperature record “based on measurements of δ18O in Neogloboquadrina pachyderma, 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…..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.” [Sejrup, H. P., S. J. Lehman, H. Haflidason, D. Noone, R. Muscheler, I. M. Berstad, and J. T. Andrews 2010: J. Geophys. Res]