Read here. Analyzing sediment cores from the northeast Atlantic Ocean, researchers looked back some 2,400 years. Roman and Medieval Warming peak temperatures were significantly warmer than current period, while atmospheric CO2 was substantially lower. Why was Atlantic so much warmer in past?
"pervasive multidecadal- to centennial-scale variability throughout the sedimentary proxy records can be partly attributed to solar forcing and/or variable heat extraction from the surface ocean caused by shifts in the prevailing state of the North Atlantic Oscillation," as well as to "internal (unforced) fluctuations."