Read here. View full map source here and/or click on image to enlarge. From the Sargasso Sea, researchers worked with a combination of "biweekly hydrographic data, a long history of sediment trap collections to document the rain of particles from the sea surface to the seafloor, and exceptional deep sea cores of sediment" to reconstruct sea surface temperatures stretching back some 3,000 years. The empirical evidence confirms that the Bermuda sub-tropical area experienced warmer temperatures during both the Roman and Medieval warming periods versus modern temperatures. (click on image to enlarge)
"The core-top data indicate temperatures of nearly 23 degrees, very close to the average temperature at Station S over the past 50 years. However, during the Little Ice Age of about 300 years ago sea surface temperatures were at least a full degree lower than today, and there was an earlier cool event centered on 1,700 years ago. Events warmer than today occurred about 500 and 1,000 years ago, during the Medieval Warm Period, and it was even warmer than that prior to about 2,500 years ago."
"These results are exciting for a few reasons. First, events as young and as brief as the Little Ice Age and the Medieval Warm Period have never before been resolved in deep sea sediments from the open ocean. Because the Sargasso Sea has a rather uniform temperature and salinity distribution near the surface, it seems that these events must have had widespread climatic significance. The Sargasso Sea data indicate that the Medieval Warm Period may have actually been two events separated by 500 years, perhaps explaining why its timing and extent have been so controversial."